ASTRONOMY
THE SCIENCE OF THE HEAVENLY BODIES
DAVID TODD
DIBECTOR EMERITUS, AMHERST COLLEGE OBSERVATORY
NEW YORK AND LONDON
HARPER & BROTHERS
PUBLISHERS MCMXXII
PREFACE
SIR WILLIAM ROWAN HAMILTON, the emi-
nent mathematician of Dublin, has, of all writers
ancient and modern, most fittingly characterized
the ideal science of astronomy as man's golden chain
connecting the heavens to the earth, by which we
"learn the language and interpret the oracles of the
universe."
The oldest of the sciences, astronomy is also the
broadest in its relations to human knowledge and
the interests of mankind. Many are the cognate
sciences upon which the noble structure of astronomy
has been erected : foremost of all, geometry and the
higher mathematics, which tell us of motions, mag-
nitudes and distances; physics and chemistry, of
the origin, nature, and destinies of planets, sun, and
star; meteorology, of the circulation of their at-
mospheres ; geology, of the structure of the moon's
surface; mineralogy, of the constitution of mete-
orites; while, if we attack, even elementally, the
fascinating, though perhaps forever unsolvable,
problem of life in other worlds, the astronomer must
invoke all the resources that his fellow biologists
and their many-sided science can afford him.
The progress of astronomy from age to age has
been far from uniform rather by leaps and bounds :
from the earliest epoch when man's planet earth
was the center about which the stupendous cosmos
wheeled, for whom it was created, and for whose
edification it was maintained down to the modern
3
?* *
4 PREFACE
age whose discoveries have ascertained that even
our stellar universe, the vast region of the solar
domain, is but one of the thousands of island uni-
verses that tenant the inconceivable immensities of
space.
Such results have been attainable only through
the successful construction and operation of monster
telescopes that bring to the eye and visualize on
photographic plates the faintest of celestial objects
which were the despair of astronomers only a few
years ago.
But the end is not yet; astronomy to-day is but
passing from infancy to youth. And with new and
greater telescopes, with new photographic processes
of higher sensitivity, with the help of modern in-
vention in overcoming the obstacle of the air that
constant foe of the astronomer who will presume
to set down any limit to the leaps and bounds of
astronomy in the future?
So rapid, indeed, has been the progress of as-
tronomy in very recent years that the present is
especially favorable for setting forth its salient
features; and this book is an attempt to present
the wide range of astronomy in readable fashion,
as if a story with a definite plot, from its origin
with the shepherds of ancient Chaldea down to
present-day ascertainment of the actual scale of the
universe, and definite measures of the huge volume
of supersolar giants among the stars.
DAVID TODD
AMHERST COLLEGE OBSERVATORY
November, 1921
CONTENTS
CHAPTER PAGE
I. ASTRONOMY A LIVING SCIENCE .... 9
II. THE FIRST ASTRONOMERS 19
III. PYRAMID, TOMB, AND TEMPLE .... 23
IV. ORIGIN OF GREEK ASTRONOMY .... 27
V. MEASURING THE EARTH ERATOSTHENES . 30
VI. PTOLEMY AND His GREAT Boox ... 33
VII. ASTRONOMY OF THE MIDDLE AGES ... 37
VIII. COPERNICUS AND THE NEW ERA .... 42
IX. TYCHO, THE GREAT OBSERVER 45
X. KEPLER, THE GREAT CALCULATOR .... 49
XL GALILEO, THE GREAT EXPERIMENTER ... 53
XII. AFTER THE GREAT MASTERS 57
XIII. NEWTON AND MOTION 62
XIV. NEWTON AND GRAVITATION 66
XV. AFTER NEWTON 73
XVI. HALLEY AND His COMET 83
XVII. BRADLEY AND ABERRATION 90
XVIII. THE TELESCOPE 93
XIX. REFLECTORS MIRROR TELESCOPES .... 102
XX. THE STORY OF THE SPECTROSCOPE . . . Ill
XXI. THE STORY OF ASTRONOMICAL PHOTOGRAPHY 125
XXII. MOUNTAIN OBSERVATORIES 139
XXIII. THE PROGRAM OF A GREAT OBSERVATORY . 152
XXIV. OUR SOLAR SYSTEM 162
XXV. THE SUN AND OBSERVING IT 165
XXVI. SUN SPOTS AND PROMINENCES .... 174
XXVII. THE INNER PLANETS 189
XXVIII. THE MOON AND HER SURFACE . . . . 193
XXIX. ECLIPSES OF THE MOON 206
XXX. TOTAL ECLIPSES OF THE SUN 209
XXXI. THE SOLAR CORONA 219
5
G
CONTENTS
CHAPTER PAGE
XXXII. THE RUDDY PLANET 227
XXXIII. THE CANALS OF MARS . . . . ' . .235
XXXIV. LIFE IN OTHER WORLDS 242
XXXV. THE LITTLE PLANETS 254
XXXVI. THE GIANT PLANET 260
XXXVII. THE RINGED PLANET 264
XXXVIII. THE FARTHEST PLANETS 267
XXXIX. THE TRANS-NEPTUNIAN PLANET . . . 270
XL. COMETS THE HAIRY STARS . ... . 273
XLI. WHERE Do COMETS COME FROM? . . . 279
XLII. METEORS AND SHOOTING STARS .... 283
XLIII. METEORITES 290
XLIV. THE UNIVERSE OF STARS 294
XLV. STAR CHARTS AND CATALOGUES .... 300
XLVI. THE SUN'S MOTION TOWARD LYRA . . 304
XLVII. STARS AND THEIR SPECTRAL TYPE . . . 307
XLVIII. STAR DISTANCES 311
XLIX. THE NEAREST STARS 319
L. ACTUAL DIMENSIONS OF THE STARS . . 321
LI. THE VARIABLE STARS ....... 324
LII. THE NOV^E, OR NEW STARS . . . .331
LIII. THE DOUBLE STARS 334
LIV. THE STAR CLUSTERS 336
LV. MOVING CLUSTERS 341
LVI. THE Two STAR STREAMS 345
LVII. THE GALAXY OR MILKY WAY .... 350
LVIII. STAR CLOUDS AND NEBULAE 357
LIX. THE SPIRAL NEBULA 361
LX. COSMOGONY 366
LXI. COSMOGONY IN TRANSITION 380
LIST OF ILLUSTRATIONS
ACTIVE PROMINENCE OF THE SUN, 140,000 MILES HIGH
Frontispiece
FACING PAGE
NICHOLAS COPERNICUS 64
GALILEO GALILEI 64
JOHANN KEPLER 65
SIR ISAAC NEWTON 65
THE HUNDRED-INCH REFLECTING TELESCOPE AT MOUNT
WILSON 96
THE FORTY-INCH REFRACTING TELESCOPE, YERKES OB-
SERVATORY 96
150-FooT TOWER, MOUNT WILSON, A DIAGRAM OF TOWER
AND PIT 97
150-FooT TOWER EXTERIOR VIEW 97
VIEW LOOKING DOWN INTO THE PIT BENEATH 150-FooT
TOWER 97
MOUNT WILSON SOLAR OBSERVATORY THE 100-FooT
DOME 128
MOUNT CHIMBORAZO, THE BEST SITE IN THE WORLD FOR
AN OBSERVATORY 128
LICK OBSERVATORY, MOUNT HAMILTON, CALIFORNIA . 129
PHOTOGRAPHING WITH THE 40-INCH REFRACTOR . . 129
GREAT SUNSPOT GROUP OF AUGUST 8, 1917 .... 160
CALCIUM FLOCCULI ON THE SUN 161
ECLIPSE OF THE MOON, WITH THE LUNAR SURFACE VISIBLE 161
MOON'S SURFACE IN THE REGION OF COPERNICUS . . 192
SOUTH CENTRAL PORTION OF THE MOON, AT LAST
QUARTER 193
7
8 LIST OF ILLUSTRATIONS
FACING PAGB
CORONA OF THE SUN DURING AN ECLIPSE .... 224
VENUS, IN THE CRESCENT PHASE 225
MARS, SHOWING BRIGHT POLAR CAP 225
JUPITER, THE GIANT PLANET 256
NEPTUNE AND ITS SATELLITES 256
SATURN, WITH EDGE OP RINGS ONLY IN VIEW . . . 257
SATURN, WITH RINGS DISPLAYED TO FULLEST EXTENT . 257
Two VIEWS OF HALLEY'S COMET 288
SWIFT'S COMET, WHICH SHOWED REMARKABLE TRANS-
FORMATIONS 288
METEOR TRAIL IN FIELD WITH FINE NEBULAE . . . 289
RING NEBULA IN LYRA 320
DUMB-BELL NEBULA 321
STAR CLOUDS AND BLACK HOLES IN SAGITTARIUS . . 352
GREAT NEBULA IN ANDROMEDA . 353
CHAPTER I
ASTRONOMY A LIVING SCIENCE
TIKE life itself we do not know when astronomy
-L-^ began; we cannot conceive a time when it was
not. Man of the early stone age must have begun
to observe sun, moon, and stars, because all the
bodies of the cosmos were there, then as now. With
his intellectual birth astronomy was born.
Onward through the childhood of the race he
began to think on the things he observed, to make
crude records of times and seasons; the Chaldeans
and Chinese began each their own system of
astronomy, the causes of things and the reasons
underlying phenomena began to attract attention,
and astronomy was cultivated not for its own sake,
but because of its practical utility in supplying the
data necessary to accurate astrological prediction.
Belief in astrology was universal.
The earth set in the midst of the wonders of the
sky was the reason for it all. Clearly the earth
was created for humanity ; so, too, the heavens were
created for the edification of the race. All was sub-
servient to man; naturally all was geocentric, or
earth-centered. From the savage who could count
only to five, the digits of one hand, civilized man
very slowly began to evolve; he noted the progress
of the seasons; the old records of eclipses showed
Thales, an early Greek, how to predict their
happenings, and true science had its birth when
9
10 ASTRONOMY TO-DAY
man acquired the power to make forecasts that
always came true.
Few ancient philosophers were greater than
Pythagoras, and his conceptions of the order of
the heavens and the shape and motion of the
earth were so near the truth that we sometimes
wonder how they could have been rejected for
twenty centuries. We must remember, however,
that man had not yet learned the art of measuring
things, and the world could not be brought into
subjection to him until he had. To measure he must
have tools instruments; to have instruments he
must learn the art of working in metals, and all
this took time ; it was a slow and in large part
imperceptible process ; it is not yet finished.
The earliest really sturdy manifestation of
astronomical life came with the birth of Greek
science, culminating with Aristarchus, Hipparchus
and Ptolemy. The last of these great philosophers,
realizing that only the art of writing prevents man's
knowledge from perishing with him, set down all
the astronomical knowledge of that day in one of
the three greatest books on astronomy ever written,
the Almagest, a name for it derived through the
Arabic, and really meaning "the greatest."
The system of earth and heaven seemed as if
finished, and the authority of Ptolemy and his Alma-
gest were as Holy Writ for the unfortunate cen-
turies that followed him. With fatal persistence
the fundamental error of his system delayed the
evolutionary life of the science through all that
period.
But man had begun to measure. Geometry had
been born and Eratosthenes had indeed measured
the size of the earth. Tools in bronze and iron were
ASTRONOMY A LIVING SCIENCE 11
fashioned closely after the models of tools of stone;
astrolabes and armillary spheres were first built
on geometric spheres and circles; and science was
then laid away for the slumber of the Dark
Ages.
Nevertheless, through all this dreary period the
life of the youthful astronomical giant was main-
tained. Time went on, the heavens revolved; sun,
moon, and stars kept their appointed places, and
Arab and Moor and the savage monarchs of the East
were there to observe and record, even if the world-
mind was lying fallow, and no genius had been born
to inspire anew that direction of human intellect on
which the later growth of science and civilization
depends. With the growth of the collective mind
of mankind, from generation to generation, we note
that ordered sequence of events which characterizes
the development of astronomy from earliest peoples
down to the age of Newton, Herschel, and the
present. It is the unfolding of a story as if with a
definite plot from the beginning.
Leaving to philosophical writers the great funda-
mental reason underlying the intellectual lethargy
of the Dark Ages, we only note that astronomy and
its development suffered with every other depart-
ment of human activity that concerned the intel-
lectual progress of the race. To knowledge of every
sort the medieval spirit was hostile. But with the
founding and growth of universities, a new era
began. The time was ripe for Copernicus and a
new system of the heavens. The discovery of the
New World and the revival of learning through
the universities added that stimulus and inspiration
which marked the transition from the Middle Ages
to our modern era, and the life of astronomy, long
12 ASTRONOMY TO-DAY
dormant, was quickened to an extraordinary de-
velopment.
It fell to the lot of Copernicus to write the second
great book on astronomy, "De Revolutionibus Or-
bium Coelestium." But the new heliocentric or sun-
centered system of Copernicus, while it was the true
system bidding fair to replace the false, could not
be firmly established except on the basis of accurate
observation.
How fortunate was the occurrence of the new
star of 1572, that turned the keen intellect of Tycho
Brahe toward the heavens! Without the observa-
tional labors of Tycho's lifetime, what would the
mathematical genius of Kepler have availed in dis-
covery of his laws of motion of the planets ?
Historians dwell on the destruction and violent
conflicts of certain centuries of the Middle Ages,
quite overlooking the constructive work in progress
through the entire era. Much of this was of a nature
absolutely essential to the new life that was to
manifest itself in astronomy. The Arabs had made
important improvements in mathematical processes,
European artisans had made great advances in the
manufacture of glass and in the tools for working
in metals.
Then came Galileo with his telescope revealing
anew the universe to mankind. It was the north of
Italy where the Renaissance was most potent, re-
calling the vigorous life of ancient Greece. Coperni-
cus had studied here; it was the home of Galileo.
Columbus was a Genoese, and the compass which
guided him to the Western World was a product
of deft Italian artisans whose skill with that of
their successors was now available to construct the
instruments necessary for further progress in the
ASTRONOMY A LIVING SCIENCE 13
accurate science of astronomical observation. Even
before Copernicus, Johann Miiller, better known as
Regiomontanus, had imbibed the learning of the
Greeks while studying in Italy, and founded an ob-
servatory and issued nautical almanacs from Nu-
remberg, the basis of those by which Columbus was
guided over untraversed seas.
About this time, too, the art of printing was
invented, and the interrelation of all the movements
then in progress led up to a general awakening of
the mind of man, and eventually an outburst in
science and learning, which has continued to the
present day. Naturally it put new life into astron-
omy, and led directly up from Galileo and his experi-
mental philosophy to Newton and the "Principia,"
the third in the trinity of great astronomical books
of all time.
To get to the bottom of things, one must study
intimately the history of the intellectual develop-
ment of Europe through the fifteenth and sixteenth
centuries. Many of the western countries were ruled
by sovereigns of extraordinary vigor and force of
character, and their activities tended strongly to-
ward that firm basis on which the foundations of
modern civilization were securely laid.
Contemporaneously with this era, and following
on through the seventeenth century, came the
measurements of the earth by French geodesists,
the construction of greater and greater telescopes
and the wonderful discoveries with them by Huy-
gens, Cassini, and many others.
Most important of all was the application of
telescopes to the instruments with which angles are
measured. Then for the first time man had begun
to find out that by accurate measures of the heavenly
14 ASTRONOMY TO-DAY
bodies, their places among the stars, their sizes and
distances, he could attain to complete knowledge of
them and so conquer the universe.
But he soon realized the insufficiency of the
mathematical tools with which he worked how un-
suited they were to the solution of the problem of
three bodies (sun, earth, and moon) under the New-
tonian law of gravitation, let alone the problem of
n-bodies, mutually attracting each the other; and
every one perturbing the motion of every other one.
So the invention of new mathematical tools was
prosecuted by Newton and his rival Leibnitz, who,
by the way, showed himself as great a man as
mathematician : "taking mathematics," wrote Leib-
nitz, "from the beginning of the world to the times
when Newton lived, what he had done was much
the better half." Newton was the greatest of astron-
omers who, since the revival of learning, had ob-
served the motions of the heavenly bodies and
sought to find out why they moved.
Copernicus, Tycho Brahe, Galileo, Kepler, New-
ton, all are bound together as in a plot. Not one of
them can be dissociated from the greatest of all
discoveries. But Newton, the greatest of them all,
revealed his greatness even more by saying: "If I
have seen further than other men, it is because I
have been standing on the shoulders of giants."
Elsewhere he says : "All this was in the two plague
years of 1665 and 1666 [he was then but twenty-
four] , for in those days I was in the prime of my
age for invention, and minded mathematics and
philosophy more than at any time since." All school
children know these as the years of the plague and
the fire; but very few, in school or out, connect
these years with two other far-reaching events in
ASTRONOMY A LIVING SCIENCE 15
the world's history, the invention of the infinitesimal
calculus and the discovery of the law of gravitation.
We have passed over the name of Descartes, al-
most contemporary with Galileo, the founder of
modern dynamics, but his initiation of one of the
greatest improvements of mathematical method
cannot be overlooked. This era was the beginning
of the Golden Age of Mathematics that embraced
the lives of the versatile Euler, equally at home in
dynamics and optics and the lunar theory; of La
Grange, author of the elegant "Mecanique Ana-
lytique"; and La Place, of the unparalleled "Meca-
nique Celeste." With them and a fully elaborated
calculus Newton's universal law had been extended
to all the motions of the cosmos. Even the tides and
precession of the equinoxes and Bradley's nutation
were accounted for and explained. Mathematical or
gravitational astronomy had attained its pinnacle
it seemed to be a finished science: all who were to
come after must be but followers.
The culmination of one great period, however,
proved to be but the inception of another epoch in
the development of the living science.
The greatest observer of all time, with a tele-
scope built by his own hands, had discovered a great
planet far beyond the then confines of the solar sys-
tem. Mathematicians would take care of Uranus,
and Herschel was left free to build bigger telescopes
still, and study the construction of the stellar uni-
verse. Down to his day astronomy had dealt almost
wholly with the positions and motions of the celes-
tial bodies astronomy was a science of where.
To inquire what the heavenly bodies are, seemed
to Herschel worthy of his keenest attention also.
While "a knowledge of the construction of the
16 ASTRONOMY TO-DAY
heavens has always been the ultimate object of my
observations," as he said, and his ingenious method
of star-gauging was the first practicable attempt to
investigate the construction of the sidereal universe,
he nevertheless devoted- much time to the descrip-
tion of nebulae and their nature, as well as their
distribution in space. He was the founder of double-
star astronomy, and his researches on the light of
the stars by the simple method of sequences were
the inception of the vast fields of stellar photometry
and variable stars. The physics of the sun, also, was
by no means neglected; and his lifework earned
for him the title of father of descriptive astronomy.
While progress and discovery in the earlier fields
of astronomy were going on, the initial discoveries
in the vast group of small planets were made at the
beginning of the nineteenth century. The great
Bessel added new life to the science by revolution-
izing the methods and instruments of accurate
observation, his work culminating in the measure
of the distance of 61 Cygni, first of all the stars
whose distance from the sun became known.
Wonderful as was this achievement, however, a
greater marvel still was announced just before the
middle of the century a new planet far beyond
Uranus, whose discovery was made as a direct re-
sult of mathematical researches by Adams and Le
Verrier, and affording an extraordinary verification
of the great Newtonian law. These were the days of
great discoveries, and about this time the giant
of all the astronomical tools of the century was
erected by Lord Rosse, the "Leviathan" reflector
with a speculum six feet in diameter, which re-
mained for more than half a century the greatest
telescope in the world, and whose epochal discovery
ASTRONOMY A LIVING SCIENCE 17
of spiral nebulae has greater significance than we
yet know or perhaps even surmise.
The living science was now at the height of a
vigorous development, when a revolutionary dis-
covery was announced by Kirchhoff which had been
hanging fire nearly half a century the half cen-
tury, too, which had witnessed the invention of
photography, the steam engine, the railroad, and the
telegraph: three simple laws by which the dark
absorption lines of a spectrum are interpreted, and
the physical and chemical constitution of sun and
stars ascertained, no matter what their distance
from us.
Huggins in England and Secchi in Italy were
quick to apply the discovery to the stars, and Draper
and Pickering by masterly organization have photo-
graphed and classified the spectra of many hundred
thousand stars of both hemispheres, a research of
the highest importance which has proved of unique
service in studies of stellar movements and the
structure of the universe by Eddington and Shapley,
Campbell and Kapteyn, with many others who are
still engaged in pushing our knowledge far beyond
the former confines of the universe.
Few are the branches of astronomy that have not
been modified by photography and the spectroscope.
It has become a measuring tool of the first order of
accuracy; measuring the speed of stars and nebulae
toward and from us ; measuring the rotational speed
of sun and planets, corona and Saturnian ring;
measuring the distances of whole classes of stars
from the solar system; measuring afresh even the
distance of the sun the yardstick of our immediate
universe; measuring the drift of the sun with his
entire family of planets twelve miles every second
18 ASTRONOMY TO-DAY
in the direction of Alpha Lyrae; and discovering
and measuring the speed of binary suns too close
together for our telescopes, and so making real
the astronomy of the invisible.
Impatient of the handicap of a turbulent atmos-
phere, the living science has sought out mountain
tops and there erected telescopes vastly greater than
the "Leviathan" of a past century. There the sun
in every detail of disk and spectrum is photographed
by day, and stars with their spectra and the nebulse
by night. Great streams of stars are discovered
and the speed and direction of their drift ascer-
tained. The marvels of the spiral nebulse are un-
folded, their multitudinous forms portrayed and
deciphered.
And their distances ? And the distances of the still
more wonderful clusters? Far, inconceivably far
beyond the Milky Way. And are they "island uni-
verses"? And can man, the measurer, measure the
distance of the "mainland" beyond?
CHAPTER II
THE FIRST ASTRONOMERS
WHO were the first astronomers? And who
wrote the first treatise on astronomy, oldest
of the sciences?
Questions not easy to answer in our day. With
the progress of archaeological research, or inquiry
into the civilization and monuments of early
peoples, it becomes certain that man has lived on
this planet earth for tens of thousands of years in
the past as an intelligent, observing, intellectual
being; and it is impossible to assign any time so
remote that he did not observe and philosophize
upon the firmament above.
We can hardly imagine a people so primitive that
they would fail to regard the sun as "Lord of the
Day," and therefore all important in the scheme of
things terrestrial. Says Anne Bradstreet of the
sun in her "Contemplations" :
What glory's like to thee?
Soul of this world, this universe's eye,
No wonder some made thee deity.
To the Babylonians belongs the credit of the
oldest known work on astronomy. It was written
nearly six thousand years ago, about B. c. 3800, by
their monarch Sargon the First, King of Agade.
Only the merest fragments of this historic treatise
have survived, and they indicate the reverence of
19
20 ASTRONOMY TO-DAY
the Babylonians for the sun. Another work by
Sargon is entitled "Omens," which shows the inti-
mate relationship of astronomy to mysticism and
superstitious worship at this early date, and which
persists even at the present day.
As remotely as B. c. 3000, the sun-god Shamash
and his wife Aa are carved upon the historic
cylinders of hematite and lapis lazuli, and one of
the oldest designs on these cylinders represents the
sun-god coming out of the Door of Sunrise, while a
porter is opening the Gate of the East. The
Semitic religion had as its basis a reverence for the
bodies of the sky; and Samson, Hebrew for sun,
was probably the sun-god of the Hebrews. The
Phoenician deity, Baal, was a sun-god under differ-
ing designations ; and at the epoch of the Shepherd
Kings, about B. C. 1500, during the Hyksos dynasty,
the sun-god was represented by a circle or disk
with extended rays ending in hands, possibly the
precursor of the frequently recurring Egyptian
design of the winged disk or winged solar globe.
Hittites, Persians, and Assyrians, as well as the
Phoenicians, frequently represented the sun-god
in similar fashion in their sacred glyphs or
carvings.
For a long period in early human history, as-
tronomy and astrology were pretty much the same.
We can trace the history of astrology back as far
as B. C. 3000 in ancient Babylonia. The motions of
the sun, moon, and the five lucid planets of that
time indicated the activity of the various gods
who influenced human affairs. So the Babylonian
priests devised an elaborate system of interpreting
the phenomena of the heavens; and attaching the
proper significance in human terms to everything
THE FIRST ASTRONOMERS 21
that took place in the sky. In Babylonia and As-
syria it was the king and his people for whom the
prognostications were made out. It was the same
in Egypt. Later, about the fifth century B. c.,
astrology spread through Greece, where astrologers
developed the idea of the influence of planets upon
individual concerns. Astrology persisted through
the Dark Ages, and the great astronomers Coper-
nicus, Tycho, Kepler, Gassendi, and Huygens were
all astrologers as well. Milton makes many refer-
ences to planetary influence, our language has many
words with a direct origin in astrology, and in our
great cities to-day are many astrologers who pre-
pare individual horoscopes of more than ordinary
interest.
It is difficult to assign the antiquity of the
Chinese astronomy with any approach to definite-
ness. Their earliest records appear to have been
total eclipses of the sun, going back nearly 2,200
years before the Christian era; and nearly a
thousand years earlier the Hindu astronomy sets
down a conjunction of all the planets, concerning
which, however, there is doubt whether it was
actually observed or merely calculated backward.
Owing to a colossal misfortune, the burning of all
native scientific books by order of the Emperor
Tsin-Chi-Hwang-Ti, in B. c. 221, excepting only
the volumes relating to agriculture, medicine, and
astrology, the Chinese lost a precious mass of astro-
nomical learning, accumulated through the ages. No
less an authority than Wells Williams credits them
with observing 600 solar eclipses between B. C. 2159
and A. D. 1223, and there must have been some cen-
turies of eclipses observed and recorded anterior to
B. C. 2159, as this is the date assigned to the eclipse
22 ASTRONOMY TO-DAY
which came unheralded by the astronomers royal,
Hi and Ho, who had become intoxicated and forgot
to warn the Court, in accord with their duty. China
was thereby exposed to the anger of the gods, and
Hi and Ho were executed by his Majesty's com-
mand. It is doubtful if there is an earlier record
of any celestial phenomenon.
CHAPTER III
PYRAMID, TOMB, AND TEMPLE
TNQUIRY into the beginnings of astronomy in
JL ancient Egypt reveals most interesting relations
of the origins of the science to the life and work and
worship of the people. Their astronomers were
called the "mystery teachers of heaven"; their
monuments indicate a civilization more or less ad-
vanced; and their temples were built on astronom-
ical principles and dedicated to purpose of wor-
ship. The Egyptian records carry us back many
thousands of years, and we find that in Egypt, as
in other early civilizations observation of the
heavenly bodies may be embraced in three pretty
distinct stages. Awe, fear, wonder and worship
were the first. Then came utility : a calendar was
necessary to tell men when "to plow and sow, to
reap and mow," and a calendar necessitated astro-
nomical observations of some sort. Following this,
the third direction required observations of celestial
positions and phenomena also, because astrology, in
which the potentates of every ancient realm believed,
could only thrive as it was based on astronomy.
Sun worship was preeminent in early Egypt as
in India, where the primal antithesis between night
and day struck terror in the unformed mind of man.
In one of the Vedas occurs this significant song to
the god of day: "Will the Sun rise again? Will our
old friend the Dawn come back again? Will the
23
24 ASTRONOMY TO-DAY
power of Darkness be conquered by the God of
Light?"
Quite different from India, however, is Egypt in
matters of record: in India, records in papyrus,
but no monuments of very great antiquity; in
Egypt, no papyrus, but monuments of exceeding an-
tiquity in abundance. Herodotus and Pliny have
told us of the great antiquity of these monuments,
even in their own day, and research by archaeologist
and astronomer has made it certain that the pyra-
mids were built by a race possessing great knowledge
of astronomy. Their temples, too, were constructed
in strict relation to stars. Not only are the tem-
ples, as Edfu and Denderah, of exceeding interest
in themselves, but associated with them are often
huge monoliths of syenite, obelisks of many hundred
tons in weight, which the astronomer recognizes as
having served as observation pillars or gnomons.
Specimens of these have wandered as far from
home as Central Park and the bank of the Thames.
But there is an even more remarkable wealth of
temple inscriptions, zodiacs especially.
Next to the sun himself was the worship of the
Dawn and Sunrise, the great revelations of nature.
There were numerous hymns to the still more
numerous sun-gods and the powers of sunlight.
Ra was the sun-god in his noontide strength ; Osiris,
the dying sun of sunset. Only two gods were as-
sociated with the moon, and for the stars a special
goddess, Sesheta. Sacrifices were made at day-
break; and the stars that heralded the dawn were
the subjects of careful observation by the sacrificial
priests, who must therefore have possessed a good
knowledge of star places and names, doubtless in
belts of stars extending clear around the heavens.
PYRAMID, TOMB, AND TEMPLE 25
These decans, as they were called, are the exact
counterparts of the moon stations devised by the
Arabians, Indians, and other peoples for a like
purpose.
The plane or circle of observation, both in Egypt
and India, was always the horizon, whether the sun
was observed or moon or stars. So the sun was
often worshiped by the ancient Egyptians as the
"Lord of the Two Horizons." It is sometimes
difficult to keep in mind the fact, in regard to all
temples of the ancients, whether in Egypt or else-
where, that in studying them we must deal with
the risings or settings of the heavenly bodies in
quite different fashion from that of the astrono-
mer of to-day, who is mainly concerned only with
observing them on the meridian. The axis of the
temple shows by its direction the place of rising or
setting: if the temple faces directly east or west,
its amplitude is 0. Now the sun, moon, and planets
are, as everyone knows, very erratic as to their
amplitudes (i. e., horizon points) of rising and set-
ting ; so it must have been the stars that engrossed
the attention of the earliest builders of temples.
After that, temples were directed to the rising sun,
at the equinox or solstices. Then came the neces-
sity of finding out about the inclination or obliquity
of the ecliptic, and this is where the gnomon was
employed.
At Karnak are many temples of the solstitial
order: the wonderful temple of Amen-Ra is so or-
iented that its axis stands in amplitude 26 degrees
north of west, which is the exact amplitude of the
sun at Thebes at sunset of the summer solstice.
The axis of a lesser temple adjacent points to 26
degrees south of east, which is the exact amplitude
26 ASTRONOMY TO-DAY
of sunrise at the winter solstice. At Gizeh we find
the temples oriented, not solstitially, but by the
equinoxes, that is, they face due east and west.
Peoples who worshiped the sun at the solstice must
have begun their year at the solstice ; and Sir Nor-
man Lockyer shows how the rise of the Nile, which
took place at the summer solstice, dominated not
only the industry but the astronomy and religion of
Egypt.
Looking into the question of temple orientation
in other countries, as China, for example, Lockyer
finds that the most important temple of that
country, the Temple of the Sun at Peking, is oriented
to the winter solstice; and Stonehenge, as has long
been known, is oriented to sunrise at the summer
solstice.
In like fashion the rising and setting of many
stars were utilized by the Egyptians, in both temple
and pyramid ; and no astronomer who has ever seen
these ancient structures and studied their orienta-
tions can doubt that they were built by astronomers
for use by astronomers of that day. The priests
were the astronomers, and the temples had a deep
religious significance, with a ceremony of exceeding
magnificence wherever observations of heavenly
bodies were undertaken, whether of sun or stars.
Hindu and Persian astronomy must be passed
over very briefly. Interesting as their systems are
historically, there were few, if any, original contri-
butions of importance, and the Indian treatises bear
strong evidence of Greek origin.
CHAPTER IV
ORIGIN OF GREEK ASTRONOMY
WHILE the Greeks laid the foundations of mod-
ern scientific astronomy, they were not as a
whole observers: rather philosophers, we should
say. The later representatives of the Greek School,
however, saw the necessity of observation as a basis
of true induction; and they discovered that real
progress was not possible unless their speculative
ideas were sufficiently developed and made definite
by the aid of geometry, so that they became capable
of detailed comparison with observation. This was
the necessary and ultimate test with them, and the
same is true to-day. The early Greek philosophers
were, however, mainly interested, not in observa-
tions, but in guessing the causes of phenomena.
Thales of Miletus, founder of the Ionian School,
introduced the system of Egyptian astronomy into
Greece, about the end of the seventh century B. c.
He is universally known as the first astronomer who
ever predicted a total eclipse of the sun that
happened when he said it would : the eclipse of B. C.
585. This he did by means of the Chaldean eclipse
cycle of 18 years known as the Saros.
Aristarchus of Samos was the first and most
eminent of the Alexandrian astronomers, and his
treatise "On the Magnitudes and Distances of the
Sun and Moon" is still extant. This method of
ascertaining how many times farther the sun is
27
28 ASTRONOMY TO-DAY
than the moon is very simple, and geometrically
exact. Unfortunately it is impossible, even to-day,
to observe with accuracy the precise time when the
moon "quarters," (an observation essential to his
method), because the moon's terminal, or line be-
tween day and night, is not a straight line as re-
quired by theory, but a jagged one. By his observa-
tion, the sun was only twenty times farther away
than the moon, a distance which we know to be
nearly twenty times too small.
His views regarding other astronomical questions
were right, although they found little favor among
contemporaries. Not only was the earth spherical,
he said, but it rotated on its axis and also traveled
round the sun. Aristarchus was, indeed, the true
originator of the modern doctrine of motions in the
solar system, and not Copernicus, seventeen cen-
turies later; but Seleucus appears to have been his
only follower in these very advanced conceptions.
Aristarchus made out the apparent diameters of
sun and moon as practically equal to one another,
and inferred correctly that their real diameters are
in proportion to their distances from the earth.
Also he estimated, from observations during an
eclipse of the moon, that the moon's diameter is
about one-third that of the earth. Aristarchus
appears to have been one of the clearest and most
accurate thinkers among the ancient astronomers;
even his views concerning the distances of the stars
were in accord with the fact that they are immeasur-
ably distant as compared with the distances of the
sun, moon, and planets.
Practically contemporary with Aristarchus were
Timocharis and Aristillus, who were excellent ob-
servers, and left records of position of sun and
ORIGIN OF GREEK ASTRONOMY 29
planets which were exceedingly useful to their suc-
cessors, Hipparchus and Ptolemy in particular.
Indeed their observations of star positions were
such that, in a way, they deserve the fame of hav-
ing made the first catalogue, rather than Hipparchus,
to whom is universally accorded that honor.
Spherical astronomy had its origin with the
Alexandrian school, many famous geometers, and in
particular Euclid, pointing the way. Spherics, or
the doctrine of the sphere, was the subject of nu-
merous treatises, and the foundations were securely
laid for that department of astronomical research
which was absolutely essential to farther advance.
The artisans of that day began to build rude mechan-
ical adaptations of the geometric conceptions as
concrete constructions in wood and metal, and it
became the epoch of the origin of astrolabes and
armillary spheres.
CHAPTER V
MEASURING THE EARTH-
ERATOSTHENES
ALL told, the Greek philosophers were probably
<* the keenest minds that ever inhabited the planet,
and we cannot suppose them so stupid as to reject
the doctrine of a spherical earth. In fact so certain
were they that the earth's true figure is a sphere
that Eratosthenes in the third century B. c. made
the first measure of the dimensions of the terrestrial
sphere by a method geometrically exact.
At Syene in Upper Egypt the sun at the summer
solstice was known to pass through the zenith at
noon, whereas at Alexandria Eratosthenes esti-
mated its distance as seven degrees from the zenith
at the same time. This difference being about one-
fiftieth of the entire circumference of a meridian,
Eratosthenes correctly inferred that the distance
between Alexandria and Syene must be one-fiftieth
of the earth's circumference. So he measured the
distance between the two and found it 5,000 stadia.
This figured out the size of the earth with a per-
centage of error surprisingly small when we con-
sider the rough means with which Eratosthenes
measured the sun's zenith distance and the distance
between the two stations.
Greatest of all the Greek astronomers and one
of the greatest in the history of the science was
Hipparchus who had an observatory at Rhodes in
30
MEASURING THE EARTH 31
the middle of the second century B. c. His activi-
ties covered every department of astronomy; he
made extensive series of observations which he
diligently compared with those handed down to
him by the earlier astronomers, especially Aristillus
and Timocharis. This enabled him to ascertain the
motion of the equinoxial points, and his value of
the constant of precession of the equinoxes is ex-
ceedingly accurate for a first determination.
In 134 B. C. a new star blazed out in the constella-
tion Scorpio, and this set Hipparchus at work on a
catalogue of the brighter stars of the firmament, a
monumental work of true scientific conception, be-
cause it would enable the astronomers of future
generations to ascertain what changes, if any, were
taking place in the stellar universe. There were
1,080 stars in his catalogue, and he referred their
positions to the ecliptic and the equinoxes. Also he
originated the present system of stellar magnitudes
or orders of brightness, and his catalogue was in
use as a standard for many centuries.
Hipparchus was a great mathematician as well,
and he devoted himself to the improvement of the
method of applying numerical calculations to geo-
metrical figures: trigonometry, both plane and
spherical, that is ; and by some authorities he is re-
garded as the inventor of original methods in trigo-
nometry. The system of spheres of Eudoxus did not
satisfy him, so he devised a method of representing
the paths of the heavenly bodies by perfectly uni-
form motion in circles. There is slight evidence that
Apollonius of Perga may have been the originator
of the system, but it was reserved for Hipparchus
to work it out in final form. This enabled him to
ascertain the varying length of the seasons, and he
32 ASTRONOMY TO-DAY
fixed the true length of the year as 3651/4 days. He
had almost equal success in dealing with the ir-
regularities of the moon's motion, although the
problem is much more complicated. The distance
and size of the moon, by the method of Aristarchus,
were improved by him, and he worked out, for the
distance of the sun, 1,200 radii of the earth a
classic for many centuries.
Hipparchus devoted much attention to eclipses
of both sun and moon, and we owe to him the first
elucidation of the subject of parallax, or the effect
of difference of position of an observer on the earth's
surface as affecting the apparent projection of the
moon against the sun when a solar eclipse takes
place ; whereas an eclipse of the moon is unaffected
by parallax and can be seen at the same time by
observers everywhere, no matter what their loca-
tion on the earth. Indeed, with all that Hipparchus
achieved, we need not be surprised that astronomy
was regarded as a finished science, and made prac-
tically no progress whatever for centuries after his
time.
Then came Claudius Ptolemseus, generally known
as Ptolemy, the last great name in Greek astronomy.
He lived in Alexandria about the middle of the
second century A. D. and wrote many minor as-
tronomical and astrological treatises, also works on
geography and optics, in the last of which the
atmospheric refraction of rays of light from the
heavenly bodies, apparently elevating them toward
the zenith, is first dealt with in true form.
Sci. Vol. 21
CHAPTER VI
PTOLEMY AND HIS GREAT BOOK
PTOLEMY was an observer of the heavens,
though not of the highest order; but he had
all the work of his predecessors, best of all
Hipparchus, to build upon. Ptolemy's greatest
work was the "Megale Syntaxis," generally known
as the Almagest. It forms a nearly complete
compendium of the ancient astronomy, and
although it embodies much error, because built
on a wrong theory, the Almagest nevertheless
is competent to follow the motions of all the
bodies in the sky with a close approach to ac-
curacy, even at the present day. This marvel-
ous work written at this critical epoch became as
authoritative as the philosophy of Aristotle, and
for many centuries it was the last word in the
science. The old astrology held full sway, and
the Ptolemaic theory of the universe supplied
everything necessary: further progress, indeed,
was deemed impossible.
The Almagest comprises in all thirteen books, the
first two of which deal with the simpler observations
of the celestial sphere, its own motion and the ap-
parent motions of sun, moon, and planets upon it. He
discusses, too, the postulates of his system and ex-
hibits great skill as an original geometer and mathe-
matician. In the third book he takes up the length
of the year, and in the fourth book similarly the
33 Sci. Vol. 22 J
34 ASTRONOMY TO-DAY
moon and the length of the month. Here his mathe-
matical powers are at their best, and he made a dis-
covery of an inequality in the moon's motion known
as the evection. Book five describes the construc-
tion and use of the astrolabe, a combination of grad-
uated circles with which Ptolemy made most of his
observations. In the sixth book he follows mainly
Hipparchus in dealing with eclipses of sun and
moon. In the seventh and eighth books he discusses
the motion of the equinox, and embodies a catalogue
of 1,028 stars, substantially as in Hipparchus. The
five remaining books of the Almagest deal with the
planetary motions, and are the most important of all
of Ptolemy's original contributions to astronomy.
Ptolemy's fundamental doctrines were that the heav-
ens are spherical in form, all the heavenly motions
being in circles. In his view, the earth too is spheri-
cal, and it is located at the center of the universe,
being only a point* as it were, in comparison. All was
founded on mere appearance combined with the phil-
osophical notion that the circle being the only
perfect curve, all motions of heavenly bodies must
take place in earth-centered circles. For fourteen
or fifteen centuries this false theory persisted, on
the authority of Ptolemy and the Almagest, render-
ing progress toward the development of the true
theory impossible.
Ptolemy correctly argued that the earth itself is a
sphere that is curved from east to west, and from
north to south as well, clinching his argument, as we
do to-day, by the visibility of objects at sea, the
lower portions of which are at first concealed from
our view by the curved surface of the water which
intervenes. To Ptolemy also the earth is at the
center of the celestial sphere, and it has no motion
PTOLEMY AND HIS GREAT BOOK 35
of translation from that point; but his argument
fails to prove this. Truth and error, indeed, are so
deftly intermingled that one is led to wonder why
the keen intelligence of this great philosopher per-
mitted him to reject the simple doctrine of the
earth's rotation on its axis. But if we reflect that
there was then no science of natural philosophy or
physics proper, and that the age was wholly unde-
veloped along the lines of practical mechanics, we
shall see why the astronomers of Ptolemy's time and
subsequent centuries were content to accept the doc-
trines of the heavens as formulated by him.
When it came to explaining* the movements of the
"wandering stars," or planets, as we term them, the
Ptolemaic theory was very happy in so far as ac-
curacy was concerned, but very unhappy when it had
to account for the actual mechanics of the cosmos in
space. Sun and moon were the only bodies that went
steadily onward, easterly: whereas all the others,
Mercury, Venus, Mars, Jupiter, Saturn, although
they moved easterly most of the time, nevertheless
would at intervals slow down to stationary points,
where for a time they did not move at all, and then
actually go backward to the west, or retrograde, then
become stationary again, finally resuming their regu-
lar onward motion to the east.
To help out of this difficulty, the worst possible
mechanical scheme was invented, that known as the
epicycle. Each of the five planets was supposed to
have a fictitious "double," which traveled eastward
with uniformity, attached to the end of a huge but
mechanically impossible bar. The earth-centered
circle in which this traveled round was called the
"deferent." What this bar was made of, what
stresses it would be subjected to, or what its size
36 ASTRONOMY TO-DAY
would have to be in order to keep from breaking
none of these questions seems to have agitated the
ancient and medieval astronomers, any more than
the flat-earth astronomy of the Hindu is troubled
by the necessity of something to hold up the
tortoise that holds up the elephant that holds up
the earth.
But at the end of this bar is jointed or swiveled
another shorter bar, to the revolving end of which is
attached the actual planet itself ; and the second bar,
by swinging once round the end of the primary ad-
vancing bar, would account for the backward or ret-
rograde motion of the planet as seen in the sky. For
every new irregularity that was found, in the motion
of Mars, for instance, a new and additional bar was
requisitioned, until interplanetary space was hope-
lessly filled with revolving bars, each producing one
of the epicycles, some large, some small, that were
needed to take up the vagaries of the several planets.
The Arabic astronomers who kept the science alive
through the Middle Ages added epicycle to epicycle,
until there was every justification for Milton's verses
descriptive of the sphere:
With Centric and Eccentric scribbled o'er,
Cycle and Epicycle, Orb in Orb.
CHAPTER VII
ASTRONOMY OF THE MIDDLE AGES
WITH the fall of Alexandria and the victory of
Mohammed throughout the West, and a conse-
quent decline in learning, supremacy in science
passed to the East and centered round the caliphs
of Bagdad in the seventh and eighth centuries.
They were interested in astronomy only as a prac-
tical, and to them useful, science, in adjusting the
complicated lunar calendar of the Mohammedans, in
ascertaining the true direction of Mecca which
every Mohammedan must know, and in the revival
of astrology, to which the Greeks had not attached
any particular significance.
Harun-al-Rashid ordered the Almagest and
many other Greek works translated, of which the
modern world would otherwise no doubt never have
heard, as the Greek originals are not extant.
Splendid observatories were built at Damascus
and Bagdad, and fine instruments patterned after
Greek models were continuously used in observing.
The Arab astronomers, although they had no clocks,
were nevertheless so fully impressed with the im-
portance of time that they added extreme value to
their observations of eclipses, for example, by setting
down the altitudes of sun or stars at the same time.
On very important occasions the records were certi-
fied on oath by a body of barristers and astrono-
mers conjointly a precedent which fortunately has
never been followed.
37
38 ASTRONOMY TO-DAY
About the middle of the ninth century, the Caliph
Al-Mamun directed his astronomers to revise the
Greek measures of the earth's dimensions, and they
had less reverence for the Almagest than existed in
later centuries: indeed, Tabit ben Korra invented
and applied to the tables of the Almagest a theoreti-
cal fluctuation in the position of the ecliptic which
he called "trepidation," which brought sad confu-
sion into astronomical tables for many succeeding
centuries.
Albategnius was another Arab prince whose
record in astronomy in the ninth and tenth centuries
was perhaps the best: the Ptolemaic values of the
precession of the equinoxes and of the obliquity of
the ecliptic were improved by new observations, and
his excellence as mathematician enabled him to make
permanent improvements in the astronomical ap-
plication of trigonometry.
Abul Wefa was the last of the Bagdad astrono-
mers in the latter half of the tenth century, and his
great treatise on astronomy known as the Alma-
gest is sometimes confused with Ptolemy's work.
Following him was Ibn Yunos of Cairo, whose
labors culminated in the famous Hakemite Tables,
which became the standard in mathematical and
astronomical computations for several centuries.
Mohammedan astronomy thrived, too, in Spain
and northern Africa. Arzachel of Toledo published
the Toledan Tables, and his pupils made improve-
ments in instruments and the methods of calcula-
tion. The Giralda was built by the Moors in Seville
in 1196, the first astronomical observatory on the
continent of Europe; but within the next half cen-
tury both Seville and Cordova became Christian
again, and Arab astronomy was at an end.
ASTRONOMY OF THE MIDDLE AGES 39
Through many centuries, however, the science had
been kept alive, even if no great original advances
had been achieved; and Arab activities have modi-
fied our language very materially, adding many such
words as almanac, zenith, and radii, and a wealth of
star names, as Aldebaran, Rigel, Betelgeuse, Vega,
and so on.
Meanwhile, other schools of astronomy had de-
veloped in the East, one at Meraga near the modern
Persia, where Nassir Eddin, the astronomer of
Hulagu Khan, grandson of the Mongol emperor
Genghis Khan, built and used large and carefully
constructed instruments, translated all the Greek
treatises on astronomy, and published a laborious
work known as the Ilkhanic Tables, based on the
Hakemite Tables of Ibn Yunos.
More important still was the Tartar school of as-
tronomy under Ulugh Begh, a grandson of Tamer-
lane, who built an observatory at Samarcand in 1420,
published new tables of the planets, and made with
his excellent instruments the observations for a new
catalogue of stars, the first since Hipparchus, the
star places being recorded with great precision.
The European astronomy of the Middle Ages
amounted to very little besides translation from the
Arabic authors into Latin, with commentaries. As-
tronomers under the patronage of Alfonso X of Leon
and Castile published in 1252 the Alfonsine Tables,
which superseded the Toledan tables and were ac-
cepted everywhere throughout Europe. Alfonso
published also the "Libros del Saber," perhaps the
first of all astronomical cyclopedias, in which is said
to occur the earliest diagram representing a planet-
ary orbit as an ellipse: Mercury's supposed path
round the earth as a center.
40 ASTRONOMY TO-DAY
Purbach of Vienna about the middle of the 15th
century began his "Epitome of Astronomy" based
on the "Almagest" of Ptolemy, which was finished by
his collaborator Regiomontanus, who was an expert
in mathematics and published a treatise on trigo-
nometry with the first table of sines calculated for
every minute from to 90, a most helpful contribu-
tion to theoretical astronomy.
Regiomontanus had a very picturesque career,
finally taking up his residence in Nuremberg, where
a wealthy citizen named Walther became his patron,
pupil, and collaborator. The artisans of the city
were set at work on astronomical instruments of the
greatest accuracy, and the comet of 1472 was the
first to be observed and studied in true scientific
fashion. Regiomontanus was very progressive and
the invention of the new art of 'printing gave him an
opportunity to publish Purbach's treatise, which
went through several editions and doubtless had
much to do in promoting dissatisfaction with the
ancient Ptolemaic system, and was thus most sig-
nificant in preparing a background for the coming
of the new Copernican order.
The Nuremberg presses popularized astronomy
in other important ways, issuing almanacs, the first
precursors of our astronomical Ephemerides. Re-
giomontanus was practical as well, and invented a
new method of getting a ship's position at sea, with
tables so accurate that they superseded all others
in the great voyages of discovery, and it is probable
that they were employed by Columbus in his dis-
covery of the American continent. Regiomontanus
had died several years earlier, in 1475 at Rome, where
he had gone by invitation of the Pope to effect a re-
formation in the calendar. He was only forty, and
ASTRONOMY OF THE MIDDLE AGES 41
his patron Walther kept on with excellent observa-
tions, the first probably to be corrected for the effect
of atmospheric refraction, although its influence had
been known since Ptolemy. The Nuremberg School
lasted for nearly two centuries.
Nearly contemporary with Regiomontanus were
Fracastoro and Peter Apian, whose original observa-
tions on comets are worthy of mention because they
first noticed that the tails of these bodies always
point away from the sun. Leonardo da Vinci was
the first to give the true explanation of earth-shine
on the moon, and similarly the moon-illumination
of the earth ; and this no doubt had great weight in
disposing of the popular notion of an essential dif-
ference of nature between the earth and celestial
bodies all of which helped to prepare the way for
Copernicus and the great revolution in astronomical
thought.
CHAPTER VIII
COPERNICUS AND THE NEW ERA
rpHROUGHOUT the Middle Ages the progress of
-* astronomy was held back by a combination of
untoward circumstances. A prolonged reaction from
the heights attained by the Greek philosophers was
to be expected. The uprising of the Mohammedan
world, and the savage conquerors in the East did not
produce conditions favorable to the origin and devel-
opment of great ideas.
At the birth of Copernicus, however, in 1473, the
time was ripening for fundamental changes from
the ancient system, the error of which had helped to
hold back the development of the science for cen-
turies. The fifteenth century was most fruitful in
a general quickening of intelligence, the invention of
printing had much to do with this, as it spread a
knowledge of the Greek writers, and led to conflict
of authorities. Even Aristotle and Ptolemy were
not entirely in harmony, yet each was held inviolate.
It was the age of the Reformation, too, and near the
end of the century the discovery of America exerted
a powerful stimulus in the advance of thought.
Copernicus searched the works of the ancient
writers and philosophers, and embodied in this new
order such of their ideas as commended themselves
in the elaboration of his own system.
Pythagoras alone and his philosophy looked in the
true direction. Many believe that he taught that the
sun, not the earth, is at the center of our solar sys-
42
COPERNICUS AND THE NEW ERA 43
tern; but his views were mingled with the specula-
tive philosophy of the Greeks, and none of his writ-
ings, barring a few meager fragments, have come
down to our modern age.
To many philosophers, through all these long cen-
turies, the true theory of the celestial motions must
have been obvious, but their views were not formu-
lated, nor have they been preserved in writing. So
the fact remains that Copernicus alone first proved
the truth of the system which is recognized to-day.
This he did in his great treatise entitled "De Revolu-
tionibus Orbium Coelestium," the first printed copy
of which was dramatically delivered to him on his
deathbed, in May, 1543. The seventy years of his
life were largely devoted to the preparation of this
work, which necessitated many observations as
well as intricate calculations based upon them.
Being ,a canon in the church, he naturally hesi-
tated about publishing his revolutionary views, his
friend Rheticus first doing this for him in outline
in 1540.
So simple are the great principles that they may
be embodied in very few words; what appears to
us as the daily revolution of the heavens is not a
real motion, but only an apparent one; that is, the
heavens are at rest, while the earth itself is in
motion, turning round an axis which passes through
its center. And the second proposition is that the
earth is simply one of the six known planets; and
they all revolve round the sun as the true center.
The solar system, therefore, is "heliocentric," or
sun-centered, not "geocentric" or earth-centered, as
taught by the Ptolemaic theory.
Copernicus demonstrates clearly how his system
explains the retrograde motion of the planets and
44 ASTRONOMY TO-DAY
their stationary points, no matter whether they are
within the orbit of the earth, as Mercury and Venus,
or outside of it, as Mars, Jupiter, and Saturn. His
system provides also the means of ascertaining with
accuracy the proportions of the solar system, or
the relative distances of the planets from the sun
and from each other. In this respect also his sys-
tem possessed a vast advantage over that of
Ptolemy, and the planetary distances which Coper-
nicus computed are very close approximations to
the measures of the present day.
Reinhold revised the calculations of Copernicus
and prepared the "Tabulae Prutenicse," based on the
"De Revolutionibus," which proved far superior to
the Alfonsine Tables, and were only supplanted
by the Rudolphine Tables of Kepler. On the whole
we may regard the lifework of Copernicus as
fundamentally the most significant in the history
and progress of astronomy.
CHAPTER IX
TYCHO, THE GREAT OBSERVER
LEAR as Copernicus had made the demonstration
of the truth of his new system, it nevertheless
failed of immediate and universal acceptance. The
Ptolemaic system was too strongly intrenched, and
the motions of all the bodies in the sky were too
well represented by it. Accurate observations were
greatly needed, and the Landgrave William IV. of
Hesse built the Cassel Observatory, which made a
new catalogue of stars, and introduced the use of
clocks to carry on the time as measured by the uni-
form motion of the celestial sphere. Three years
after the death of Copernicus, Tycho Brahe was
born, and when he was 30 the King of Denmark
built for him the famous observatory of Uraniborg,
where the great astronomer passed nearly a quarter
of a century in critically observing the positions of
the stars and planets. Tycho was celebrated as a
designer and constructor of new types of astro-
nomical instruments, and he printed a large volume
of these designs, which form the basis of many in
use at the present day. Unfortunately for the
genius of Tycho and the significance of his work,
the invention of the telescope had not yet been
made, so that his observations had not the modern
degree of accuracy. Nevertheless, they were des-
tined to play a most important part in the progress
of astronomy.
45
46 ASTRONOMY TO-DAY
Tycho was sadly in error in his rejection of the
Copernican system, -although his reasons, in his day,
seemed unanswerable. If the outer planets were
displaced among the stars by the annual motion of
the earth round the sun, he argued, then the fixed
stars must be similarly displaced unless indeed
they be at such vast distances that their motions
would be too slight to be visible. Of course we know
now that this is really true, and that no instruments
that Tycho was able to build could possibly have
detected the motions, the effects of which we now
recognize in the case of the nearer fixed stars in
their annual, or parallactic, orbits.
The remarkably accurate instruments devised by
Tycho Brahe and employed by him in improving
the observations of the positions of the heavenly
bodies were no doubt built after descriptions of
astrolabes such as Hipparchus used, as described
by Ptolemy. In his "Astronomise Instaurata Meca-
nica" we find illustrations and descriptions of many
of them.
One is a polar astrolabe, mounted somewhat as
a modern equatorial telescope is, and the meridian
circle is adjustable so that it can be used in any
place, no matter what its latitude might be. There
is a graduated equatorial ring at right angles to
the polar axis, so that the astrolabe could be used
for making observations outside the meridian as
well as on it. This equatorial circle slides through
grooves, and is furnished with movable sights, and
a plumb line from the zenith or highest point of
the meridian circle makes it possible to give the
necessary adjustment in the vertical. Screws for
adjustment at the bottom are provided, just as in
our modern instruments, and two observers were
TYCHO, THE GREAT OBSERVER 47
necessary, taking their sights simultaneously; un-
less, as in one type of the instrument, a clock, or
some sort of measure of time, was employed.
Another early type of instrument is called by
Tycho the ecliptic astrolabe (Armillse Zodiacales,
or the Zodiacal Rings) . It resembles the equatorial
astrolabe somewhat, but has a second ring inclined
to the equatorial one at an angle equal to the
obliquity of the ecliptic. In observing, the equa-
torial ring was revolved round till the ecliptic ring
came into coincidence with the plane of the ecliptic
in the sky. Then the observation of a star's longi-
tude and latitude, as referred to the ecliptic plane,
could be made, quite as well as that of right ascen-
sion and declination on the equatorial plane. But it
was necessary to work quickly, as the adjustment on
the ecliptic would soon disappear and have to be
renewed.
Tycho is often called the father of the science of
astronomical observation, because of the improve-
ments in design and construction of the instruments
he used. His largest instrument was a mural quad-
rant, a quarter-circle of copper, turning parallel
to the north-and-south face of a wall, its axis turn-
ing on a bearing fixed in the wall. The radius of
this quadrant was nine feet, and it was graduated
or divided so as to read the very small angle of ten
seconds of arc an extraordinary degree of pre-
cision for his day.
Tycho built also a very large alt-azimuth quad-
rant, of six feet radius. Its operation was very
much as if his mural quadrant could be swung
round in azimuth. At several of the great observa-
tories of the present day, as Greenwich and Wash-
ington, there are instruments of a similar type,
48 ASTRONOMY TO-DAY
but much more accurate, because the mechanical
work in brass and steel is executed by tools that are
essentially perfect, and besides this the power of
the telescope is superadded to give absolute direc-
tion, or pointing on the object under observation.
Excellent clocks are necessary for precise ob-
servation with such an instrument; but neither
Tycho Brahe, nor Hevelius was provided with such
accessories. Hevelius did not avail himself of the
telescope as an aid to precision of observation,
claiming that pinhole sights gave him more accu-
rate results. It was a dispute concerning this ques-
tion that Halley was sent over from London to
Danzig to arbitrate.
There could be but one way to decide; the tele-
scope with its added power magnifies any displace-
ment of the instrument, and thereby enables the
observer to point his instrument more exactly. So
he can detect smaller errors and differences of
direction than he can without it. And what is of
great importance in more modern astronomy, the
telescope makes it possible to observe accurately the
position of objects so faint that they are wholly
invisible to the naked eye.
CHAPTER X
KEPLER, THE GREAT CALCULATOR
MOST fortunate it was for the later development
of astronomical theory that Tycho Brahe not
only was a practical or observational astronomer of
the highest order, but that he confined himself studi-
ously for years to observations of the places of the
planets. Of Mars he accumulated an especially long
and accurate series, and among those who assisted
him in his work was a young and brilliant pupil
named Johann Kepler.
Strongly impressed with the truth of the Coperni-
can System, Kepler was free to reject the erroneous
compromise system devised by Tycho Brahe, and
soon after Tycho's death Kepler addressed himself
seriously to the great problem that no one had ever
attempted to solve, viz : to find out what the laws of
motion of the planets round the sun really are. Of
course he took the fullest advantage of all that
Ptolemy and Copernicus had done before him, and
he had in addition the splendid observations of
Tycho Brahe as a basis to work upon.
Copernicus, while he had effected the tremendous
advance of substituting the sun for the earth as the
center of motion, nevertheless clung to the errone-
ous notion of Ptolemy that all the bodies of the sky
must perforce move at uniform speeds, and in cir-
cular curves, the circle being the only "perfect
curve." Kepler was not long in finding out that
49
50 ASTRONOMY TO-DAY
this could not be so, and he found it out because
Tycho Brahe's observations were much more ac-
curate than any that Copernicus had employed.
Naturally he attempted the nearest planet first,
and that was Mars the planet that Tycho had
assigned to him for research. How fortunate that
the orbit of Mars was the one, of all the planets, to
show practically the greatest divergence from the
ancient conditions of uniform motion in a perfectly
circular orbit! Had the orbit of Mars chanced to
be as nearly circular as is that of Venus, Kepler
might well have been driven to abandon his search
for the true curve of planetary motion.
However, the facts of the cosmos were on his
side, but the calculations essential in testing his
various hypotheses were of the most tedious nature,
because logarithms were not yet known in his day.
His first discovery was that the orbit of Mars is
certainly not a circle, but oval or elliptic in figure.
And the sun, he soon found, could not be in the
center of the ellipse, so he made a series of trial
calculations with the sun located in one of the
foci of the ellipse instead.
Then he found he could make his calculated places
of Mars agree quite perfectly with Tycho Brahe's
observed positions, if only he gave up the other
ancient requisite of perfectly uniform motion. On
doing this, it soon appeared that Mars, when in
perihelion, or nearest the sun, always moved
swiftest, while at its greatest distance from the
sun, or aphelion, its orbital velocity was slowest.
Kepler did not busy himself to inquire why these
revolutionary discoveries of his were as they were ;
he simply went on making enough trials on Mars,
and then on the other planets in turn, to satisfy
KEPLER, THE GREAT CALCULATOR 51
himself that all the planetary orbits are elliptical,
not circular in form, and are so located in space
that the center of the sun is at one of the two foci
of each orbit. This is known as Kepler's first law
of planetary motion.
The second one did not come quite so easy; it
concerned the variable speed with which the planet
moves at every point of the orbit. We must remem-
ber how handicapped he was in solving this prob-
lem : only the geometry of Euclid to work with, and
none of the refinements of the higher mathematics
of a later day. But he finally found a very simple
relation which represented the velocity of the planet
everywhere in its orbit. It was this : if we calculate
the area swept, or passed over, by the planet's
radius vector (that is, the line joining its center to
the sun's center) during a week's time near peri-
helion, and then calculate the similar area for a
week near aphelion, or indeed for a week when
Mars is in any intermediate part of its orbit, we
shall find that these areas are all equal to each
other. So Kepler formulated his second great law of
planetary motion very simply: the radius vector of
any planet describes, or sweeps over, equal areas in
equal times. And he found this was true for all the
planets.
But the real genius of the great mathematician
was shown in the discovery of his third law, which
is more complex and even more significant than
the other two a law connecting the distances of
the planets from the sun with their periods of revo-
lution about the sun. This cost Kepler many addi-
tional years of close calculation, and the resulting
law, his third law of planetary motion is this: The
cubes of the mean or average distances of the
52 ASTRONOMY TO-DAY
planets from the sun are proportional to the squares
of their times of revolution around him.
So Kepler had not only disposed of the sacred
theories of motion of the planets held by the
ancients as inviolable, but he had demonstrated the
truth of a great law which bound all the bodies of
the solar system together. So accurately and com-
pletely did these three laws account for all the mo-
tions, that the science of astronomy seemed as if
finished; and no matter how far in the future a
time might be assigned, Kepler's laws provided
the means of calculating the planet's position for
that epoch as accurately as it would be possible
to observe it. Kepler paused here, and he died
in Ij630.
CHAPTER XI
GALILEO, THE GREAT EXPERIMENTER
HHHE fifteenth and sixteenth centuries, containing
JL the lives and work of Copernicus, Tycho, Galileo,
Kepler, Huygens, Halley, and Newton, were a veri-
table Golden Age of astronomy. All these men were
truly great and original investigators.
None had a career more picturesque and popular
than did Galileo. Born a few years earlier and
dying a few years later than Kepler, the work of
each of these two great astronomers was wholly in-
dependent of the other and in entirely different
fields. Kepler was discovering the laws of planetary
motion, while Galileo was laying the secure founda-
tions of the new science of dynamics, in particular
the laws of falling bodies, that was necessary before
Kepler's laws could be fully understood. When only
eighteen Galileo's keen power of observation led to
his discovery of the laws of pendulum motion, sug-
gested by the oscillation to and fro of a lamp in the
cathedral of Pisa.
The world-famous leaning tower of this place,
where he was born, served as a physical laboratory
from the top of which he dropped various objects,
and thus was led to formulate the laws of falling
bodies. He proved that Aristotle was all wrong in
saying that a heavy body must fall swifter in pro-
portion to its weight than a lighter one. These and
other discoveries rendered him unpopular with his
associates, who christened him the "Wrangler."
53
54 ASTRONOMY TO-DAY
The new system of Copernicus appealed to him ;
and when he, first of all men, turned a telescope on
the heavenly bodies, there was Venus with phases
like those of the moon, and Jupiter with satellites
traveling about it a Copernican system in min-
iature. Nothing could have happened that would
have provided a better demonstration of the truth
of the new system and the falsity of the old. His
marvelous discoveries caused the greatest excite-
ment consternation even, among the anti-Coperni-
cans. Galileo published the "Sidereus Nuncius," with
many observations and drawings of the moon,
which he showed to be a body not wholly dissimilar
to the earth: this, too, was obviously of great mo-
ment in corroboration of the Copernican order and
in contradiction to the Ptolemaic, which maintained
sharp lines of demarcation between things terres-
trial and things celestial.
His telescopes, small as they were, revealed to him
anomalous appearances on both sides of the planet
Saturn which he called ansae, or handles. But their
subsequent disappearance was unaccountable to
him, and later observers, who kept on guessing
ineffectively till Huygens, nearly a half century
after, showed that the true nature of the appendage
was a ring. Spots on the sun were frequently ob-
served by Galileo and led to bitter controversies.
He proved, however, that they were objects on the
sun itself, not outside it, and by noticing their re-
peated transits across the sun's disk, he showed that
the sun turned round on his axis in a little less than
a month another analogy to the like motion of the
earth on the Copernican plan.
Galileo's appointment in 1610 as "First Philoso-
pher and Mathematician" to the Grand Duke of
GALILEO, THE GREAT EXPERIMENTER 55
Tuscany gave him abundant time for the pursuit
of original investigations and the preparation of
books and pamphlets. His first visit to Rome the
year following was the occasion of a reception with
great honor by many cardinals and others of high
rank. His lack of sympathy with others whose
views differed from his, and his naturally contro-
versial spirit, had begun to lead him headlong into
controversies with the Jesuits and the church, which
culminated in his censure by the authorities of the
church and persecution by the Inquisition.
In 1618 three comets appeared, and Galileo was
again in controversial hot water with the Jesuits.
But it led to the publication five years later of "II
Saggiatore" (The Assayer), of no great scientific
value, but only a brilliant bit of controversial litera-
ture dedicated to the newly elevated Pope, Urban
VIII. Later he wrote through several years a great
treatise, more or less controversial in character,
entitled a "Dialogue on the Two Chief Systems of
the World" between three speakers, and extending
through four successive days. Simplicio argues for
the Aristotelians, Salviati for the Copernicans,
while Sagredo does his best to be neutral. It will
always be a very readable book, and we are for-
tunate to have a recent translation by Professor
Crew of Evanston.
Here we find the first suggestion of the
modern method of getting stellar parallaxes, the
relative parallax, that is, of two stars in the same
field a method not put into service till BessePs
time, two centuries later. But the most important
chapters of the "Dialogue" deal with Galileo's inves-
tigations of the laws of motion of bodies in general,
which he applied to the problem of the earth's
56 ASTRONOMY TO-DAY
motion. In this he really anticipated Newton in the
first of his three laws of motion, and in a subse-
quent work, dealing with the theory of projectiles,
he reaches substantially the results of Newton's
second law of motion, although he gave no general
statement of the principle. Nevertheless, in the
epoch where his life was lived and his work done,
his telescopic discoveries, combined with his dyna-
mic researches in untrodden fields, resulted in the
complete and final overthrow of the ancient system
of error, and the secure establishment of the Co-
pernican system beyond further question and dis-
cussion. Only then could the science of astronomy
proceed unhampered to the fullest development by
the master minds of succeeding centuries.
CHAPTER XII
AFTER THE GREAT MASTERS
T710LLOWING Kepler and Galileo was a half cen-
T tury of great astronomical progress along many
lines laid out by the work of the great masters. The
telescope seemed only a toy, but its improvement in
size and quality showed almost inconceivable pos-
sibilities of celestial discoveries.
Hevelius of Danzig took up the study of the moon,
and his "Selenographia" was finely illustrated by
plates which he not only drew but engraved himself.
Lunar names of mountains, plains, and craters we
owe very largely to him. Also he published among
other works two on comets, the second of which was
published in 1668 and called the "Cometographia,"
the first detailed account of all the comets observed L
and recorded to date.
Many were the telescopes turned on the planet
Saturn, and every variety of guess was made as to
the actual shape and physical nature of the weird
appendages discovered by Galileo. The true solution
was finally reached by Huygens, whose mechanical
genius had enabled him to grind and polish larger
and better lenses than his contemporaries; in 1659
he published the "Systema Saturnium" interpreting
the ring and the cause of its various configurations,
and the first discovery of a Saturnian satellite is
due to him.
Gascoigne in England about 1640 was the first
to make the important application of the microm-
57
58 ASTRONOMY TO-DAY
eter to enhance the accuracy of measurement of
small angles in the telescopic field; an invention
made and applied independently many years later
by Huygens in Holland and Auzout and Picard in
France, where the instrument was first regularly
employed as an accessory in the work of an
observatory.
Another Englishman, Jeremiah Horrocks, was
the first observer of a transit of Venus over the
disk of the sun, in 1639. Horrocks was possessed
of great ability in calculational astronomy also.
This was about the time of the invention of the
pendulum clock by Huygens, which in conjunction
with the later invention of the transit instrument
by Roemer wrought a revolution in the exacting
art of practical astronomy. This was because it
enabled the time to be carried along continuously,
and the revolution of the earth could be utilized
in making precise measures of the position of
sun, moon, and stars. Louis XIV had just founded
the new Observatory at Paris in 1668, and Picard
was the first to establish regular time-observations
there.
Huygens followed up the motion of the pendulum
in theory as well as practice in his "Oscillatorium
Horologium" (1673), showing the way to measure
the force of gravity, and his study of circular
motion showed the fundamental necessity of some
force directed toward the center in planetary
motions.
The doctrine of the sphericity of the earth being
no longer in doubt, the great advance in accuracy of
astronomical observation indicated to Willebrord
Snell in Holland the best way to measure an arc
of meridian by triangulation. Picard repeated the
AFTER THE GREAT MASTERS 59
measurements near Paris with even greater ac-
curacy, and his results were of the utmost sig-
nificance to Newton in establishing his law of
gravitation.
Domenico Cassini, an industrious observer,
voluminous writer, and a strong personality, devised
telescopes of great size, discovered four Saturnian
satellites and the main division in the ring of
Saturn, determined the rotation periods of Mars
and Jupiter, and prepared tables of the eclipses of
Jupiter's satellites. At his suggestion Richer under-
took an expedition to Cayenne in latitude 5 degrees
north, where it was found that the intensity of
gravity was less than at Paris, and his clock there-
fore lost time, thus indicating that the earth was
not a perfect sphere as had been thought, but a
spheroid instead.
The planet Mars passed a near opposition, and
Richer's observations of it from Cayenne, when
combined with those of Cassini and others in
France, gave a new value of the sun's parallax and
distance, really the first actual measurement worth
the name in the history of astronomy.
To close this era of signal advance in astronomy
we may cite a discovery by Roemer of the first
order : no"less than that of the velocity of transmis-
sion of light through space. At the instigation of
Picard, Roemer in studying the motions of Jupiter's
satellites found that the intervals between eclipses
grew less and less as Jupiter and the earth ap-
proached each other, and greater and greater than
the average as the two planets separated farther
and farther. Roemer correctly attributed this dif-
ference to the progressive motion of light and a
rough value of its velocity was calculated, though
60 ASTRONOMY TO-DAY
not accepted oy astronomers generally for more
than a century.
Why the laws of Kepler should be true, Kepler
himself was unable to say. Nor could anyone else
in that day answer these questions: (1) The planets
move in orbits that are elliptical not circular why
should they move in an impeifect curve, rather than
the perfect one in which it had always been taught
that they moved? (2) Why should our planet vary
its velocity at all, and travel now fast, now slow;
especially why should the speed so vary that the
line of varying length, joining the planet to the sun,
always passes over areas proportional to the time
of describing them? And (3) Why should there be
any definite relation of the distances of planets from
the sun to their times of revolution about him?
Why should it be exactly as the cube of one to the
square of the other?
We must remember that the Copernican sys-
tem itself was not yet, in the beginning of the
seventeenth century, accepted universally; and
the great minds of that period were most con-
cerned in overturning the erroneous theory of
Ptolemy.
The next step in logical order was to find a basic
explanation of the planetary motions, and Des-
cartes and his theory of vortices are worthy of men-
tion, among many unsuccessful attempts in this
direction. Descartes was a brilliant French philos-
opher and mathematician, but his hypothesis of a
multitude of whirlpools in the ether, while ingenious
in theory, was too vague and indefinite to account for
the planetary motions with any approach to the
precision with which the laws of Kepler represented
them.
AFTER THE GREAT MASTERS 61
Another great astronomer whose labors helped
immensely in preparing the way for the signal dis-
coveries that were soon to come was Huygens, a
man of versatility as natural philosopher, mechani-
cian, and astronomical observer. Huygens was born
thirteen years before the death of Galileo, and to
the discovery of the laws of motion by the latter
Huygens added researches on the laws of action of
centrifugal forces. Neither of them, however, ap-
peared to see the immediate bearing on the great
general problem of celestial motions in its true light,
and it was reserved for another generation, and an
astronomer of another country, to make the one
fundamental discovery that should explain the
whole by a single simple law.
CHAPTER XIII
NEWTON AND MOTION
OW is it that you are able to make these great
discoveries ?" was once asked of Sir Isaac New-
ton, facile princeps of all philosophers, and the dis-
coverer of the great law of universal gravitation.
"By perpetually thinking about them," was New-
ton's terse and illuminating reply. He had set for
himself the definite problem of Kepler's laws : why
is it that they are true, and is there not some single,
general law that will embody all the circumstances
of the planetary motions?
Newton was born in 1643, the year after the
death of Galileo. He had a thorough training in
the mathematics of his day, and addressed himself
first to an investigation and definite formulation of
the general laws of motion, which he found to be
three in number, and which he was able to put in
very simple terms. The first one is: Any body,
once it is set in motion, will continue to move for-
ward in a straight line with a uniform velocity for-
ever, provided it is acted upon by no force what-
ever. In other words, a state of motion is as natural
as a state of rest (rest in relation to things every-
where adjacent) in which we find all things in
general.
Here on earth where gravity itself pulls all ob-
jects downward toward the earth, and where re-
sistance of the air tends to hold a moving body
62
NEWTON AND MOTION 63
back and bring it to rest, and where friction from
contact with whatever material substance may be
in its path is perpetually tending to neutralize all
motion with all three of these forces always at
work to stop a moving body, the truth of this first
and fundamental law of motion was not apparent on
the surface.
Till Galileo's time everyone had made the mis-
take of supposing that some force or other must
be acting continually on every moving body to keep
it in motion. Ptolemy, Copernicus, Kepler, Leo-
nardo da Vinci all failed to see the truth of this
law which Newton developed in the immortal
Principia. And at the present day it is not always
easy to accept at first, although the progress of
mechanical science, by reducing friction and resist-
ance, has produced machines in which motion of
large masses may be kept up indefinitely with the
application of only the merest minimum of force.
Once a planet is set in motion round the sun,
it would go on forever through frictionless, non-
resistant space; but there must be a central force,
as Huygens saw clearly, to hold it in its orbit.
Otherwise it would at any moment take the direc-
tion of a tangent to the orbit. Here is where New-
ton's second law of motion comes in, and he formu-
lated it with great definiteness. When any force
acts on a moving body, its deviation from a straight
line will be in the direction of the force applied and
proportional to that force.
In accord with this law, Newton first began to
inquire whether the force of attraction here on
earth, which everyone commonly recognizes as
gravity, drawing all things down toward the center
of the earth, might not extend upward indefinitely.
64 ASTRONOMY TO-DAY
It is found in operation on the summits of mountain
peaks, and the clouds above them and the rain
falling from them are obviously drawn downward
by the same force. May it not extend outward into
space, even as far as the moon?
This was an audacious question, but Newton not
only asked, but tried to answer it in the year 1665,
when he was only twenty-three. On the surface of
the earth this attraction is strong enough to draw
a falling body downward through a vertical space
of sixteen feet in a second of time. What ought it
to be at the distance of the moon. The distance of
the moon in Newton's time was better known in
terms of the earth's size than was the size of the
earth itself : the earth's radius was known to be one-
sixtieth of the moon's distance, but the earth's
diameter was thought to be something under- 7,000
miles, so that Newton's first calculations were most
disappointing, and he laid them aside for nearly
twenty years.
Meanwhile the French astronomers led by Picard
had measured the earth anew, and showed it to be
nearly 8,000 miles in diameter. As soon as Newton
learned of this, he revised his calculations, and
found that by the law of the inverse square the
moon, in one second, should fall away from a tan-
gent to its orbit one thirty-six hundredth of sixteen
feet.
This accorded exactly with his original supposi-
tion that the earth's attraction extended to the
moon. So he concluded that the force which makes a
stone fall, or an apple, as the story goes, is the same
force that holds the moon in its orbit, and that this
force diminishes in the exact proportion that the
square of the distance from the earth's center in-
NEWTON AND MOTION 65
creases. The moon, indeed, becomes a falling body ;
only, as Kingdon Clifford puts it: "She is going so
fast and is so far off that she falls quite around to
the other side of the earth, instead of hitting it;
and so goes on forever."
Newton goes on in the Principia to explain the
extension of gravitation to the other bodies of the
solar system beyond the earth and moon. Clearly
the same gravitation that holds the moon in its
orbit round the earth, must extend outward from
the sun also, and hold all the planets in their orbits
centered about him. Newton demonstrates by cal-
culation based on Kepler's third law that (1) the
forces drawing the planets toward the sun are in-
versely as the squares of their mean distances from
him; and (2) if the force be constantly directed
toward the sun, the radius vector in an elliptic orbit
must pass over equal areas in equal times.
Sci. Vol. 23
CHAPTER XIV
NEWTON AND GRAVITATION
SO all of Kepler's laws could be embodied in a
single law of gravitation toward a central body,
whose force of attraction decreases outward in
exact proportion as the square of the distance in-
creases.
Only one farther step had to be taken, and this
the most complicated of all: he must make all the
bodies of the sky conform to his third law of motion.
This is : Action and reaction are equal, or the
mutual actions of any two bodies are always equal
and oppositely directed. There must be mutual at-
tractions everywhere: earth for sun as well as sun
for earth, moon for sun and sun for moon, earth for
Venus and Venus for earth, Jupiter for Saturn and
Saturn for Jupiter, and so on.
The motions of the planets in the undisturbed
ellipses of Kepler must be impossible. As observa-
tions of the planets became more accurate, it was
found that they really did fail to move in exact
accord with Kepler's laws unmodified. Newton
was unable, with the imperfect processes of the
mathematics of his day to ascertain whether the de-
viations then known could be accounted for by his
law of gravitation; but he nevertheless formulated
the law with entire precision, as follows :
Every particle of matter in the universe attracts
every other particle with a force exactly propor-
66
NEV/TON AND GRAVITATION 67
tioned to the product of their masses, and inversely
as the square of the distance between their centers.
The centuries of astronomical research since
Newton's day, however, have verified the great law
with the utmost exactness. Practically every ir-
regularity of lunar and planetary motion is ac-
counted for; indeed, the intricacies of the problems
involved, and the nicety of their solution, have led
to the invention of new mathematical processes ade-
quate to the difficulties encountered.
And about the middle of the last century, when
Uranus departed from the path laid out for it by
the mathematical astronomers, its orbital devia-
tions were made the basis of an investigation which
soon led to the assignment of the position where a
great planet could be found that would account for
the unexplained irregularities of the motion of
Uranus. And the immediate discovery of this
planet, Neptune, became the most striking verifica-
tion of the Newtonian law that the solar system
could possibly afford.
The astronomers of still later days investigating
the statelier motions of stellar systems find the
Newtonian law regnant everywhere among the
stars where our most powerful telescopes have as
yet reached. So that Newton's law is known as the
law of Universal Gravitation, and its author is
everywhere held as the greatest scientist of the
ages.
Newton's Principia may be regarded as the cul-
minating research of the inductive method, and
further outline of its contents is desirable. It is
divided into three books following certain intro-
ductory sections. The first book treats of the prob-
lems of moving bodies, the solutions being worked
68 ASTRONOMY TO-DAY
out generally and not with special reference to
astronomy. The second book deals with the motion
of bodies through resistant media, as fluids, and
has very little significance in astronomy. The third
book is the all important one, and applies his gen-
eral principles to the case of the actual solar system,
providing a full explanation of the motions of all
the bodies of the system known in his day. Any-
one who critically reads the Principia of Newton
will be forced to conclude that its author was a
genius in the highest sense of the word. The ele-
gance and thoroughness of the demonstrations, and
the completeness of application of the law of gravi-
tation are especially impressive.
The universality of his new law was the feature
to which he gave particular attention. It was clear
to him that the gravitation of a planet, although
it acted as if wholly concentrated at the center,
was nevertheless resident in every one of the par-
ticles of which the planet is composed. Indeed, his
universal law was so formulated as to make every
particle attract every other particle ; and an investi-
gation known as the Cavendish experiment a re-
search of great delicacy of manipulation not only
proves this, but leads also to a measurement of
the earth's mean density, from which we can cal-
culate approximately how much the earth actually
weighs.
Another way to attack the same problem is by
"measuring the attraction of mountains, as Maske-
lyne, Astronomer Royal of Scotland did on Mount
Schehallien in Scotland, which was selected because
of its sheer isolation. The attraction of the moun-
tain deflected the plumb-lines by measurable
amounts, the volume of the mountain was care-
NEWTON AND GRAVITATION 69
fully ascertained by surveys, and geologists found
out what rocks composed it. So the weight of the
entire mountain became pretty well known, and
combining this with the observed deflection, an in-
dependent value of the earth's weight was found.
Still other methods have been applied to this
question, and as an average it is found that the
materials composing the earth are about five and
a half times as heavy as water, and the total
weight of the earth is something like six sex-
tillions of tons.
What is the true shape of the earth? And does
the earth's turning round on its axis affect this
shape? Newton saw the answer to these questions
in his law of gravitation. A spherical figure fol-
lowed as a matter of course from the mutual
attraction of all materials composing the earth, pro-
viding it was at rest, or did not turn round on its
axis. But rotation bulges it at the equator and
draws it in at the poles, by an amount which
calculation shows to be in exact agreement with
the amount ascertained by actual measurement of
the earth itself.
Another curious effect, not at first apparent, was
that all bodies carried from high latitudes toward
the equator would get lighter and lighter, in conse-
quence of the centrifugal force of rotation. This
was unexpectedly demonstrated by Richer when
the French Academy sent him south to observe
Mars in 1672. His clock had been regulated exactly
in Paris, and he soon found that it lost time when
set up at Cayenne. The amount of loss was found by
observation, and it was exactly equal to the calcu-
lated effect that the reduction of gravity by cen-
trifugal action should produce.
70 ASTRONOMY TO-DAY
Also Newton saw that his law of gravitation
would afford an explanation of the rise and fall of
the tides. The water on the side of the earth toward
the moon, being nearer to the moon, would be more
strongly attracted toward it, and therefore raised
in a tide. And the water on the farther side of the
earth away from the moon, being at a greater dis-
tance than the earth itself, the moon would attract
the earth more strongly than this mass of water,
tending therefore to draw the earth away from the
water, and so raising at the same time a high tide
on the side of the earth away from the moon. As
the earth turns round on its axis, therefore, two
tidal waves continually follow each other at inter-
vals of about twelve hours.
The sun, too, joins its gravitating force with that
of the moon, raising tides nearly half as high as
those which the moon produces, because the sun's
vaster mass makes up in large part for its much
greater distance. At first and third quarters of
the moon, the sun acts against the moon, and the
difference of their tide-producing forces gives us
"neap tides" ; while at new moon and full, sun and
moon act together, and produce the maximum effect
known as "spring tides."
Newton passed on to explain, by the action of
gravitation also, the precession of the equinoxes,
a phenomenon of the sky discovered by Hipparchus,
who pretty well ascertained its amount, although
no reason for it had ever been assigned. The plane
of the earth's equator extended to the celestial
sphere marks out the celestial equator, and the two
opposite points where it intersects the plane of the
ecliptic, or the earth's path round the sun, are
called the equinoctial points, or simply the equi-
NEWTON AND GRAVITATION 71
noxes. And precession of the equinoxes is the motion
of these points westward or backward, about 50
seconds each year, so that a complete revolution
round the ecliptic would take place in about 26,000
years.
Newton saw clearly how to explain this: it is
simply due to the attraction of the sun's gravitation
upon the protuberant bulge around the earth's
equator, acting in conjunction with the earth's rota-
tion on its axis, the effect being very similar to that
often seen in a spinning top, or in a gyroscope. The
moon moving near the ecliptic produces a preces-
sional effect, as also do the planets to a very slight
degree; and the observed value of precession is the
same as that calculated from gravitation, to a high
degree of precision.
Newton died in 1727, too early to have witnessed
that complete and triumphant verification of his
law which ultimately has accounted for practically
every inequality in the planetary motions caused by
their mutual attractions. The problems involved are
far beyond the complexity of those which the mathe-
matical astronomer has to deal with, and the mathe-
maticians of France deserve the highest credit for
improving the processes of their science so that
obstacles which appeared insuperable were one
after another overcome.
Newton's method of dealing with these problems
was mainly geometric, and the insufficiency of this
method was apparent. Only when the French
mathematicians began to apply the higher methods
of algebra was progress toward the ultimate goal
assured. D'Alembert and Clairaut for a time were
foremost in these researches, but their places were
soon taken by Lagrange, who wrote the "Mecanique
72 ASTRONOMY TO-DAY
Analytique," and Laplace, whose "Mecanique
Celeste" is the most celebrated work of all. In large
part these works are the basis of the researches of
subsequent mathematical astronomers who, strictly
speaking, cannot as yet be said to have arrived at
a complete and rigorous solution of all the problems
which the mutual attractions of all the bodies of
the solar system have originated.
It may well be that even the mathematics of the
present day are incompetent to this purpose. When
the brilliant genius of Sir William Hamilton in-
vented quaternion analysis and showed the marvel-
ous facility with which it solved the intricate prob-
lems of physics, there was the expectation that its
application to the higher problems of mathematical
astronomy might effect still greater advances; but
nothing in that direction has so far eventuated.
Some astronomers look for the invention of new
functions with numerical tables bearing perhaps
somewhat the relation to present tables of log-
arithms, sines, tangents, and so on, that these tables
do to the simple multiplication table of Pythagoras.
CHAPTER XV
AFTER NEWTON
WE have said that practically all the motions in
the solar system have been accounted for by the
Newtonian law of gravitation. It will be of in-
terest to inquire into the instances that lead to
qualification of this absolute statement.
One relates to the planet Mercury, whose orbit
or path round the sun is the most elliptical of all
the planetary orbits. This will be explained a little
later.
The moon has given the mathematical astron-
omers more trouble than any other of the celestial
bodies, for one reason because it is nearest to us
and very minute deviations in its motion are there-
fore detectible. Halley it was who ascertained two
centuries ago that the moon's motion round the
earth was not uniform, but subject to a slight accel-
eration which greatly puzzled Lagrange and La-
place, because they had proved exactly this sort of
thing to be impossible, unless indeed^ the body in
question should be acted on by some other force
than gravitation. But Laplace finally traced the
cause to the secular or very slow reduction in the
eccentricity of the earth's own orbit. The sun's
action on the moon was indeed progressively chang-
ing from century to century in such manner as to
accelerate the moon's own motion in its orbit round
the earth.
73
74 ASTRONOMY TO-DAY
Adams, the eminent English astronomer, revised
the calculations of Laplace, and found the effect in
question only half as great as Laplace had done;
and for years a great mathematical battle was on
between the greatest of astronomical experts in this
field of research. Adams, in conjunction with De-
launay, the greatest of the French mathematicians
a half century ago, won the battle in so far as the
mathematical calculations were concerned; but the
moon continues to the present day her slight and
perplexing deviation, as if perhaps our standard
time-keeper, the earth, by its rotation round its axis,
were itself subject to variation. Although many in-
vestigations have been made of the uniformity of
the earth's rotation, no such irregularity has been
detected, and this unexplained variation of the
moon's motion is one of the unsolved problems of
the gravitational astronomer of to-day.
But we are passing over the most impressive of
all the earlier researches of Lagrange and Laplace,
which concerned the exceedingly slow changes,
technically called the secular variations of 'the ele-
ments of the planetary orbits. These elements are
geometrical relations which indicate the form of
the orbit, the size of the orbit, and its position
in space; and it was found that none of these re-
lations or quantities are constant in amount or
direction, but that all, with but one exception,
are subject to very slow, or secular, change,
or oscillation.
This question assumed an alarming significance
at an early day, particularly as it affected the eccen-
tricity of the earth's orbit round the sun. Should
it be possible for this element to go on increasing
for indefinite ages, clearly the earth's orbit would
AFTER NEWTON 76
become more and more elliptical, and the sun would
come nearer and nearer at perihelion, and the earth
would drift farther and farther from the sun at
aphelion, until the extremes of temperature would
bring all forms of life on the earth to an end. The
refined and powerful analysis of Lagrange, how-
ever, soon allayed the fears of humanity by account-
ing for these slow progressive changes as merely
part of the regular system of mere oscillations, in
entire accord with the operation of the law of gravi-
tation; and extending throughout the entire plane-
tary system. Indeed, the periods of these oscilla-
tions were so vast that none of them were shorter
than 50,000 years, while they ranged up to two
million years in length "great clocks of eternity
which beat ages as ours beat seconds."
About a century ago, an eminent lecturer on
astronomy told his audience that the problem of
weighing the planets might readily be one that
would seem wholly impossible to solve. To measure
their sizes and distances might well be done, but
actually to ascertain how many tons they weigh >
never !
Yet if a planet is fortunate enough to have one
satellite or more, the astronomer's method of weigh-
ing the planet is exceedingly simple; and all the
major planets have satellites except the two in-
terior ones, Mercury and Venus. As the satellite
travels round its primary, just as the moon does
round the earth, two elements of its orbit need to
be ascertained, and only two. First, the mean dis-
tance of the satellite from its primary, and second
the time of revolution round it.
Now it is simply a case of applying Kepler's
third law. First take the cube of the satellite's dis-
76 ASTRONOMY TO-DAY
tance and divide it by the square of the time of
revolution. Similarly take the cube of the planet's
distance from the sun and divide by the square of
the planet's time of revolution round him. The
proportion, then, of the first quotient to the second
shows the relation of the mass (that is the weight)
of the planet to that of the sun. In the case of
Jupiter, we should find it to be 1,050, in that of
Saturn 3,500, and so on.
The range of planetary masses, in fact, is very
curious, and is doubtless of much significance in
the cosmogony, with which we deal later. If we
consider the sun and his eight planets, the mass or
weight of each of the nine bodies far exceeds the
combined mass of all the others which are lighter
than itself.
To illustrate: suppose we take as our unit of
weight the one-billionth part of the sun's weight;
then the planets in the order of their masses will be
Mercury, Mars, Venus, Earth, Uranus, Neptune,
Saturn, and Jupiter. According to their relative
masses, then, Mercury being a five-millionth part
the weight of the sun will be represented by 200;
similarly Venus, a four hundred and twenty-five
thousandth part by 2,350, and so on. Then we have
Mercury .
Mars .
Sum of weights of Mercury and Mars
Venus
Sura of weights of Mercury, Mars,
and Venus 2,890
The Earth 3,060
Sum of weights of four inner planets 5,950
AFTER NEWTON 77
Uranus . 44,250
Sum of weights of five planets 50,200
Neptune 51,600
Sum of weights of six planets 101,800
Saturn . 285,580
Sum of weights of seven planets 387,380
Jupiter 954,300
Sum of weights of all the planets. . 1,341,680
Mass or weight of the sun 1,000,000,000
Curious and interesting it is that Saturn is
nearly three times as heavy as the six lighter
planets taken together, Jupiter between two and
three times heavier than all the other planets com-
bined, while the sun's mass is 750 times that of all
the great planets of his system rolled into one.
All the foregoing masses, except those of Mer-
cury and Venus, are pretty accurately known be-
cause they were found by the satellite method just
indicated. Mercury's mass is found by its disturb-
ing effects on Encke's comet whenever it approaches
very near. The mass of Venus is ascertained by
the perturbations in the orbital motion of the
earth. In such cases the Newtonian law of gravita-
tion forms the basis of the intricate and tedious
calculations necessary to find out the mass by this
indirect method.
Its inferiority to the satellite method was strik-
ingly shown at the Observatory in Washington soon
after the satellites of Mars were discovered in 1877.
The inaccurate mass of that planet, as previously
known by months of computation based upon years
and years of observation, was immediately dis-
78 ASTRONOMY TO-DAY
carded in favor of the new mass derived from the
distance and period of the outer satellite by only
a few minutes' calculation.
In weighing the planets, astronomers always use
the sun as the unit. What then is the sun's own
weight? Obviously the law of gravitation answers
this question, if we compare the sun's attraction
with the earth's at equal distances. First we con-
ceive of the sun's mass as if all compressed into
a globe the size of the earth, and calculate how far
a body at the surface of this globe would fall in one
second. The relation of this number to 16.1 feet, the
distance a body falls in one second on the actual
earth, is about 330,000, which is therefore the num-
ber of times the sun's weight exceeds that of the
earth.
A word may be added regarding the force of
gravitation and what it really is. As a matter of
fact Newton did not concern himself in the least
with this inquiry, and says so very definitely. What
he did was to discover the law according to which
gravitation acts everywhere throughout the solar
system. And although many physicists have en-
deavored to find out what gravitation really is, its
cause is not yet known. In some manner as yet
mysterious it acts instantaneously over distances
great and small alike, and no substance has been
found which, if we interpose it between two bodies,
has in any degree the effect of interrupting their
gravitational tendency toward each other.
While the Newtonian law of gravitation has been
accepted as true because it explained and accounted
for all the motions of the heavenly bodies, even in-
cluding such motions of the stars as have been sub-
jected to observation, astronomers have for a long
AFTER NEWTON 79
time recognized that quite possibly the law might
not be absolutely exact in a mathematical sense,
and that deviations from it would surely make their
appearance in time.
A crude instance of this was suggested about a
century ago, when the planet Uranus was found td
be deviating from the path marked out for it by
Bouvard's tables based on the Newtonian law; and
the theory was advocated by many astronomers
that this law, while operant at the medium dis-
tances from the sun where the planets within
Jupiter and Saturn travel, could not be expected to
hold absolutely true at the vast distance of Uranus
and beyond. The discovery of Neptune in 1846,
however, put an end to all such speculation, and has
universally been regarded as an extraordinary
verification of the law, as indeed it is.
When, however, Le Verrier investigated the orbit
of Mercury he found an excess of motion in the
perihelion point of the planet's orbit which neither
he nor subsequent investigators have been able to
account for by Newtonian gravitation, pure and
simple. If Newton's theory is absolutely true, the
excess motion of Mercury's perihelion remains a
mystery.
Only one theory has been advanced to account for
this discrepancy, and that is the Einstein theory
of gravitation. This ingenious speculation was first
propounded in comprehensive form nearly fifteen
years ago, and its author has developed from it
mathematical formulae which appear to yield results
even more precise than those based on the New-
tonian theory.
In expressing the difference between the law of
gravitation and his own conception, Einstein says :
80 ASTRONOMY TO-DAY
"Imagine the earth removed, and in its place sus-
pended a box as big as a moon or a whole house and
inside a man naturally floating in the center, there
being no force whatever pulling him. Imagine,
further, this box being, by a rope or other con-
trivance, suddenly jerked to one side, which is
scientifically termed 'difform motion/ as opposed
to 'uniform motion/ The person would then natur-
ally reach bottom on the opposite side. The re-
sult would consequently be the same as if he
obeyed Newton's law of gravitation, while, in fact,
there is no gravitation exerted whatever, which
proves that difform motion will in every case pro-
duce the same effects as gravitation The term
relativity refers to time and Space. According to
Galileo and Newton, time and space were absolute
entities, and the moving systems of the universe
were dependent on this absolute time and space. On
this conception was built the science of mechanics.
The resulting formulas sufficed for all motions of
a slow nature; it was found, however, that they
would not conform to the rapid motions apparent
in electrodynamics .... Briefly the theory of special
relativity discards absolute time and space, and
makes them in every instance relative to moving
systems. By this theory all phenomena in electro-
dynamics, as well as mechanics, hitherto irreducible
by the old formulae, were satisfactorily explained."
Natural phenomena, then, involving gravitation
and inertia, as in the planetary motions, and
electro-magnetic phenomena, including the motion
of light, are to be regarded as interrelated, and not
independent of one another. And the Einstein
theory would appear to have received a striking
verification in both these fields. On this theory the
AFTER NEWTON 81
Newtonian dynamics fails when the velocities con-
cerned are a near approach to that of light. The
Newtonian theory, then, is not to be considered as
wrong, but in the light of a first approximation.
Applying the new theory to the case of the motion
of Mercury's perihelion, it is found to account for
the excess quite exactly.
On the electro-magnetic side, including also the
motion of light, a total eclipse of the sun affords
an especially favorable occasion for applying the
critical test, whether a huge mass like the sun would
or would not deflect toward itself the rays of light
from stars passing close to the edge of its disk, or
limb. A total eclipse of exceptional duration oc-
curred on May 29, 1919, and the two eclipse parties
sent out by the Royal Society of London and the
Royal Astronomical Society were equipped espe-
cially with apparatus for making this test. Their
stations were one on the east coast of Brazil and the
other on the west coast of Africa.
Accurate calculation beforehand showed . just
where the sun would be among the stars at the time
of the eclipse ; so that star plates of this region were
taken in England before the expeditions went out.
Then, during the total eclipse, the same regions were
photographed with the eclipsed sun and the corona
projected against them. To make doubly sure, the
stars were a third time photographed some weeks
after the eclipse, when the sun had moved away
from that particular region.
Measuring up the three sets of plates, it was
found that an appreciable deflection of the light of
the stars nearest alongside the sun actually exists ;
and the amount of it is such as to afford a fair
though not absolutely exact verification of the
82 ASTRONOMY TO-DAY
theory. The observed deflection may of course be
due to other causes, but the English astronomers
generally regard the near verification as a triumph
for the Einstein theory. Astronomers are already
beginning preparations for a repetition of the
eclipse programme with all possible refinement of
observation, when the next total eclipse of the sun
occurs, September 20, 1922, visible in Australia and
the islands of the Indian Ocean.
A third test of the theory is perhaps more critical
than either of the others, and this necessitates a
displacement of spectral lines in a gravitational
field toward the red end of the spectrum; but the
experts who have so far made measures for de-
tecting such displacement disagree as to its actual
existence. The work of St. John at Mt. Wilson is
unfavorable to the theory, as is that of Ever shed
of Kodiakanal, who has made repeated tests on the
spectrum of Venus, as well as in the cyanogen
bands of the sun.
The enthusiastic advocates of the Einstein theory
hold that, as Newton proved the three laws of
Kepler to be special cases of his general law, so
the "universal relativity theory" will enable eventu-
ally the Newtonian law to be deduced from the Ein-
stein theory. "This is the way we go on in science,
as in everything else," wrote Sir George Airy,
Astronomer Royal ; "we have to make out that some-
thing is true; then we find out under certain cir-
cumstances that it is not quite true; and then we
have to consider and find out how the departure can
be explained." Meanwhile, the prudent person
keeps the open mind.
CHAPTER XVI
HALLEY AND HIS COMET
H ALLEY is one of the most picturesque charac-
ters in all astronomical history. Next to Newton
himself he was most intimately concerned in giving
the Newtonian law to the world.
Edmund Halley was born (1656) in stirring
times. Charles I. had just been executed, and it
was the era of CromwelPs Lord Protectorate and
the wars with Spain and Holland. Then followed
(1660) the promising but profligate Charles II.
(who nevertheless founded at Greenwich the great-
est of all observatories when Halley was nineteen),
the frightful ravages of the Black Plague, the
tyrannies of James II., and the Revolution of 1688
all in the early manhood of Halley, whose scien-
tific life and works marched with much of the vigor
of the contending personalities of state.
The telescope had been invented a half century
earlier, and Galileo's discoveries of Jupiter's moons
and the phases of Venus had firmly established the
sun-centered theory of Copernicus.
The sun's distance, though, was known but
crudely; and why the stars seemed to have no yearly
orbits of their own corresponding to that of the
earth was a puzzle. Newton was well advanced
toward his supreme discovery of the law of uni-
versal gravitation; and the authority of Kepler
taught that comets travel helter-skelter through
83
84 ASTRONOMY TO-DAY
space in straight lines past the earth, a perpetual
menace to humanity.
"Ugly monsters," that comets always were to the
ancient world, the medieval church perpetuated this
misconception so vigorously that even now these
harmless, gauzy visitors from interstellar space
possess a certain "wizard hold upon our imagina-
tion." This entertaining phase of the subject is
excellently treated in President Andrew D. White's
"History of the Doctrine of Comets," in the Papers
of the American Historical Association. Halley's
brilliant comet at its earlier apparitions had been
no exception.
Halley's father was a wealthy London soap
maker, who took great pride in the growing intel-
lectuality of his son. Graduating at Queen's College,
Oxford, the latter began his astronomical labors at
twenty by publishing a work on planetary orbits;
and the next year he voyaged to St. Helena to cata-
logue the stars of the southern firmament, to
measure the force of terrestrial gravity, and observe
a transit of Mercury over the disk of the sun.
While clouds seriously interfered with his obser-
vations on that lonely isle, what he saw of the
transit led to his invention of "Halley's method,"
which, as applied to the transit of Venus, though
not till long after his death, helped greatly in the
accurate determination of the sun's distance from
the earth. Halley's researches on the proper motions
of the stars of both hemispheres soon made him fa-
mous, and it was said of him, "If any star gets dis-
placed on the globe, Halley will presently find it out."
His return to London and election to the Royal
Society (of which he was many years secretary)
added much to his fame, and he was commissioned
HALLEY AND HIS COMET 85
by the society to visit Danzig and arbitrate an
astronomical controversy between Hooke and He-
velius, both his seniors by a generation.
On the continent he associated with other great
astronomers, especially Cassini, who had already
found three Saturnian moons; and it was then he
observed the great comet of 1680, which led up to
the most famous event of Halley's life.
The seerlike Seneca may almost be said to have
predicted the advent of Halley, when he wrote
("Quaestiones Naturales," vii) : "Some day there
will arise a man who will demonstrate in what region
of the heavens comets pursue their way; why they
travel apart from the planets ; and what their sizes
and constitution are. Then posterity will be amazed
that simple things of this sort were not explained
before."
To Newton it appeared probable that cometary
voyagers through space might have orbits of their
own; and he proved that the comet of 1680 never
swerved from such a path. As it could nowhere
approach within the moon's orbit, clearly threats
of its wrecking the earth and punishing its inhab-
itants ought to frighten no more.
Halley then became intensely interested in
comets, and gathered whatever data concerning the
paths of all these bodies he could find. His first
great discovery was that the comets seen in 1531 by
Apian, and in 1607 by Kepler, traveled round the
sun in identical paths with one he had himself
observed in 1682. A still earlier appearance of
Halley's comet (1456) seems to have given rise to
a popular and long-reiterated myth of a papal bull
excommunicating "the Devil, the Turk, and the
Comet."
86 ASTRONOMY TO-DAY
No longer room for doubt : so certain was Halley
that all three were one and the same comet, com-
pleting the round of its orbit in about seventy-six
years, that he fearlessly predicted that it would be
seen again in 1758 or 1759. And with equal con-
fidence he might have foretold its return in 1835
and 1910; for all three predictions have come true
to the letter.
Halley's span of existence did not permit his
living to see even the first of these now historic
verifications. But we in our day may emphatically
term the epoch of the third verified return Annus
Halleianus.
Says Turner, Halley's successor in the Savilian
chair at Oxford to-day: "There can be no more
complete or more sensational proof of a scientific
law, than to predict events by means of it. Halley
was deservedly the first to perform this great service
for Newton's Law of Gravitation, and he would
have rejoiced to think how conspicuous a part Eng-
land was to play in the subsequent prediction of
the existence of Neptune."
Halley rose rapidly among the chief astronomical
figures of his day. But he had little veneration for
mere authority, and the significant veering of his
religious views toward heterodoxy was for years
an obstacle to his advance.
Still Halley the astronomer was great enough to
question any contemporary dicta that seemed to
rest on authority alone. Everyone called the stars
"fixed" stars; but Halley doubting this, made the
first discovery of a star's individual motion proper
motion, as astronomers say. To-day, two hundred
years after, every star is considered to be in motion,
and astronomers are ascertaining their real motions
HALLEY AND HIS COMET 87
in the celestial spaces to a nicety undreamed of by
even the exacting Halley.
The moon, of priceless service to the early navi-
gator, was regarded by all astronomers as endowed
with an average rate of motion round the earth
that did not vary from age to age. But Halley
questioned this too; and on comparing with the
ancient value from Chaldean eclipses, he made an-
other discovery the secular acceleration of the
moon's mean motion, as it is technically termed.
This was a colossal discovery in celestial dynamics ;
and the reason underlying it lay hidden in Newton's
law for yet another century, till the keener mathe-
matics of Laplace detected its true origin.
With Newton, Halley laid down the firm foun-
dations of celestial mechanics, and they pushed the
science as far as the mathematics of their day
would permit. Halley, however, was not content
with elucidating the motion of bodies nearest the
earth, and pressed to the utmost confines of the
solar system known to him. Here, too, he made a
signal discovery of that mutual disturbance of the
planets in their motion round the sun, called the
great inequality of Jupiter and Saturn.
Halley's versatile genius attacked all the great
problems of the day. His observation of the sun's
total eclipse in 1715 is the earliest reliable account
of such a phenomenon by a trained astronomer.
He described the corona minutely and was the first
to see that other interesting phenomenon which
only an alert observer can detect, which a great
astronomer of a later day compared to the "ignition
of a fine train of gunpowder," and which has ever
since borne the name of "Baily's beads."
88 ASTRONOMY TO-DAY
Besides being a great astronomer, Halley was a
man of affairs as well, which Newton, although the
greater mathematician, was not. Without Halley,
Newton's superb discovery might easily have been
lost to the age and nation, for the latter was bent
merely on making discoveries, and on speculative
contemplation of them, with never a thought of
publishing to the world.
Halley, more practical and businesslike, insisted
on careful writing out and publication. Newton
was then only forty-two, and Halley fully fourteen
years his junior. But the philosophers of that day
were keenly alive to the mystery of Kepler's laws,
and Halley was fully conscious of the grandeur and
far-reaching significance of Newton's great gen-
eralization which embodied all three of Kepler's laws
in one.
Newton at last yielded, though reluctantly, and
the "Principia" was given to the world, though
wholly at Halley's private charges.
But Halley was far from being completely en-
grossed with the absorbing problems of the sky;
things terrestrial held for years his undivided at-
tention. Imagine present-day Lords Commissioners
of the Admiralty intrusting a ship of the British
navy to civilian command. Yet such was their con-
fidence in Halley that he was commissioned as cap-
tain of H. M.'s pink Paramour in 1698, with instruc-
tions to proceed to southern seas for geographical
discoveries, and for improving knowledge of the
longitude problem, and of the variations of the com-
pass. Trade winds and monsoons, charts of mag-
netic variation, tides and surveys of the Channel
coast, and experiments with diving bells were prac-
tical activities that occupied his attention.
HALLEY AND HIS COMET 89.
Halley in 1720 became Astronomer Royal. He
was the second incumbent of this great office, but
the first to supply the Royal Observatory with in-
struments of its own, some of which adorn its walls
even to-day. His long series of lunar observations
and his magnetic researches were of immense
practical value in navigation.
Halley lived to a ripe old age and left the world
vastly better than he found it. His rise from hum-
blest obscurity was most remarkable, and he lived
to gratify all the ambitions of his early manhood.
"Of attractive appearance, pleasing manners, and
ready wit/' says one of his biographers, "loyal,
generous, and free from self-seeking, he was one
of the most personally engaging men who ever held
the office of Astronomer Royal.
He died in office at Greenwich in 1742.
"Halley was buried," says Chambers, "in the
churchyard of St. Margaret's, Lee, not far from
Greenwich, and it has lately been announced that
the Admiralty have decided to repair his tomb at
the public expense, no descendants of his being
known." There is no suitable monument in England
to the memory of one of her greatest scientific men.
In any event the collection and republication of
his epoch-making papers would be welcomed by
astronomers of every nation.
CHAPTER XVII
BRADLEY AND ABERRATION
EVING at Kew in London early in the 18th cen-
tury was an enthusiastic young astronomer,
James Bradley. He is famous chiefly for his accurate
observations of star places which have been invalu-
able to astronomers of later epochs in ascertaining
the proper motions of stars.
The latitude of Bradley's house in Kew was very
nearly the same as the declination of the bright star
Gamma Draconis, so that it passed through his
zenith once every day. Bradley had a zenith sector,
and with this he observed with the greatest care the
zenith distance of Gamma Draconis at every possible
opportunity. This he did by pointing the telescope
on the star and then recording the small angle of its
inclination to a fine plumb line. So accurate were
his measures that he was probably certain of the
, star's position to the nearest second of arc.
What he hoped to find was the star's motion round
a very slight orbit once each year, and due to the
earth's motion in its orbit round the sun. In other
words, he sought to find the star's parallax if it
turned out to be a measurable quantity.
It is just as well now that his method of observa-
tion proved insufficiently delicate to reveal the paral-
lax of Gamma Draconis ; but his assiduity in observa-
tion led him to an unexpected discovery of greater
moment at that time. What he really found was
90
BRADLEY AND ABERRATION 91
that the star had a regular annual orbit ; but wholly
different from what he expected, and very much
larger in amount. This result was most puzzling to
Bradley. The law of relative motion would require
that the star's motion in its expected orbit should
be opposite to that of the earth in its annual orbit ;
instead of which the star was all the time at right
angles to the earth's motion.
Bradley was a frequent traveler by boat on the
Thames, and the apparent change in the direction
of the wind when the boat was in motion is said to
have suggested to him what caused the displacement
of Gamma Draconis. The progressive motion of
light had been roughly ascertained by Roemer: let
that be the velocity of the wind. And the earth's
motion in its orbit round the sun, let that be the
speed of the boat. Then as the wind (to an observer
on the moving boat) always seems to come from a
point in advance of the point it actually proceeds
from (to an observer at rest) , so the star should be
constantly thrown forward by an angle given by the
relation of the velocity of light to the speed of the
earth in orbital revolution round the sun.
The apparent places of all stars are affected in
this manner, and this displacement is called the
aberration of light. Astronomers since Bradley's
discovery of aberration in 1726 have devoted a great
deal of attention to this astronomical constant, as it
is called, and the arc value of it is very nearly 20".5.
This means that light travels more than ten thousand
times as fast as the earth in its orbit (186,330 miles
per second as against the earth's 18.5) . And we can
ascertain the sun's distance by aberration also be-
cause the exact values of the velocity of light and of
the constant of aberration when properly combined
92 ASTRONOMY TO-DAY
give the exact orbital speed of the earth; and this
furnishes directly by geometry the radius of the
earth's orbit, that is the distance of the sun.
In fact, this is one of the more accurate modern
methods of ascertaining the distance of the sun. As
early as 1880 it enabled the writer to calculate the
sun's parallax equal to 8".80, a value absolutely
identical with that adopted by the Paris Confer-
ence of 1896, and now universally accepted as the
standard.
In whatever part of the sky we observe, every star
is affected by aberration. At the poles of the ecliptic,
23% degrees from the earth's poles, the annual aber-
ration orbits of the stars are very small circles, 41"
in diameter. Toward the ecliptic the aberration
orbits become more and more oval, ellipses in fact
of greater and greater eccentricity, but with their
major axes all of the same length, until we reach the
ecliptic itself; and then the ellipse is flattened into
a straight line 41" in length, in which the star travels
forth and back once a year. Exact correspondence
of the aberration ellipses of the stars with the annual
motion of the earth round the sun affords indis-
putable proof of this motion, and as every star par-
takes of the movement, this proof of our motion
round the sun becomes many million fold.
Indeed, if we were to push a little farther the re-
finement of our analysis of the effect of aberration
on stellar positions, we could prove also the rotation
of the earth on its axis, because that motion is swift
enough to bear an appreciable ratio to the velocity
of light. Diurnal aberration is the term applied to
this slight effect, and as every star partakes of it,
demonstration of the earth's turning round on its
axis becomes many millionfold also.
CHAPTER XVin
THE TELESCOPE
HAD anyone told Ptolemy that his earth-centered
system of sun, moon, and stars would ultimately
be overthrown, not by philosophy but by the over-
whelming evidence furnished by a little optical in-
strument which so aided the human eye that it could
actually see systems of bodies in revolution round
each other in the sky, he would no doubt have
vehemently denied that any such thing was possible.
To be sure, it took fourteen centuries to bring this
about, and the discovery even then was without
much doubt due to accident.
Through all this long period when astronomy may
>be said to have merely existed, practically without
any forward step or development, its devotees were
unequipped with the sort of instruments which were
requisite to make the advance possible. There were
astrolabes and armillary spheres, with crudely di-
vided circles, and the excellent work done with them
only shows the genius of many of the early astrono-
mers who had nothing better to work with. Re-
garding star-places made with instruments fixed in
the meridian, Bessel, often called the father of prac-
tical astronomy, used to say that, even if you pro-
vided a bad observer with the best of instruments,
a genius could surpass him with a gun barrel and a
cart wheel.
Before the days of telescopes, that is, prior to the
seventeenth century, it was not known whether any
93
94 ASTRONOMY TO-DAY
of the planets except the earth had a moon or not ;
consequently the masses of these planets were but
very imperfectly ascertained ; the phases of Mercury
and Venus were merely conjectured ; what were the
actual dimensions of the planets could only be
guessed at ; the approximate distances of sun, moon,
and planets were little better than guesses ; the dis-
tances of the stars were wildly inaccurate; and the
positions of the stars on the celestial sphere, and of
sun, moon, and planets among them were far re-
moved from modern standards of precision all be-
cause the telescope 'had not yet become available as
an optical adjunct to increase the power of the
human eye and enable it to see as if distances were
in considerable measure annihilated.
Galileo almost universally is said to have been the
inventor of the telescope, but intimate research into
the question would appear to give the honor of that
original invention to another, in another country.
What Gfalileo deserves the highest praise for, how-
ever, is the reinvention independently of an "op-
tick tube" by which he could bring distant objects
apparently much nearer to him; and being an as-
tronomer, he was by universal acknowledgment first
of all men to turn a telescope on the heavenly bodies.
This was in the year 1609, and his first discovery
was the phase of Venus, his second the four Medicean
moons or satellites of Jupiter, discoveries which at
that epoch were of the highest significance in es-
tablishing the truth of the Copernican system be-
yond the shadow of doubt.
But the first telescopes of which we have record
were made, so far as can now be ascertained, in
Holland very early in the 17th century. Metius, a
professor of mathematics, and Jansen and Lipper-
THE TELESCOPE 95
hey, who were opticians in Middelburg all three
are entitled to consideration as claimants of the orig-
inal invention of the telescope. But that such an
instrument was pretty well known would appear to
be shown by his government's refusal of a patent to
Lipperhey in 1608 ; while the officials recognizing the
value of such an instrument for purposes of war,
got him to construct several telescopes and ordered
him to keep the invention a secret.
Within a year Galileo heard that an instrument
was in use in Holland by which it was possible to
see distant objects as if near at hand. Skilled in
optics as he was, the reinvention was a task neither
long nor difficult for him. One of his first instru-
ments magnified but three times ; still it made a great
sensation in Venice where he exhibited the little
tube to the authorities of that city, in which he first
invented it.
Galileo's telescope was of the simplest type, with
but two lenses; the one a double convex lens with
which an image of the distant object is formed, the
other a double concave lens, much smaller which
was the eye-lens for examining the image. It is this
simple form of Gahlean telescope that is still used
in opera glasses and field glasses, because of the
shorter tube necessary.
Galileo carried on the construction of telescopes,
all the time improving their quality and enlarging
their power until he built one that magnified thirty
times. What the diameter of the object glass was
we do not know, perhaps two inches or possibly a
little more. Glass of a quality good enough to make
a telescope of cannot have been abundant or even
obtainable except with great difficulty in those early
days.
96 ASTRONOMY TO-DAY
Other discoveries by this first of celestial ob-
servers were the spots on the sun, the larger moun-
tains of the moon, the separate stars of which the
Milky Way is composed, and, greatest wonder of all,
the anomalous "handles" (ansae, he called them) of
Saturn, which we now know as the planet's ring, the
most wonderful of all the bodies in the sky.
Since Galileo's time, only three centuries past,
the progress in size and improvement in quality
of the telescope have been marvelous. And
this advance would not have been possible except
for, first, the discoveries still kept in large part
secret by the makers of optical glass which have
enabled them to make disks of the largest size;
second, the consummate skill of modern opticians
in fashioning these disks into perfect lenses; and
third, the progress in the mechanical arts and en-
gineering, by which telescope tubes of many tons'
weight are mounted or poised so delicately that the
thrust of a finger readily swerves them from one
point of the heavens to another.
As the telescope is the most important of all
astronomical instruments, it is necessary to
understand its construction and adjustment and
how the astronomer uses it. Telescopes are optical
instruments, and nothing but optical parts would be
requisite in making them, if only the optical con-
ditions of their perfect working could be obtained
without other mechanical accessories.
In original principle, all telescopes are as simple
as Galileo's; first, an object glass to form the image
of the distant object; second the eyepiece usually
made of two lenses, but really a microscope, to
magnify that image, and working in the same way
that any microscope magnifies an object close at
THE 150-Fi. TOWER AT THE MT. WILSON SOLAR OBSERVATORY. At the
left is a diagram of tower, telescope and pit. At the upper right is an
exterior view of the tower ; below a view looking down into the pit, 75
ft. deep. (Photo, Mt. Wilson Solar Observatory.)
THE TELESCOPE 97
hand; and third, a tube to hold all the necessary
lenses in the true relative positions.
The focal lengths of object glass and eyepiece
will determine just what distance apart the lenses
must be in order to give perfect vision. But it is
quite as important that the axes of all the lenses be
adjusted into one and the same straight line, and
then held there rigidly and permanently. Otherwise
vision with the telescope will be very imperfect
and wholly unsatisfactory. The distance from the
objective, or object glass to its focal point is called
its focal length ; and if we divide this by the focal
length of the eyepiece, we shall have the magnifying
power of the telescope. The eyepiece will usually
be made of two lenses, or more, and we use its focal
length considered as a single lens, in getting the
magnifying power. A telescope will generally have
many eyepieces of different focal lengths, so that
it will have a corresponding range of magnifying
powers. The lowest magnifying power will be not
less than four or five diameters for each inch of
aperture of the objective ; otherwise the eye will fail
to receive all the light which falls upon the glass.
A 4-inch telescope will therefore have no eyepiece
with a lower magnifying power than about 20
diameters. The highest magnifying power advan-
tageous for a glass of this size will be about 250 to
300, the working rule being about 70 diameters to
each inch of aperture, although the theoretical
limit is regarded as 100.
The reason for a variety of eyepieces with dif-
ferent magnifying powers soon becomes apparent
on using the telescope. Comets and nebulae call for
very low powers, while double stars and the plane-
tary surfaces require the higher powers, provided
Sci. Vol. 2 *
98 ASTRONOMY TO-DAY
the state of the atmosphere at the moment will
allow it. If there is much quivering and unsteadi-
ness, nothing is gained by trying the higher powers,
because all the waves of unsteadiness are magnified
also in the same proportion, and sharpness of vision,
or fine definition, or "good seeing," as it is called,
becomes impossible. The vibrations and tremors of
the atmosphere are the greatest of all obstacles to
astronomical observation, and the search is always
in order for regions of the world, in deserts or on
high mountains, where the quietest atmosphere is
to be found.
Quite another power of the telescope is dependent
on its objective solely: its light-gathering power.
Light by which we see a star or planet is admitted
to the retina of the eye through an adjustable aper-
ture called the pupil. In the dark or at night, the
pupil expands to an average diameter of one-fourth
of an inch. But the object-glass of a telescope, by
focusing the rays from a star, pours into the eye,
almost as a funnel acts with water, all the light
which falls on its larger surface. And as geometry
has settled it for us that areas of surfaces are pro-
portioned to the squares of their diameters, a two-
inch object glass focuses upon the retina of the eye
64 times as much light as the unassisted eye would
receive. And the great 40-inch objective of the
Yerkes telescope would, theoretically, yield 25,600
times as much light as the eye alone. But there
would be a noticeable percentage of this lost through
absorption by the glasses of the telescope and
scattering by their surfaces.
The first makers of telescopes sooti encountered
a most discouraging difficulty, because it seemed to
them absolutely insuperable. This is known as
THE TELESCOPE 99
chromatic aberration, or the scattering of light in
a telescope due simply to its color or wave length.
When light passes through a prism, red is refracted
the least and violet the most. Through a lens it is
the same, because a lens may be regarded as an in-
definite system of prisms. The image of a star or
planet, then, formed by a single lens cannot be
optically perfect ; instead it will be a confused inter-
mingling of images of various colors. With low
powers this will not be very troublesome, but great
indistinctness results from the use of high magnify-
ing powers.
The early makers and users of telescopes in the
latter part of the seventeenth century found that the
troublesome effects of chromatic aberration could
be much reduced by increasing the focal length of
the objective. This led to what we term engineer-
ing difficulties of a very serious nature, because the
tubes of great length were very awkward in point-
ing toward celestial objects, especially near the
zenith, where the air is quietest. And it was next
to impossible to hold an object steadily in the field,
even after all the troubles of getting it there had
been successfully overcome.
Bianchini and Cassini, Hevelius and Huygens
were among the active observers of that epoch who
built telescopes of extraordinary length, a hundred
feet and upward. One tube is said to have been
built 600 feet in length, but quite certainly it could
never have been used. So-called aerial telescopes
were also constructed, in which the objective was
mounted on top of a tower or a pole, and the eyepiece
moved along near the ground. But it is difficult to
see how anything but fleeting glimpses of the
heavenly bodies could have been obtained with such
100 ASTRONOMY TO-DAY
contrivances, even if the lenses had been perfect.
Newton indeed, who was expert in optics, gave up
the problem of improving the refracting telescope,
and turned his energies toward the reflector.
In 1733, half a century after Newton and a cen-
tury and a quarter after Galileo, Chester More Hall,
an Englishman, found by experiment that chro-
matic aberration could be nearly eliminated by
making the objective of two lenses instead of one,
and the same invention was made independently by
Dollond, an English optician, who took out letters
patent about 1760. So the size of telescopes seemed
to be limited only by the skill of the glassmaker
and the size of disks that he might find it practi-
cable to produce.
What Hall and Dollond did was to make the outer
or crown lens of the objective as before, and place
behind it a plano-concave lens of dense flint glass.
This had the effect of neutralizing the chromatic
effect, or color aberration, while at the same time
only part of the refractive effect of the crown lens
was destroyed. This ingenious but costly combina-
tion prepared the way for the great refracting
telescopes of the present day, because it solved, or
seemed to solve, the important problem of getting
the necessary refraction of light rays without harm-
ful dispersion or decomposition of them.
Through the 18th century and the first years of
the 19th many telescopes of a size very great for
that day were built, and their success seemed com-
plete. With large increase in the size of the disks,
however, a new trouble arose, quite inherent in the
glass itself. The two kinds of glass, flint and
crown, do not decompose white light with uniform-
ity, so that when the so-called achromatic objec-
THE TELESCOPE
live was composed of flint and crown, there was an
effect known as irrationality of dispersion, or
secondary spectrum, which produced a very trouble-
some residuum of blue light surrounding the images
of bright objects. This is the most serious defect
of all the great refractors of the day, and effectively
it limits their size to about 60 inches of aperture,
with present types of flint and crown. It is ex-
pected by present experimenters, however, that
further improvements in optical glass will do much
to extend this limit; so that a refracting telescope
of much greater size than any now in existence will
be practicable.
Improvements in mounting telescopes, too, are
still possible. Within recent years, Hartness, of
Springfield, Vermont, has erected a new and ingen-
ious type of turret telescope which protects the ob-
server from wind and cold while his instrument is
outside. It affords exceptional facilities for rapid
and convenient observing, as for variable stars, and
is adaptable to both refractors and reflectors.
The captivating study of the heavens can of
course be begun with the naked eye alone, but very
moderate optical assistance is a great help and
stimulates. An opera-glass affords such assistance;
a field-glass does still better, and best of all, for
certain purposes, is a modern prism-binocular.
CHAPTER XIX
REFLECTORS MIRROR TELESCOPES
/CHERISHED with the utmost care in the rooms
\J of the Royal Society of London is a world-
famous telescope, a diminutive reflector made by the
hands of Sir Isaac Newton. We have already men-
tioned his connection with the refractor; and how
he abandoned that type of telescope in favor of the
reflecting mirror, or reflector in which the obstacles
to great size appeared to be purely mechanical. By
many, indeed, Newton is regarded as the inventor
of the reflector.
By the principles of optics, all the rays from a
star that strike a concave mirror will be reflected
to the geometric focal point, provided a section of
that mirror is a parabola. Such a mirror is called
a speculum, and is an alloy of tin, copper, and bis-
muth. Its surface takes a Very high polish, reflect-
ing when newly polished nearly 90 per cent of the
light that falls upon it.
But the focus where the eyepiece must be used is
in front of the mirror, and if the eye were placed
there, the observer's head would intercept all or
much of the light that would otherwise reach the
mirror. Gregory, probably the real inventor of the
reflector, was the first to dodge this difficulty by
perforating the mirror at the center and applying
the eyepiece there, at the back of the speculum;
but it was necessary to first send the rays to that
102
MIRROR TELESCOPES 103
point by reflection from a second or smaller mirror,
in the optical axis of the speculum. This reflects
the rays backward down the tube to the eyepiece, or
spectroscope, or camera,,
Another English optician, Cassegrain, improved
on this design somewhat by placing the secondary
mirror inside the focus of the speculum, or nearer
to it, so that the tube is shorter. This form is pref-
erable for many kinds of astronomical work, es-
pecially photography. Herschel sought to do away
with the secondary reflector entirely and save the
loss of light by tilting the speculum slightly, so as
to throw the image at one side of the tube; but this
modification introduces bad definition of the image
and has never been much used.
A better plan is that of Newton, who placed a
small plane speculum at an angle of 45 degrees in
the optical axis where the secondary mirror of the
Gregory-Cassegrainian type is placed. The rays are
then received by the eyepiece at the side of the upper
end of the tube, the observer looking in at right
angles to the axis. And a modern improvement first
used by Draper is a small rectangular prism in place
of the little plane speculum, effecting a saving of
five to ten per cent of the light.
It is not easy to say which type of telescope, the
refractor or the reflector, is the more famous. Nor
which is the better or more useful, or the more
likely to lead in the astronomy of the future. When
the successors of Dollond had carried the achro-
matic refractor to the limit enforced by the size of
the glass disks they were able to secure, they found
these instruments not so great an improvement
after all. The single-lens telescopes of great focal
length were nearly as good optically, though much
104 ASTRONOMY TO-DAY
more awkward to handle. But the quality of the
glass obtainable in that day appeared to set an
arbitrary limit to that great amplification of size
and power which progress in observational as-
tronomy demanded.
Then came the elder Herschel, best known and
perhaps the greatest of all astronomers. At Bath,
England, music was his profession, especially the
organ. But he was dissatisfied with his little
Gregorian reflector, and being a very clever me-
chanician he set out to build a reflector for himself.
It is said that he cast and polished nearly 200
mirrors, in the course of experiments on the most
highly reflective type of alloys, and the sort of
mechanism that would enable him to give them the
highest polish. In all his work he was ably and
enthusiastically aided by his sister, Caroline Her-
schel, most famous of all women astronomers.
Upward in size of his mirrors he advanced, till
he had a speculum of two feet diameter with a tube
20 feet long. Twelve to fifteen years had elapsed
when in 1781, while testing one of these reflectors
on stars in the constellation Gemini, he made
the first discovery of a planet since the invention
of the telescope the great planet now known as
Uranus.
Under the patronage of King George, he advanced
to telescopes of still greater size, his largest being
no less than forty feet in length, with a speculum of
four feet in diameter. Two new satellites of
Saturn were discovered with this giant reflector,
which was dismantled by Sir John Herschel with
appropriate ceremonies, including the singing of an
ode by the Herschel family assembled inside of the
tube, on New Year's Eve, 1839-40.
MIRROR TELESCOPES 105
We have record of but few attempts to improve
the size and definition of great reflectors by the
continental astronomers during this era. In Eng-
land and Ireland, however, great progress was
made. About 1860 Lassell built a two-foot reflector,
with which he discovered two new satellites of
Uranus, and which he subsequently set up in the
island of Malta. Ten years later Thomas Grubb
and Son of Dublin constructed a four-foot reflector,
now at the Observatory in Melbourne, Australia.
Calver in conjunction with Common of Esling, Lon-
don, about 1880-95 built several large reflectors, the
largest of five feet diameter, now owned by Harvard
College Observatory; and, rather earlier, Martin of
Paris completed a four-foot reflector.
The mirrors of these latter instruments were not
made of speculum metal, but of solid glass, which
must be very thick (one-seventh their diameter) in
order to prevent flexure or bending by their own
weight. So sensitive is the optical surface to dis-
tortion that unless a complicated series of levers
and counterpoises is supplied, to support the under
surface of the mirror, the perfection of its optical
figure disappears when the telescope is directed to
objects at different altitudes in the sky. The upper
or outer surface of the glass is the one which re-
ceives the optical polish on a heavy coat of silver
chemically deposited on the polished glass after its
figure has been tested and found satisfactory.
But far and away the most famous reflecting
telescope of all is the "Leviathan" of Lord Rosse,
built at Birr Castle, Parsonstown, Ireland, about the
middle of the last century. His Lordship made
many ingenious improvements in grinding the
mirror, which was of speculum metal, six feet in
106 ASTRONOMY TO-DAY
diameter and weighed seven tons. It was ground
to a focal length of fifty-four feet and mounted be-
tween heavy walls of masonry, so that the motion of
the great tube was restricted to a few degrees on
both sides of the meridian. The huge mechanism
was very cumbersome in operation, and photog-
raphy was not available in those days ; nevertheless
Lord Rosse's telescope made the epochal discovery
of the spiral nebulae, which no other telescope of
that day could have done.
In America the reflector has always kept at least
even pace with the refractor. As early as 1830,
Mason and Smith, two students at Yale College,
enthused by Denison Olmsted, built a 12-inch specu-
lum with which they made unsurpassed observations
of the nebulae. Dr. Henry Draper, returning from a
visit to Lord Rosse, began about 1865 the construc-
tion of two silver-on-glass reflectors, one of 15
inches diameter, the other of 28 inches, with which
he did important work for many years in photog-
raphy and spectroscopy, and his mirrors are now
the property of Harvard College Observatory. Alvan
Clark and Sons have in later years built a 40-inch
mirror for the Lowell Observatory in Arizona, and
very recently a 6-foot silver-on-glass mirror has
been set up in the Dominion of Canada Astrophy-
sical Observatory at Victoria, British Columbia,
where it is doing excellent work in the hands of
Plaskett, its designer.
The huge glass disk for the reflector weighs two
tons, and it must be cast so that there are no in-
ternal strains; otherwise it is liable to burst in
fragments in the process of grinding. It should be
free from air-bubbles, too; so the glass is cast in
one melting, if possible. This disk was made by
MIRROR TELESCOPES 107
the St. Gobain Plate Glass Company, whose works
have been ruthlessly destroyed by the enemy during
the war; but fortunately the great disk had been
shipped from Antwerp only a week before declara-
tion of hostilities.
Brashear of Allegheny was intrusted with the
optical parts, which occupied many months of
critical work. The finished mirror is 73 inches in
diameter, its focal length is 30 feet, and its thickness
12 inches. A central hole 10 inches in diameter
makes possible its use as a Gregorian or Cassegrain-
ian type, as well as Newtonian. The mechanical parts
of this great telescope are by Warner and Swasey of
Cleveland, after the well-known equatorial mount-
ing of the Melbourne reflector by Grubb of Dublin.
Friction of the polar and declination axes is re-
duced by ball bearings. The 66-foot dome has an
opening 15 feet wide and extending six feet beyond
the zenith. All motions of the telescope, dome
shutters, and observing platform are under com-
plete control by electric motors. Spectroscopic
binaries form one of the special fields of research
with this powerful instrument, and many new bin-
aries have already been detected.
The great reflectors designed and constructed by
Ritchey, formerly of Chicago and now of Pasadena,
deserve especial mention. While connected with
the Yerkes Observatory he constructed a two-foot
reflector for that institution, with which he had ex-
ceptional success in photography of the stars and
nebulae. Later he built a 5-foot reflector, now at
the Carnegie Observatory on Mount Wilson, Cali-
fornia, with which the spiral nebulae and many
other celestial objects have been especially well
photographed. Ritchey's later years have been
108 ASTRONOMY TO-DAY
spent on the construction of an even. greater mirror,
no less than 100 inches in diameter, which was com-
pleted in 1919, and has already yielded photographic
results dealt with farther on, and far surpassing
anything previously obtained. Theoretically this
huge mirror, if its surface were perfectly reflective
so that it would transmit all the rays falling upon
it, would gather 160,000 times as much light as the
unaided eye alone.
Whether a 72-inch refractor, should it ever be
constructed, would surpass the 100-inch reflector as
an all-round engine for astronomical research, is a
question that can only be fully answered by build-
ing it and trying the two instruments alongside.
Probably three-quarters of all the really great
astronomical work in the past has been done by re-
fractors. They are always ready and convenient
for use, and the optical surfaces rarely require
cleaning and readjustment. With increase of size,
however, the secondary spectrum becomes very
bothersome in the great lenses ; and the larger they
are, the more light is lost by absorption on account
of the increasing thickness of the lenses. With the
reflector on the other hand, while there is clearly
a greater range of size, the reflective surface re-
tains its high polish only a brief period, so that
mere tarnish effectively reduces the aperture; and
the great mirror is more or less ineffective in con-
sequence of flexure uncompensated by the lever
system that supports the back of the mirror.
Both types of telescope still have their enthusi-
astic devotees; and the next great reflector would
doubtless be a gratifying success, if mounted in some
elevated region of the world, like the Andes of
northern Chile, where the air is exceptionally steady
MIRROR TELESCOPES 109
and the sky very clear a large part of the year. The
highest magnifying powers suitable for work with
such a telescope could then be employed, and new
discoveries added as well as important work done in
extension of lines already begun on the universe of
stars.
On the authority of Clark, even a six-foot objec-
tive would not necessitate a combined thickness of
its glasses in excess of six inches. Present disks
are vastly superior to the early ones in transpar-
ency, and there is reason to expect still greater im-
provement. The engineering troubles incident to
execution of the mechanical side of the scheme need
not stand in the way; they never have, indeed the
astronomer has but just begun to invoke the fertile
resources of the modern engineer,. Not long before
his death the younger Clark who had just finished
the great lenses of the 40-inch Yerkes telescope,
ventured this prevision, already in part come true:
"The new astronomy, as well as the old, demands
more power. Problems wait for their solution, and
theories to be substantiated or disproved. The hori-
zon of science has been greatly broadened within
the last few years, but out upon the borderland I
see the glimmer of new lights that await for their
interpretation, and the great telescopes of the future
must be their interpreters."
Practically all the great telescopes of the world
have in turn signalized the new accession of power
by some significant astronomical discovery: to
specify, one of Herschel's reflectors first revealed
the planet Uranus; Lord Rosse's "Leviathan" the
spiral nebulas ; the 15-inch Cambridge lens the crape,
or dusky ring of Saturn; the 18V2-i n ch Chicago
refractor the companion of Sirius ; the Washington
110 ASTRONOMY TO-DAY
26-inch telescope the satellites of Mars ; the 30-inch
Pulkowa glass the nebulosities of the Pleiades;
and the 36-inch Lick telescope brought to light a
fifth satellite of Jupiter. At the time these dis-
coveries were made, each of these great telescopes
was the only instrument then in existence with
power enough to have made the discovery possible.
So we may advance to still farther accessions of
power with the expectation that greater discoveries
will continue to gratify our confidence.
CHAPTER XX
THE STORY OF THE SPECTROSCOPE
SIR ISAAC NEWTON ought really to have been
the inventor of the spectroscope, because he
began by analyzing light in the rough with prisms,
was very expert in optics, and was certainly enough
of a philosopher to have laid the foundations of the
science.
What Newton did was to admit sunlight into a
darkened room through a small round aperture,
then pass the rays through a glass prism and re-
ceive the band of color on a screen. He noticed the
succession of colors correctly violet, indigo, blue,
green, yellow, orange, red; also that they were not
pure colors, but overlapping bands of color. Ap-
parently neither he nor any other experimenter for
more than a century went any further, when the
next essential step was taken by Wollaston about
1802 in England. He saw that by receiving the light
through a narrow slit instead of a round hole, he
got a purer spectrum, spectrum being the name
given to the succession of colors into which the
prism splits up or decomposes the original beam of
white sunlight. This seemingly insignificant change,
a narrow slit replacing the round hole, made Wol-
laston and not Newton the discoverer of the dark
lines crossing the spectrum at various irregular in-
tervals, and these singularly neglected lines meant
the basis of a new and most important science.
ill
112 ASTRONOMY TO-DAY
Even Wollaston, however, passed them by, and
it was Fraunhofer who in 1814-1815 first made a
chart of them. Consequently they are known as
Fraunhofer lines, or dark absorption lines. Send-
ing the beam of light through a succession of prisms
gives greater dispersion and increases the power of
the spectroscope. The greater the dispersion the
greater the number of absorption lines ; and it is the
number and intensity of these lines, with their ac-
curate position throughout the range of the spectrum
which becomes the basis of spectrum analysis.
The half century that saw the invention of the
steam engine, photography, the railroad and the
telegraph elapsed without any farther developments
than mere mapping of the fundamental lines, A, B,
C, D, E, F, G, H of the solar spectrum. The moon,
too, was examined and its spectrum found the same,
as was to be expected from sunlight simply reflected.
Sir John Herschel and other experimenters came
near guessing the significance of the dark lines, but
the problem of unraveling their mystery was finally
solved by Bunsen and Kirchhoff who ascertained
that an incandescent gas emits rays of exactly the
same degree of ref rangibility which it absorbs when
white light is passed through it. This great dis-
covery was at once received as the secure basis of
spectrum analysis, and Kirchhoff in 1858 put in
compact and comprehensive form the three follow-
ing principles underlying the theory of the science :
(1) Solid and liquid bodies, also gases under high
pressure, give when incandescent a continous
spectrum, that is one with a mere succession of
colors, and neither bright nor dark lines ;
(2) Gases under low pressure give a discontinuous
spectrum, crossed by bright lines whose number and
STORY OF THE SPECTROSCOPE 113
position in the spectrum differ according to the sub-
stances vaporized;
(3) When white light passes through a gas, this
medium absorbs or quenches rays of identical wave-
length with those composing its own bright-line
spectrum.
Clearly then it makes no difference where the light
originates whether it comes from sun or star. Only
it must be bright enough so that we can analyze it
with the spectroscope. But our analysis of sun and
star could not proceed until the chemist had
vaporized in the laboratory all the elements, and
charted their spectra with accuracy. When this had
been done, every substance became at once recogniz-
able by the number and position of its lines, with
practical certainty.
How then can we be sure of the chemical and
physical composition of sun and stars? Only by de-
tailed and critical comparison of their spectra with
the laboratory spectra of elements which chemical
and physical research have supplied. As in the sun,
so in the stars, each of which is encircled by a
gaseous absorptive layer or atmosphere, the light
rays from the self-luminous inner sphere must pass
through this reversing layer, which absorbs light of
exactly the same wave length as the lines that make
up its own bright line spectrum. Whatever sub-
stances are here found in gaseous condition, the same
will be evident by dark lines in the spectrum of sun
or star, and the position of these dark lines will show,
by coincidence with the position of the laboratory
bright lines, all the substances that are vaporized in
the atmospheres of the self -luminous bodies of the sky.
Here then originated the science of the new as-
tronomy: the old astronomy had concerned itself
114 ASTRONOMY TO-DAY
mainly with positions of the heavenly bodies, where
they are ; the new astronomy deals with their chemi-
cal composition and physical constitution, and what
they are. Between 1865 and 1875 the fundamental
application of the basic principles was well advanced
by the researches of Sir William Huggins in Eng-
land, of Father Angelo Secchi in Rome, of Jules
Janssen in Paris, and of Dr. Henry Draper in New
York.
In analyzing the spectrum of the sun, many
thousands of dark absorption lines are found, and
their coincidences with the bright lines of terrestrial
elements show that iron, for instance, is most prom-
inently identified, with rather more than 2,000 co-
incidences of bright and dark lines. Calcium, too,
is indicated by peculiar intensity of its lines, as well
as their great number. Next in order are hydrogen,
nickel and sodium. By prolonged and minute com-
parison of the solar spectrum with spectra of ter-
restrial elements, something like forty elemental
substances are now known to exist in the sun. Row-
land's splendid photographs of the solar spectrum
have contributed most effectively. About half of
these elements, though not in order of certainty, are
aluminum, cadmium, calcium, carbon, chromium,
cobalt, copper, hydrogen, iron, magnesium, man-
ganese, nickel, scandium, silicon, silver, sodium,
titanium, vanadium, yttrium, zinc, and zirconium.
Oxygen, too, is pretty surely indicated; but certain
elements abundant on earth, as nitrogen and chlo-
rine, together with gold, mercury, phosphorus, and
sulphur, are not found in the sun.
The two brilliant red stars, Aldebaran in Taurus,
and Betelgeuse in Orion, were the first stars whose
chemical constitution was revealed to the eye of man,
STORY OF THE SPECTROSCOPE 115
and Sir William Huggins of London was the astrono-
mer who achieved this epoch-making result. Father
Secchi of the Vatican Observatory proceeded at
once with the visual examination of the spectra of
hundreds of the brighter stars, and he was the first
to provide a classification of stellar spectra. There
were four types.
Secchi's type I is characterized chiefly by the
breadth and intensity of dark hydrogen lines, to-
gether with a faintness or entire absence of metallic
lines. These are bluish or white stars and they are
very abundant, nearly half of all the stars. Vega,
Altair, and numerous other bright stars belong to
this type, and especially Sirius, which gives to the
type the name "Sirians."
Type II is characterized by a multitude of fine
dark metallic lines, closely resembling the lines of
the solar spectrum. These stars are somewhat
yellowish in tinge like the sun, and from this
similarity of spectra they are called "solars."
Arcturus and Capella are "solars," and on the whole
the solars are rather less numerous than the Sirians.
Stars nearest to the solar system are mostly of this
type, and, according to Kapteyn of Groningen, the
absolute luminous power of first type stars exceeds
that of second type stars seven fold.
Secchi's type III is characterized by many dark
bands, well defined on the side toward the blue end
of the spectrum, but shading off toward the red
a "colonnaded spectrum", as Miss Clerke aptly terms
it. Alpha Herculis, Antares, and Mira, together
with orange and reddish stars and most of the
variable stars, belong in type III.
Type IV is also characterized by dark bands, often
called "flutings," similar to those of type III, but
116 ASTRONOMY TO-DAY
reversed as to shading, that is, well defined on the
side toward the red, but fading out toward the blue.
Their atmospheres contain carbon ; but they are not?
abundant, besides being faint and nearly all blood-
red in tint.
Following up the brilliant researches of Draper,
who in 1872 obtained the first successful photograph
of a star's spectrum, that of Vega, Pickering of
Harvard supplemented Secchi's classification by
Type V, a spectrum characterized by bright lines.
They, too, are not abundant and are all found near
the middle of the Galaxy. These are usually known
as Wolf-Rayet stars, from the two Paris astrono-
mers who first investigated their spectra. Type
V stars are a class of objects seemingly apart from,
the rest of the stellar universe, and many of the
planetary nebulaB yield the same sort of a spectrum.
The late Mrs. Anna Palmer Draper, widow of Dr.
Henry Draper, established the Henry Draper Me-
morial at Harvard, and investigation of the photo-
graphic spectra of all the brighter stars of the
entire heavens has been prosecuted on a compre-
hensive scale, those of the northern hemisphere at
Cambridge, and of the southern at Arequipa, Peru.
These researches have led to a broad reclassifica-
tion of the stars into eight distinct groups, a work
of exceptional magnitude begun by the late Mrs.
Fleming and recently completed by Miss Annie
Cannon, who classified the photographic spectra of
more than 230,000 stars on the new system, as fol-
lows :
The letters O, B, A, F, G, K, M, N represent a
continuous gradation in the supposed order of stel-
lar evolution, and farther subdivision is indicated by
tenths, G5K meaning a type half way between G and
STORY OF THE SPECTROSCOPE 117
K, and usually written G5 simply. B2 would indi-
cate a type between B and A, but nearer to B than
A, and so on. On this system, the spectrum of a
star in the earliest stages of its evolution is made
up of diffuse bright bands on a faint continuous
background. As these bands become fewer and
narrower, very faint absorption lines begin to ap-
pear, first the helium lines, followed by several series
of hydrogen lines. On the disappearance of the
bright bands, the spectrum becomes wholly ab-
sorptive bands and lines. Then comes a very great
increase in intensity of the true hydrogen spectrum,
with wide and much diffused lines, and few if any
other lines. Then the H and K calcium lines and
other lines peculiar to the sun become more and
more intense. Then the hydrogen lines go through
their long decline. The calcium spectrum becomes
intense, and later the spectrum becomes quite like
that of the sun with a great wealth of lines. Fol-
lowing this stage the spectrum shortens from the
ultra violet, the hydrogen lines fade out still farther,
and bands due to metallic compounds make their ap-
pearance, the entire spectrum finally resembling that
of sun spots. To designate these types rather more
categorically :
Type bright bands on a faint continuous back-
ground, with five subdivisions, Oa, Ob, Oc, Od, Oe,
according to the varying width and intensity of the
bands.
Type B the Orion type, or helium type, with ad-
ditional lines of origin unknown as yet, but without
any of the bright bands of type 0.
Type A the Sirian type, the regular Balmer
series of hydrogen lines being very intense, with
a few other lines not conspicuously marked.
118 ASTRONOMY TO-DAY
Type F the calcium type, hydrogen lines less
strongly marked, but with the narrow calcium lines
H and K very intense.
Type G the solar type, with multitudes of metal-
lic lines.
Type K in some respects similar to G, but with
the hydrogen lines fading out, and the metallic lines
relatively more prominent.
Type M spectrum with peculiar flutings due to
titanium oxide, with subdivisions Ma and Mb, and
the variable stars of long period, with a few bright
hydrogen lines additional, in a separate class Md.
Type N similar to M, in that both are pro-
nouncedly reddish, but with characteristic flutings
probably indicating carbon compounds.
The Draper classification being based on photo-
graphic spectra, and the original Secchi classifica-
tion being visual, the relation of the two systems is
approximately as follows :
Secchi Type I includes Draper B & A
II includes Draper F, G & K
III includes Draper M
IV includes Draper N
Pickering's marked success in organization and
execution of this great programme was due to his
adoption of the "slitless spectroscope," which made
it possible to photograph stellar spectra in vast
numbers on a single plate. The first observers of
stellar spectra placed the spectroscope beyond the
focus of the telescope with which it was used, there-
by limiting the examination to but one star at a
time. In the slitless spectroscope, a large prism is
mounted in front of the objective (of short focus),
so that the star's rays pass through it first, and then
are brought to the same focus on the photographic
STORY OF THE SPECTROSCOPE 119
plate, for all the stars within the field of view, some-
times many thousand in number. This arrange-
ment provides great advantages in the comparison
and classification of stellar spectra.
When spectroscopic methods were first intro-
duced into astronomy, there was no expectation that
the field of the old or so-called exact astronomy
would be invaded. Physicists were sometimes
jocularly greeted among astronomers as "ribbon
men," and no one even dreamed that their researches
were one day to advance to equal recognition with
results derived from micrometer, meridian circle,
and heliometer.
The first step in this direction was taken in 1868
by Sir William Huggins of London, who noticed
small displacements in the lines of spectra of very
bright stars. In fact the whole spectrum appeared
to be shifted; in the case of Sirius it was shifted
toward the red, while the whole spectrum of Arc-
turus was shifted by three times this amount toward
the violet end of the spectrum. The reason was not
difficult to assign.
As early as 1842 Doppler had enunciated the
principle that when we are approaching or are ap-
proached by a body which is emitting regular vibra-
tions, then the number of waves we receive in a
second is increased, and their wave-length corre-
spondingly diminished; and just the reverse of this
occurs when the distance of the vibrating body is
increasing. It is the same with light as with sound,
and everyone has noticed how the pitch of a loco-
motive whistle suddenly rises as it passes, and falls
as suddenly on retreating from us. So Huggins
drew the immediate inference that the distance be-
tween the earth and Sirius was increasing at the
120 ASTRONOMY TO-DAY
rate of nearly twenty miles per second, while Arc-
turus was nearing us with a velocity of sixty miles
per second.
These pioneer observations of motions in the line
of sight, or radial velocities as they are now called,
led directly to the acceptance of the high value of
spectroscopic work as an adjunct of exact astron-
omy in stellar research. Nor has it been found
wanting in application to a great variety of exact
problems in the solar system which would have been
wholly impossible to solve without it.
Foremost is the sun, of course, because of the
overplus of light. Young early measured the dis-
placement of lines in the spectra of the prominences,
and found velocities sometimes exceeding 250 miles
per second. Many astronomers, Duner among them,
investigated the rotation of the sun by the spectro-
scopic method. The sun's east limb is coming to-
ward us, while the west is going from us; and by
measuring the sum of the displacements, the rate of
rotation has been calculated, not only at the sun's
equator but at many solar latitudes also, both north
and south. As was to be expected, these results
agree well with the sun's rotation as found by the
transits of sun spots in the lower latitudes where
they make their appearance.
Belopolsky has applied the same method to the
rotation of the planet Venus, and Keeler, by measur-
ing the displacement of lines in the spectrum of
Saturn, on opposite sides of the ring, provided a
brilliant observational proof of the physical con-
stitution of the rings; because he showed that the
inner ring traveled round more swiftly than the
outer one, thus demonstrating that the ring could
not be solid, but must be composed of multitudes of
STORY OF THE SPECTROSCOPE 121
small particles traveling around the ball of Saturn,
much as if they were satellites. Indeed, Keeler as-
certained the velocity of their orbital motion and
found that in each case it agreed exactly with that
required by the Keplerian law.
Even the filmy corona of the sun was investigated
in similar fashion by Deslandres at the total eclipse
of 1893, and he found that it rotates bodily with the
sun. But the complete vindication of the spectro-
scopic method as an adjunct of the old astronomy
came with its application to measurement of the
distance of the sun. The method is very interest-
ing and was first suggested by Campbell in 1892.
Spectrum-line measurements have become very ac-
curate with the introduction of dry-plate photo-
graphy, and ecliptic stars were spectrographed, to-
ward and from which the earth is traveling by its
orbital motion round the sun. By accurate measure-
ment of these displacements, the orbital velocity of
the earth is calculated; and as we know the exact
length of the year, or a complete period, the length
of the orbit itself in miles becomes known, and thus,
by simple mensuration, the length of the radius of
the orbit which is the distance of the sun.
If we pass from sun to star, the triumph of the
spectroscope has been everywhere complete and sig-
nificant. As the spectroscopic survey of the stars
grew toward completeness, it became evident that
the swarming hosts of the stellar universe are in
constant motion through space, not only athwart
the line of vision as their proper motions had long
disclosed, but some stars are swiftly moving toward
our solar system and others as swiftly from it.
Fixed stars, strictly speaking there are no such.
All are in relative motion. Exact astronomy by dis-
122 ASTRONOMY TO-DAY
cussion of the proper motions had assigned a region
of the sky toward which the sun and planets are
moving. Spectrography soon verified this direc-
tion not only, but gave a determination of the ve-
locity of our motion of twelve miles per second in
a direction approximately that of the constellation
Lyra. From corresponding radial velocities, we
draw the ready conclusion that certain groups or
clusters of stars are actually connected in space and
moving as related systems, as in the Pleiades and
Ursa Major.
Rather more than a quarter century ago, the
spectroscope came to the assistance of the telescope
in helping to solve the intricate problem of stellar
distribution. Kapteyn, by combining the proper
motions of certain stars with their classification in
the Draper catalogue of stellar spectra, drew the
conclusion that, as stars having very small proper
motions show a condensation toward the Galaxy, the
stars composing this girdle are mostly of the Sirian
type, and are at vast distances from the solar sys-
tem. The proper motion of a star near to us will
ordinarily be large, and, in the case of solar stars,
the larger their proper motion the greater their
number. So it would appear that the solar stars
are aggregated round the sun himself, and this con-
clusion is greatly strengthened by the fact that of
stars whose distances and spectral type are both
ascertained, seven of the eight nearest to us are
solar stars.
In 1889 the spectroscope achieved an unexpected
triumph by enabling the late Professor Pickering to
make the first discovery of a spectroscopic double, or
binary star, a type of object now quite abundant.
Unlike the visual binary systems whose periods are
STORY OF THE SPECTROSCOPE 123
years in length, the spectroscopic binaries have short
periods, reckoned in some cases in days, or hours
even. If the orbit of a very close binary is seen
edge on, the light of the two stars will coalesce twice
in every revolution. Halfway between these points
there are two times when the two stars will be mov-
ing, one toward the earth and the other from it. At
all times the light of the star, in so far as the tele-
scope shows it, proceeds from a single object.
Now photograph the star's spectrum at each of
the four critical points above indicated : in the first
pair the lines are sharply defined and single, because
at conjunction the stars are simply moving athwart
the line of sight, while at the intermediate points
the lines are double. Doppler's principle completely
accounts for this : the light from the receding com-
panion is giving lines displaced toward the red, while
the approaching companion yields lines displaced
toward the violet. Mizar, the double star at the
bend of the handle in the Great Dipper was the first
star to yield this peculiar type of spectrum, and the
period of its invisible companion is about 52 days.
The relative velocity of the components is 100 miles
a second, and applying Newton's law we find its
mass exceeds that of the sun f ortyf old. Capella has
been found to be a spectroscopic binary; also the
pole star. Spectroscopic binaries have relatively
short periods, one of the shortest known being only
35 hours in length. It is in the constellation Scorpio.
Beta Aurigse is another whose lines double on
alternate nights, giving a period of four days ; and
the combined mass of both stars is more than twice
that of the sun. The catalogue of spectroscopic
binaries is constantly enlarging; but thousands
doubtless exist that can never be discovered by this
124 ASTRONOMY TO-DAY
method, as is evident if their orbits are perpendicu-
lar to the line of sight or nearly so. The history
of the spectroscopic binaries is one of the most in-
teresting chapters in astronomy, and affords a mar-
velous confirmation of the prediction of Bessel who
first wrote of "the astronomy of the invisible."
Find a star's distance by the spectroscope? Im-
possible, everyone would have said, even a very few
years ago. Now, however, the thing is done, and
with increasing accuracy.
Adams of Mount Wilson has found, after pro-
tracted investigation, that the relative intensity of
certain spectral lines varies according to the absolute
brightness of a star; indeed, so close is the cor-
respondence that the spectroscopic observations are
employed to provide in certain cases a good determi-
nation of the absolute magnitude, and therefore of
the distance. To test this relation, the spectroscopic
parallaxes have been compared with the measured
parallaxes in numerous instances, and an excellent
agreement is shown. This new method is adding
extensively to our knowledge of stellar luminosities
and distances, and even the vast distances of globu-
lar clusters and spiral nebulae are becoming known.
In fact, but few departments of the old astronomy
are left which the new astronomy has not invaded,
and this latest triumph of the spectroscope in de-
termining accurately the distances of even the re-
motest stars is enthusiastically welcomed by ad-
vocates of the old and new astronomy alike.
CHAPTER XXI
THE STORY OF ASTRONOMICAL
PHOTOGRAPHY
THE most powerful ally of both telescope and spec-
troscope is photography. Without it the mar-
velous researches carried on with both these types
of instrument would have been essentially impos-
sible. Even the great telescopes of Herschel and
Lord Rosse, notwithstanding their splendid record
as optical instruments, might have achieved vastly
more had photography been developed in their time
to the point where the astronomer could have em-
ployed its wonderful capabilities as he does to-day.
And, with the spectroscope, it is hardly too much to
say that no investigator ever observes visually with
that instrument any more: practically every spec-
trum is made a matter of photographic record first.
The observing, or nowadays the measuring, is all
done afterward.
All telescopes and cameras are alike, in that each
must form or have formed within it an image by
means of a lens or mirror. In the telescope the eye
sees the fleeting image, in the camera the 'process of
registering the image on a plate or film is known as
photography. Daguerre first invented the process
(silver film on a copper plate) in 1839. The year
following it was first employed on the moon, in 1850
the first star was photographed, in 1851 the first
total eclipse of the sun ; all by the primitive daguer-
125
126 ASTRONOMY TO-DAY
reotype process, which, notwithstanding its awk-
wardness and the great length of exposure required,
was found to possess many advantages for astro-
nomical work.
About the middle of the last century the wet plate
process, so called because the sensitized collodion
film must be kept moist during exposure, came into
general use, and the astronomers of that period were
not slow to avail themselves of the advantages of
a more sensitive process, which in 1872, in the
skillful hands of Henry Draper, produced the first
spectrum of a star. In 1880 a nebula was first
photographed, and in 1881 a comet.
Before this time, however, the new dry-plate proc-
ess had been developed to the point where astrono-
mers began to avail of its greater convenience and
increased sensitiveness, even in spite of the coarse-
ness of grain of the film. Forty years of dry-plate
service have brought a wealth of advantages scarcely
dreamed of in the beginning, and nearly every de-
partment of astronomical research has been en-
hanced thereby, while many entirely new photo-
graphic methods of investigation have been worked
out.
Continued improvement in photographic proc-
esses has provided the possibility of pictures of
fainter and fainter celestial objects, and all the
larger telescopes have photographed stars and
nebulae of such exceeding faintness that the human
eye, even if applied to the same instrument, would
never be able to see them. This is because the eye,
in ten or twelve seconds of keen watching, becomes
fatigued and must be rested, whereas the action of
very faint light rays is cumulative on the highly
sensitive film ; so that a continuous exposure of many
ASTRONOMICAL PHOTOGRAPHY 127
hours' duration becomes readily visible to the eye
on development. So a supersensitive dry plate will
often record many thousand stars in a region where
the naked eye can see but one.
Perhaps the greatest amplification of photography
has taken place at the Harvard Observatory under
Pickering, where a library of many hundred thou-
sand plates has accumulated ; and at Groningen, Hol-
land, where Kapteyn has established an astronomi-
cal laboratory without instruments except such as
are necessary to measure photographic plates, when-
ever and wherever taken. So it is possible to
select the clearest of skies, all over the world, for
exposure of the plates, and bring back the photo-
graphs for expert discussion.
Of course the sun was the celestial body first
photographed, and its surpassing brilliance neces-
sitates reduction of exposure to a minimum. In
moments of exceptional steadiness of the atmos-
phere, a very high degree of magnification of the
solar surface on the photographic plate is permitted,
and the details in formation, development, and end-
ing of sun spots are faithfully registered. Neverthe-
less, it cannot be said that photography has yet
entirely replaced the eye in this work, and careful
drawings of sun spots at critical stages in their life
are capable of registering fine detail which the plate
has so far been unable to record. Janssen of Paris
took photographs of the solar photosphere so highly
magnified that the granulation or willow-leaf struc-
ture of the surface was clearly visible, and its varia-
tions traceable from hour to hour.
The advantages of sun spot photography in as-
certaining the sun's rotation, keeping count of the
spots, and in a permanent record for measurement
128 ASTRONOMY TO-DAY
of position of the sun's axis and the spot zones, are
obvious. In direct portrayal of the sun's corona
during total eclipses, photography has offered su-
perior advantages over visual sketching, in the form
and exact location of the coronal streamers ; but the
extraordinary differences of intensity between the
inner corona and its outlying extensions are such
that halation renders a complete picture on a single
plate practically impossible. The filamentous detail
of the inner corona, and the faintest outlying ex-
tensions or streamers, the eye must still reveal di-
rectly.
In solar spectrum photography, research has been
especially benefitted; indeed, exact registry of the
multitudinous lines was quite impossible without it.
Photographic maps of the spectrum by Thollon,
McClean and Rowland are so complete and accurate
that no visual charts can approach them. Rowland's
great photographic map of the solar spectrum
spread out into a band about forty feet in length;
and in the infra-red, Langley's spectrobolometer ex-
tended the invisible heat spectrum photographically
to many times that length. At the other end of the
spectrum, special photographic processes have ex-
tended the ultra-violet spectrum far beyond the
ocular limit, to a point where it is abruptly cut off by
absorption of the earth's atmosphere. On the same
plate with certain regions of the sun's spectrum, the
spectra of terrestrial metals are photographed side
by side, and exact coincidences of lines show that
about forty elemental substances known to terres-
trial chemistry are vaporized in the sun.
Young was the first to photograph a solar prom-
inence in 1870, and twenty years later Deslandres
of Paris and Hale of Chicago independently in-
A VIEW OF THE 100-FOOT DOME IN WHICH THE LARGEST TELESCOPE
IN THE WORLD is HOUSED. (Courtesy, Mt. Wilson Solar Observatory.)
MOUNT CHIMBORAZO, NEAR THE EQUATOR. An observatory located on
this mountain would make it possible to study the phenomena of
northern and southern skies from the same point. (Courtesy, Pan-
American Union.)
LICK OBSERVATORY, ON THE SUMMIT OF MT. HAMILTON, ABOUT
TWENTY-FIVE MILES S. W. OF SAN JOSE, CALIFORNIA. It contains
the famous Lick telescope, a 36-inch refractor.
NKAR VIEW OF THE EYE-END OF THE YERKES TELESCOPE. The eye-
piece is removed and its place taken by a photographic plate.
ASTRONOMICAL PHOTOGRAPHY 129
vented the spectroheliograph, by which the chromo-
sphere and prominences of the sun, as well as the
disk of the sun itself, are all photographed by mono-
chromatic light on a single plate. Hale has de-
veloped this instrument almost to the limit, first
at the Yerkes Observatory of the University of
Chicago, and more recently at the Mount Wilson
Observatory of the Carnegie Institution, where spec-
troheliograms of marvelous perfection are daily
taken. It was with this instrument that Hale dis-
covered the effect of an electromagnetic field in sun
spots which has revolutionized solar theories, a re-
search impossible to conceive of without the aid of
photography.
When we apply Doppler's principle, photography
becomes doubly advantageous, whether we deter-
mine, as Duner did and more recently Adams, the
sun's own rotation and find it to vary in different
solar latitudes, the equator going fastest; or apply
the method to the sun's corona at the east and west
limbs of the sun, which Deslandres in 1893
proved to be rotating bodily with the sun, because
of the measured displacement of spectral lines
of the corona in juxtaposition on the photo-
graphic plate. *
In the solar astronomy of measurement, too, pho-
tography has been helpfully utilized, as in register-
ing the transits of Mercury over the sun's disk, for
correcting the tables of the planet's orbital motion;
and most prominently in the action taken by the
principal governments of the world in sending out
expeditions to observe the transits of Venus in 1874
and 1882, for the purpose of determining the paral-
lax of Venus and so the distance of the earth from
the sun.
Scl. Vol. 25
130 ASTRONOMY TO-DAY
In our studies of the moon, photography has al-
most completely superseded ocular work during the
past sixty years. Rutherfurd and Draper of New
York about 1865 obtained very excellent lunar pho-
tographs with wet plates, which were unexcelled for
nearly half a century. The Harvard, Lick, and Paris
Observatories have published pretty complete photo-
graphic atlases of the moon, and the best negatives
of these series show nearly everything that the eye
can discern, except under unusual circumstances.
Later lunar photography was taken up at the Yerkes
Observatory, and exceptionally fine photographs on
a large scale were obtained with the 40-inch re-
fractor, using a color screen. More recently the
60-inch and 100-inch mirrors of the Mount Wilson
Observatory have taken a series of photographs of
the moon far surpassing everything previously done,
as was to be expected from the unique combination
of a tranquil mountain atmosphere with the ex-
traordinary optical power of the instruments, and a
special adaptation of photographic methods. Dur-
ing lunar eclipses, Pickering has made a photo-
graphic search for a possible satellite of the moon,
occultations of stars by the moon have been recorded
by photography, and Russell of Princeton has shown
how the position of the moon among the stars can
be determined by the aid of photography with a high
order of precision.
The story of planetary photography is on the
whole disappointing. Much has been done, but there
is much that is within reach, or ought to be, that
remains undone. From Mercury nothing ought per-
haps to be expected. On many of the photographs
of the transit of Venus, especially those taken under
the writer's direction at the Lick Observatory in
ASTRONOMICAL PHOTOGRAPHY 131
1882, we have unmistakable evidence of the planet's
atmosphere. Here again the wet plate process, al-
though more clumsy, demonstrated its superiority
over the dry process used by other expeditions.
In spectroscopy, Belopolsky has sought to deter-
mine the period of rotation of Venus on her axis. At
the Lowell Observatory, Douglass succeeded in pho-
tographing the faint zodiacal light, and very success-
ful photographs of Mars were taken at this institu-
tion as early as 1905 by Slipher. Two years later
these were much improved upon by the writer's ex-
pedition to the Andes of Chile, when 12,000 expos-
ures of Mars were made, many of them showing the
principal canali, and other prominent features of
the planet's disk. At subsequent oppositions of the
planet, Barnard at the Yerkes Observatory and the
Mount Wilson observers have far surpassed all these
photographs.
For future oppositions a more sensitive film is
highly desired, in connection with instruments pos-
sessing greater light-gathering power, so permitting
a briefer exposure that will be less influenced by ir-
regularities and defects of the atmosphere. The
spectrum of Mars is of course that of sunlight, very
much reduced, and modified to a slight extent by its
passing twice through the atmosphere of Mars.
What amount of aqueous vapor that atmosphere may
contain is a question that can be answered only by
critical comparison of the Martian spectrum with
the spectrum of the moon, and photography affords
the only method by which this can be done.
Many are the ways in which photography has
aided research on the asteroid group. Since 1891
more than 600 of them have been discovered by pho-
tography, and it is many times easier to find the
132 ASTRONOMY TO-DAY
new object on the photographic plate than to detect
it in the sky as was formerly done by means of star
charts. The planet by its motion during the ex-
posure of the plate produces a trail, whereas the sur-
rounding stars are all round dots or images. Or by
moving the plate slightly during exposure, as in
Metcalf's ingenious method, we may catch the
planet at that point where it will give a nearly
circular image, and thus be quite as easy to detect,
because all the stars on the same plate will then
be trails.
Photographic photometry of the asteroids has re-
vealed marked variations in their light, due perhaps
to irregularities of figure. On account of their faint
light, the asteroids are especially suited, as Mars is
not, to exact photography for ascertaining their
parallax, and from this the sun's distance when the
asteroid's distance has been found. Many asteroids
have been utilized in this way, in particular Eros
(433). In 1931 it approaches the earth within
13 million miles, when the photographic method
will doubtless give the sun's distance with the ut-
most accuracy.
Photographs of Jupiter have been very success-
fully taken at the Yerkes and Lowell Observatories
and elsewhere, but the great depth of the planet's
atmosphere is highly absorptive, so that the im-
pression is very weak in the neighborhood of the
limb, if the exposure is correctly timed for the cen-
ter of the disk. The striking detail of the belts, how-
ever, is excellently shown. Wood of Baltimore has
obtained excellent results by monochromatic photog-
raphy of Jupiter and Saturn with the 60-inch re-
flector on Mount Wilson. Jupiter's satellites have
not been neglected photographically, and Pickering
ASTRONOMICAL PHOTOGRAPHY 133
has observed hundreds of the eclipses of the satel-
lites by a sort of cinematographic method of re-
peated exposures, around the time of disappearance
and reappearance by eclipse. The newest outer
satellites of Jupiter were all discovered by photog-
raphy, and it is extremely doubtful if they would
have been found otherwise.
Saturn has long been a favorite object with the
astronomical photographer, and there are many
fine pictures in spite of his yellowish light, rela-
tively weak photographically. The marvelous ring
system with the Cassini division, the oblateness of
the ball, the occasional markings on it all are well
shown in the best photographs ; but the call is for
more light and a more sensitive photographic proc-
ess. Pickering's ninth satellite (Phoebe) was dis-
covered by photography, one of the faintest moons
in the solar system. Like the faint outer moons
of Jupiter, few existing telescopes are powerful
enough to show it. Its orbit has been found from
photographic observations, and its position is
checked up from time to time by photography.
But the crowning achievement of spectrum pho-
tography in the Saturnian system is Keeler's appli-
cation of Doppler's principle in determining the
rate of orbital motion of particles in different zones
of the rings, thereby establishing the Maxwellian
theory of the constitution of the rings beyond the
possibility of doubt. For Uranus and Neptune
photography has availed but little, except to nega-
tive the existence of additional satellites of these
planets, which doubtless would have been discovered
by the thorough photographic search which has
been made for them by W. H. Pickering without
result.
134 ASTRONOMY TO-DAY
As with the asteroids, so with comets : several of
these bodies have been discovered by photography;
none more spectacular than the Egyptian comet of
May 17th, 1882, which impressed itself on the plates
of the corona of that date. Withdrawal of the sun's
light by total eclipse made the comet visible, and it
had never been seen before, nor is it known whether
it will ever return. In cometary photography, much
the same difficulties are present as in photographing
the corona: if the plate is exposed long enough to
get the faint extensions of the tail, the fine filaments
of the coma or head are obliterated by halation and
overexposure.
No one has had greater success in this work than
Barnard, whose photographs of comets, particu-
larly at the Lick Observatory, are numerous and un-
excelled. His photographs of the Brooks Comet of
1893 revealed rapid and violent changes in the tail,
as if shattered by encounter with meteors ; and the
tail of Halley's comet in 1910 showed the rapid pro-
pagation of luminous waves down the tail, similar to
phenomena sometimes seen in streamers of the
aurora. Draper obtained the first photograph of a
comet's spectrum in 1881, disclosing an identity
with hydrocarbons burning in a Bunsen flame, also
bands in the violet due to carbon compounds. The
photographic spectra of subsequent comets have
shown bright lines due to sodium and the vapor of
iron and magnesium.
Even the elusive meteor has been caught by
photography, first by Wolf in 1891, who was ex-
posing a plate on stars in the Milky Way. On de-
veloping it, he found a fine, dark nearly uniform
line crossing it, due to the accidental flight across
the field of a meteor of varying brightness. Since
ASTRONOMICAL PHOTOGRAPHY 135
then meteor trails have been repeatedly photo-
graphed, and even the trail spectra of meteors have
been registered on the Harvard plates. At Yale
in 1894 Elkin employed a unique apparatus for
securing photographic trails of meteors : six photo-
graphic cameras mounted at different angles on a
long polar axis driven by clockwork, the whole
arranged so as to cover a large area of the sky where
meteors were expected.
When we pass from the solar system to the stellar
universe the advantages of photography and the am-
plification of research due to its employment as
accessory in nearly every line of investigation are
enormous. So extensively has photography been
introduced that plates, and to a slight extent films,
are now almost exclusively used in securing original
records. Regrettably so in case of the nebulae, be-
cause the numerous photographs of the brighter
nebulas taken since 1880 when Draper got the first
photograph of the nebula of Orion, are as a rule not
comparable with each other. Differences of instru-
ments, of plates, of exposure, and development all
have occasioned differences in portrayal of a nebula
which do not exist. When we consider faithful
accuracy of portrayal of the nebulae for purposes of
critical comparison from age to age, many of our
nebular photographs of the past forty years, fine
as they are and marvelous as they are, must fail
to serve the purpose of revealing progressive
changes in nebular features in the future.
Roberts and Common in England were among the
first to obtain nebular photographs with extraordi-
nary detail, also the brothers Henry of Paris. As
early as 1888 Roberts revealed the true nature of
the great nebula in Andromeda, which had never
136 ASTRONOMY TO-DAY
been suspected of being spiral; and Keeler and
Perrine at the Lick Observatory pushed the photo-
graphic discovery of spiral nebulse so far that their
estimates fill the sky with many hundred thousands
of these objects.
In the southern hemisphere the 24-inch Bruce
telescope of Harvard College Observatory has
obtained many very remarkable photographs of
nebulae, particularly in the vicinity of Eta Carinse.
But the great reflectors of the Mount Wilson Observ-
atory, on account of their exceptional location and
extraordinary power, have surpassed all others in
the photographic portrayal of these objects, especi-
ally of the spiral nebulas which appear to show all
stages in transition from nebula to star. No less
remarkable are the photographs of such wonderful
clusters as Omega Centauri, a perfect visual repre-
sentation of which is wholly impossible. Intercom-
parison of the photographs of clusters has afforded
Bailey of Harvard, Shapley of Mount Wilson and
others the opportunity of discovery that hundreds
of the component stars are variable.
What is the longest photographic exposure ever
made? At the Cape of Good Hope, under the direc-
tion of the late Sir David Gill, exposures on nebulae
were made, utilizing the best part of several nights,
and totaling as high as seventeen, or even twenty-
three hours. But the Mount Wilson observers have
far surpassed this duration. To study the rotation
and radial velocity of the central part of the nebula
of Andromeda, an exposure of no less than 79 hours'
total duration was made on the exceedingly faint
spectrum, and even that record has since been
exceeded. The eye cannot be removed from the
guiding star for a moment while the exposure
ASTRONOMICAL PHOTOGRAPHY 137
is in progress, and this tedious piece of work was
rewarded by determining the velocity of the center
of the nucleus as a motion of approach at the rate
of 316 kilometers per second.
But when the stars, their magnitudes and their
special peculiarities are to be investigated en masse,
photography provides the facile means for re-
searches that would scarcely have been dreamed of
without it. The international photographic chart of
the entire heavens, in progress at twenty observa-
tories since 1887, the photographic charts of the
northern heavens at Harvard and of the southern sky
at Cape Town, the manifold investigations that have
led up to the Harvard photometry, and the unpar-
alleled photographic researches of the Henry Draper
Memorial, enabling the spectra of many hundred
thousand stars to be examined and classified all
this is but a part of the astronomical work in stellar
fields that photography has rendered possible.
Then there are the stellar parallaxes, now ob-
served for many stars at once photographically,
when formerly only one star's parallax could be
measured at a time and with the eye at the telescope.
And photo-electric photometry, measuring smaller
differences of light than any other method, and pro-
viding more accurate light-curves of the variable
stars. And perhaps most remarkable of all, the
radial velocity work on both stars and nebulae, giv-
ing us the distance of whole classes of stars, dis-
covering large numbers of spectroscopic binaries
and checking up the motion of the solar system
toward Lyra within a fraction of a mile per
second.
All told, photography has been the most potent
adjunct in astronomical research, and it is impos-
138 ASTRONOMY TO-DAY
sible to predict the future with more powerful
apparatus and photographic processes of higher
sensitiveness. The field of research is almost
boundless, and the possibilities practically without
limit.
What would Herschel have done with 100,000
and photography!
CHAPTER XXII
MOUNTAIN OBSERVATORIES
THE century that has elapsed since the time of
Sir William Herschel, known as the father of
the new or descriptive astronomy, has witnessed all
the advances of the science that have been made
possible by adopting the photographic method of
making the record, instead of depending upon the
human eye. Only one eye can be looking at the
eyepiece at a time: the photograph can be studied
by a thousand eyes.
At mountain elevations telescopes are now ex-
tensively employed, and there the camera is of
especial and additional value, because the photo-
graph taken on the mountain can be brought down
for the expert to study, at ease and in the comfort
of a lower elevation. We shall next trace the move-
ment that has led the astronomer to seek the sum-
mits of mountains for his observatories, and the
photographer to follow him.
Not only did the genius of Newton discover the
law of universal gravitation, and make the first ex-
periments in optics essential to the invention of the
spectroscope, but he was the real originator also of
the modern movement for the occupation of moun-
tain elevations for astronomical observatories. His
keen mind followed a ray of light all the way from
its celestial source to the eye of the observer, and
analyzed the causes of indistinct and imperfect
vision.
139
140 ASTRONOMY TO-DAY
Endeavoring to improve on the telescope as
Galileo and his followers had left it, he found such
inherent difficulties in glass itself that he abandoned
the refracting type of telescope for the reflector, to
the construction of which he devoted many years.
But he soon found out, what every astronomer and
optician knew to their keen regret, that a telescope,
no matter how perfectly the skill of the optician's
hand may make it, cannot perform perfectly unless
it has an optically perfect atmosphere to look
through.
So Newton conceived the idea of a mountain
observatory, on the summit of which, as he thought,
the air would be not only cloudless, but so steady
and equable that the rays of light from the heavenly
bodies might reach the eye undisturbed by atmos-
pheric tremors and quiverings which are almost
always present in the lower strata of the great
ocean of air that surrounds our planet.
This is the way Newton puts the question in his
treatise on Opticks he says : "The Air through
which we look upon the Stars, is in a perpetual
Tremor; as may be seen by the tremulous Motion
of Shadows cast from high Towers, and by the
twinkling of the fix'd stars The only remedy
is a most serene and quiet Air, such as may perhaps
be found on the tops of the highest Mountains above
the grosser Clouds."
Newton's suggestion is that the highest moun-
tains may afford the best conditions for tranquillity ;
and it is an interesting coincidence that the summits
of the highest mountains, about 30,000 feet in eleva-
tion, are at about the same level where the turbu-
lence of the atmosphere most likely ceases, according
to the indications of recent meteorological research.
MOUNTAIN OBSERVATORIES 141
These heights are far above any elevations per-
manently occupied as yet, but a good beginning has
been made and results of great value have already
been reached.
Curiously, investigation of mountain peaks and
their suitability for this purpose was not under-
taken till nearly two centuries after Newton, when
Piazzi Smyth in 1856 organized his expedition to
the summit of a mountain of quite moderate eleva-
tion, and published his "Teneriffe: an Astronomer's
Experiment." Teneriffe is an accessible peak of
about 10,000 feet, on an island of the Canaries off
the African coast, where Smyth fancied that condi-
tions of equability would exist; and on reaching
the summit with his apparatus and spending a few
days and nights there, he was not disappointed.
Could he have reached an elevation of 13,000 feet,
he would have had fully one-third of all the atmos-
phere in weight below him, and that the most tur-
bulent portion of all. Nevertheless, the gain in
steadiness of the atmosphere, providing "better see-
ing," as the astronomer's expression is, even at
10,000 feet, was most encouraging, and led to at-
tempts on other peaks by other astronomers, a few
of whom we shall mention.
Davidson, an observer of the United States Coast
Survey, with a broad experience of many years in
mountain observing, investigated the summit of the
Sierra Nevada mountains as early as 1872, at an
elevation of 7,200 feet. His especial object was to
make an accurate comparison between elevated sta-
tions at different heights. He found the seeing
excellent, especially on the sun; but the excessive
snowfall at his station, 45 feet annually, was a con-
dition very adverse to permanent occupation.
142 ASTRONOMY TO-DAY
In the summer of 1872, Young spent several
weeks at Sherman, Wyoming, at an elevation ex-
ceeding 8,300 feet. He carried with him the 9.4-
inch telescope of Dartmouth College, where he was
then professor, and this was the first expedition on
which a large glass was used by a very skillful
observer at great elevation. He found the number
of good days and nights small, but the sky was ex-
ceedingly favorable when clear. Many 7th magni-
tude stars could be detected with the naked eye.
Young's observations at Sherman were mainly
spectroscopic, however, and they demonstrated the
immense advantage of a high-level station, far above
the dust and haze of the lower atmosphere. He
pronounced the 9.4-inch glass at 8,000 feet the full
equivalent of a 12-inch at sea level.
Mont Blanc of 15,000 feet elevation was another
summit where the veteran Janssen of Paris main-
tained a station for many years ; but the continental
conditions of atmospheric moisture and circulation
were not favorable on the whole. Janssen was
mainly interested in the sun, and the daylight seeing
is rarely benefited, owing to the strong upward
currents of warm air set in motion by the sun itself.
Mountains in the beautiful climate of California
were among the earliest investigated, and when in
1874 the trustees of Mr. James Lick's estate were
charged with equipping an observatory with the
most powerful telescope in existence, they wisely
located on the summit of Mount Hamilton. It is
4,300 feet above sea level, and Burnham and other
astronomers made critical tests of the steadiness of
vision there by observing double stars, which afford
perhaps the best means of comparing the optical
quality of the atmosphere of one region with an-
MOUNTAIN OBSERVATORIES 143
other. The writer was fortunate in having charge
of the observations of the transit of Venus in 1882
on the mountain, when the Observatory was in
process of construction, and the quality of the photo-
graphs obtained on that occasion demonstrated anew
the excellence of the site. Particularly at night, for
about nine months of the year, the seeing is ex-
ceptionally good, especially when fog banks rolling
in from the Pacific, cover the valleys below like a
blanket, preventing harmful radiation from the soil
below.
The great telescope mounted in 1888, a 86-inch
refractor by Alvan Clark, has fulfilled every ex-
pectation of its projectors, and justified the selection
of the site in every particular. The elevation, al-
though moderate, is still high enough to secure very
marked advantage in clearness and steadiness of
the air, and at the same time not so high that the
health and activities of the observers are appreci-
ably affected by the thinner air of the summit. This
telescope is known the world over for the monu-
mental contributions to science made by the able
astronomers who have worked with it : among them
Barnard who discovered the fifth satellite of Jupiter
in 1892; Burnham, Hussey, and Aitken, who have
discovered and measured thousands of close double
stars ; Keeler, who spent many faithful years on the
summit; and Campbell, the present director, whose
spectroscopic researches on stellar movements have
added greatly to our knowledge of the structure of
the universe. Among the many lines of research
now in progress at the Lick Observatory and in the
D. 0. Mills Observatory at Santiago, Chile, are the
discoveries of stars whose velocities in space are not
constant, but variable with the spectral type of the
144 ASTRONOMY TO-DAY
star. Mr. Lick's bequest for the Observatory was
about $700,000. So ably has this scientific trust
been administered that he might well have endowed
it with his entire estate, exceeding $4,000,000.
Another California mountain that was early in-
vestigated is Mount Whitney. Its summit eleva-
tion is nearly 15,000 feet, and in 1881 Langley made
its ascent for the purpose of measuring the solar
constant. He found conditions much more favor-
able than on Mount Etna, Sicily elevation about
10,000 feet which he had visited the year before.
But the height of Mount Whitney was such as to
occasion him much inconvenience from mountain
sickness, an ailment which is most distressing and
due partly to lack of oxygen and partly to mere
diminution of mechanical pressure. Mount Whitney
was also visited many years after by Campbell for
investigating the spectrum of Mars in comparison
with that of the moon. Langley found on Mount
Whitney an excellent station lower down, at about
12,000 feet elevation ; and by equipping the two sta-
tions with like apparatus for measuring the solar
heat, he obtained very important data on the selec-
tive absorption of the atmosphere.
Returning from the transit of Venus in 1882,
Copeland of Edinburgh visited several sites in the
Andes -of Peru, ascending on the railway from Mol-
lendo. Vincocaya was one of the highest, something
over 14,000 feet elevation. His report was most
enthusiastic, not only as to clearness and transpar-
ency of the atmosphere, but also as to its steadiness,
which for planetary and double star observations
is almost as important. Copeland's investigation
of this region of the Andes has led many other
astronomers to make critical tests in the same
MOUNTAIN OBSERVATORIES 145
general region. Climatic conditions are particu-
larly favorable, and the sites for high-level research
are among the best known, the atmosphere being
not only clear a large part of the year, but in certain
favored spots exceedingly steady.
In 1887 the writer ascended the summit of
Fujiyama, Japan, 12,400 feet elevation. The early
September conditions as to steadiness of atmosphere
were extraordinarily fine, but the mountain is
covered by cloud many months in each year. There
is a saddle on the inside of the crater that would
form an ideal location for a high-level observatory.
This expedition was undertaken at the request of the
late Professor Pickering, director of Harvard Col-
lege Observatory, which had recently received a be-
quest from Uriah A. Boyden, amounting to nearly a
quarter of a million dollars, to "establish and main-
tain, in conjunction with others, an astronomical
observatory on some mountain peak."
Great elevations were systematically investigated
in Colorado and California, the Chilean desert of
Atacama was visited, and a temporary station es-
tablished at Chosica, Peru, elevation about 5,000
feet. Atmospheric conditions becoming unfavorable,
a permanent station was established in 1891 at
Arequipa, Peru, elevation 8,000 feet, which has been
maintained as an annex to the Harvard Observ-
atory ever since. The cloud conditions have been
on the whole less favorable than was expected, but
the steadiness of the air has been very satisfactory.
In addition to planetary researches conducted there
in the earlier years by W. H. Pickering, many large
programs of stellar research have been executed,
especially relating to the magnitudes and spectra of
the stars. In conjunction with the home pbserva-
146 ASTRONOMY TO-DAY
tory in the northern hemisphere, this afforded a
vast advantage in embracing all the stars of the
entire heavens, on a scale not attempted elsewhere,
The Bruce photographic telescope of 24-inch aper-
ture has been employed for many years at Arequipa,
and with it the plates were taken which enabled
Pickering to discover the ninth satellite of Saturn
(Phoebe) , and the splendid photographs of southern
globular clusters in which Bailey has found
numerous variable stars of very short periods
very faint objects, but none the less interesting, and
of much significance in modern study of the evolu-
tion and structure of the stellar universe. The
crowning research of the observatory is the Henry
Draper catalogue of stellar spectra, now in process
of publication, which is of the first order of impor-
tance in statistical studies of stellar distribution
with reference to spectral type, and in studying the
relation of parallax and distance, proper motion,
radial velocity and its variation to the spectral
characteristics of the stars.
Perrine of Cordova is now establishing on Sierra
Chica about twenty-five miles southwest of Cor-
dova, a great reflecting telescope comparable in size
with the instruments of the northern hemisphere,
for investigation of the southern nebulae and
clusters, and motions of the stars. The elevation
of this new Argentine observatory will be 4,000 feet
above sea level.
Another observatory at mountain elevation and
in a highly favorable climate is the Lowell Obser-
vatory, located at about 7,000 feet elevation at Flag-
staff, Arizona. Many localities were visited and
the atmosphere tested especially for steadiness, an
optical quality very essential for research on the
MOUNTAIN OBSERVATORIES 147
planetary surfaces. Mexico was one of these sta-
tions, but local air currents and changes of temper-
ature there were such that good seeing was far from
prevalent, as had been expected. At Flagstaff, on
the other hand, conditions have been pretty uni-
formly good, and an enormous amount of work on
the planet Mars has been accumulated and pub-
lished. The first successful photographs of this
planet were taken there in 1905, and Jupiter,
Saturn, the zodiacal light and many other test
objects have been photographed, which demon-
strates the excellence of the site for astronomical
research. Within recent years spectrum research
by Slipher, especially on the nebulae, has been added
to the program, and the rotation and radial veloci-
ties of many nebulse have been determined.
On Mount Wilson, near Pasadena, California, at
an elevation of nearly 6,000 feet, is the Carnegie
Solar Observatory, founded and equipped under the
direction of Professor George E. Hale, as a depart-
ment of the Carnegie Institution of Washington, of
which Dr. John Campbell Merriam is President. The
climatology of the region was carefully investigated
and tests of the seeing made by Hussey and others.
Although equipped primarily for study of the sun,
the program of the observatory has been widely
amplified to include the stars and nebulge. The in-
strumental equipment is unique in many respects.
To avoid the harmful effect of unsteadiness of air
strata close to the ground a tower 150 feet high was
erected, with a dome surmounting it and covering
a ccelostat with mirror for reflecting the sun's rays
vertically downward. Underneath the tower a dry
well was excavated to a depth equal to % the height
of the tower above it. In the subterranean chamber
148 ASTRONOMY TO-DAY
is the spectroheliograph of exceptional size and
power. The sun's original image is nearly 17 inches
in diameter on the plate, and the solar chromosphere
and prominences, together with the photosphere and
f aculse, are all recorded by monochromatic light.
Connected with the observatory on Mount Wilson
are the laboratories, offices and instrument shops in
Pasadena, 16 miles distant, where the remarkable
apparatus for use on the mountain is constructed.
A reflecting telescope with silver-on-glass mirror
60 inches in diameter was first built by Ritchey and
thoroughly tested by stellar photographs. Also the
northern spiral nebulae were photographed, exhibit-
ing an extraordinary wealth of detail in apparent
star formation. The success of this instrument
paved the way for one similar in design, but with a
mirror 100 inches in diameter, provided by gift of
the late John D. Hooker of Los Angeles. The tele-
scope was completed in 1919. Notwithstanding its
huge size and enormous weight, the mounting is
very successful, as well as the mirror. Mercurial
bearings counterbalance the weight of the polar
axis in large part. This great telescope, by far the
largest and most powerful ever constructed, is now
employed on a program of research in which its
vast light-gathering power will be utilized to the
full. Under the skillful management of Hale and
his enthusiastic and capable colleagues, the confines
of the stellar heavens will be enormously extended,
and secrets of evolution of the universe and of its
structure no doubt revealed.
In all the mountain stations hitherto established,
as the Lick Observatory at 4,000 feet, the Mount
Wilson Observatory 6,000 feet, the Lowell Observa-
tory at 7,000 feet, the Harvard Observatory at 8,000
MOUNTAIN OBSERVATORIES 149
feet; and Teneriffe and Etna at 10,000, Fujiyama
at 12,000, Pike's Peak at 14,000, Mont Blanc and
Mount Whitney at 15,000, the researches that have
been carried on have fully demonstrated the vast
advantage of increased elevation in localities where
climatological conditions as well as elevation are
favorable. Nevertheless, only one-half of the ex-
treme altitude contemplated by Sir Isaac Newton
has yet been attained.
Can the greater heights be reached and perma-
nently occupied? Geographically and astronomically
the most favorably located mountain for a great
observatory is Mount Chimborazo in Ecuador. Its
elevation is 22,000 feet, and it was ascended by Ed-
ward Whymper in 1880. Situated very nearly on
the earth's equator, almost the entire sidereal
heavens are visible from this single station, and all
the planets are favored by circumzenith conditions
when passing the meridian. No other mountain in
the world approaches Chimborazo in this respect.
But the summit is perpetually snow-capped, ex-
ceedingly inaccessible, and the defect of barometric
pressure would make life impossible up there in the
open.
Only one method of occupation appears to be fea-
sible. The permanent snow line is at about 16,000
feet, where excellent water power is available. By
tunneling into the mountain at this point, and diag-
onally upward to the summit, permanent occupa-
tion could be accomplished, at a cost not to exceed
one million dollars.
The rooms of the summit observatory would need
to be built as steel caissons, and supplied with com-
pressed air at sea-level tension. The practicability
of this plan was demonstrated by the writer in
150 ASTRONOMY TO-DAY
September, 1907, at Cerro de Pasco, Peru. A steel
caisson was carried up to an elevation exceeding
14,000 feet. Patients suffering acutely with moun-
tain sickness were placed inside this caisson, and on
restoring the atmospheric pressure within it artifici-
ally all unfavorable symptoms headache, high res-
piration and accelerated pulse disappeared. There
was every indication that if persons liable to this
uncomfortable complaint were brought up to this
elevation, or indeed any attainable elevation, under
unreduced pressure, the symptoms of mountain
sickness would be unknown. Comfortable occupa-
tion of the highest mountain summits was thereby
assured.
The working of astronomical instruments from
within air-tight compartments does not present any
insurmountable difficulties, either mechanical or
physical. Since the time these experiments were
made, the Guayaquil-Quito railway has been con-
structed over a saddle of Chimborazo, at an eleva-
tion of 12,000 feet; and only six miles of railway
would need to be built from this station to the point
where the tunnel would enter the mountain.
Only by the execution of some such plan as this
can astronomers hope to overcome the baleful effects
of an ever mobile atmosphere, and secure the ad-
vantages contemplated by Sir Isaac Newton in that
tranquillity of atmosphere, which he conceived as
perpetually surrounding the summits of the highest
mountains.
In Russell's theory of the progressive development
of the stars, from the giant class to the dwarf, an
element of verification from observation is lacking,
because hitherto no certain method of measuring
the very minute angular diameters of the stars has
MOUNTAIN OBSERVATORIES 151
been successfully applied. The apparent surface
brightness corresponding to each spectral type is
pretty well known, and by dividing it into the total
apparent brightness, we have the angular area sub-
tended by the star, quite independent of the star's
distance. This makes it easy to estimate the angu-
lar diameter of a star, and Betelgeuse is the one
which has the greatest angular diameter of all (
whose distances we know. Antares is next in order
of angular diameter, 0".043, Aldebaran 0".022,
Arcturus 0".020, Pollux 0".013, and Sirius only
0".007.
Can these theoretical estimates be verified by
observation? Clearly it is of the utmost impor-
tance and the exceedingly difficult inquiry has been
undertaken with the 100-inch reflector on Mount
Wilson, employing the method of the interferometer
developed by Michelson and described later on, an
instrument undoubtedly capable of measuring much
smaller angles than can be measured by any other
known method. Unquestionably the interference
of atmospheric waves, or in other words what as-
tronomers call "poor seeing," will ultimately set the
limit to what can be accomplished. "But even if,"
says Eddington, "we have to send special expedi-
tions to the top of one of the highest mountains in
the world, the attack on this far-reaching problem
must not be allowed to languish."
CHAPTER XXIII
THE PROGRAM OF A GREAT OBSERVATORY
THE Mount Wilson Observatory has now been in
operation about fifteen years. The novelty in
construction of its instruments, the investigations
undertaken with them and the discoveries made, the
interpretation of celestial phenomena by laboratory
experiment, and the recent addition to its equipment
of a telescope 100 inches in diameter, surpassing all
others in power, directs especial attention to the ex-
tensive activities of this institution, whose budget
now exceeds a million dollars annually. Results are
only achieved by a carefully elaborated program,
such as the following, for which the reader is mainly
indebted to Dr. Hale, the director of the observa-
tory, who gives a very clear idea of the trend of
present-day research on the magnetic nature of the
sun, and the structure and evolution of the sidereal
universe.
The purpose of the observatory, as defined at its
inception, was to undertake a general study of
stellar evolution, laying especial emphasis upon the
study of the sun, considered as a typical star;
physical researches on stars and nebulae; and the
interpretation of solar and stellar phenomena by
laboratory experiments. Recognizing that the de-
velopment of new instruments and methods afforded
the most promising means of progress, well-
equipped machine shops and optical shops were pro-
.vided with this end in view.
' 152
PROGRAM OF A GREAT OBSERVATORY 153
The original program of the observatory has
been much modified and extended by the independ-
ent and striking discovery by Campbell and Kap-
teyn of an important relationship between stellar
speed and spectral type; the demonstration by
Hertzsprung and Russell of the existence of giant
and dwarf stars; the successful application of
the 60-inch reflector by Van Maanen to the measure-
ment of minute parallaxes of stars and nebulae ; the
important developments of Shapley's investigation
of globular star clusters ; the possibilities of research
resulting from Seares's studies in stellar photom-
etry; and the remarkable means of attack devel-
oped by Adams through the method of spectroscopic
parallaxes.
By this method the absolute magnitude, and hence
the distance of a star is accurately determined from
estimates of the relative intensities of certain lines
in stellar spectra. Attention was first directed to-
ward lines of this character in 1906, when it was
inferred that the weakening of some lines in the
spectra of sun spots and the strengthening of others
was the result of reduced temperature of the spot
vapors. On testing this hypothesis by laboratory
experiments, it was fully verified.
Subsequently Adams, who had thus become fa-
miliar with these lines and their variability, studied
them extensively in the spectra of other stars. In
this way was discovered the dependence of their
relative intensities on the star's absolute magnitude,
so providing the powerful method of spectroscopic
parallaxes.
This method, giving the absolute magnitude as
well as the distance of every star (excepting those
of the earliest type) whose spectrum is photo-
154 ASTRONOMY TO-DAY
graphed, is no less important from the evolutional
than from the structural point of view.
Investigations in solar physics which formerly
held chief place in the research program have
developed along unexpected lines. It could not be
foreseen at the outset that solar magnetic phenom-
ena might become a subject of inquiry, demanding
special instrumental facilities, and throwing light
on the complex question of the nature of the sun
spots and other solar problems of long standing. It
is obvious that these researches, together with those
on the solar rotation and the motions of the solar
atmosphere, developed by Adams and St. John, must
be carried to their logical conclusion, if they are to
be utilized to the fullest in interpreting stellar and
nebular phenomena.
The discovery of solar magnetism, like many
other Mount Wilson results, was the direct outcome
of a long series of instrumental developments. The
progressive improvement and advance in size of the
tools of research was absolutely necessary. Hale's
first spectroheliograph at Kenwood in 1890 was at-
tached to a 12-inch refractor, and the solar image
was but two inches in diameter. It was soon found
that a larger solar image was essential, and a spec-
trograph of much greater linear dispersion ; in fact,
the spectrograph must be made the prime element
in the combination, and the telescope so designed
as to serve as a necessary auxiliary.
Accordingly, successive steps have led through
spectrographs of 18 and 30 feet dimension to a
vertical spectrograph 75 feet in focal length. The
telescope is the 150 feet tower telescope, giving a
solar image of 16.5 inches in diameter. Its spectro-
graph is massive in construction, and by extending
PROGRAM OF A GREAT OBSERVATORY 155
deep into the earth, it enjoys the stability and con-
stancy of temperature required for the most exact-
ing work.
Another direct outgrowth of the work of sun-spot
spectra is a study of the spectra of red stars, where
the chemistry of these coolest regions of the sun is
partially duplicated. The combination of titanium
and oxygen, and the significant changes of line in-
tensity already observed in both instances, and also
in the electric furnace at reduced temperatures, give
indication of what may be expected to result from
an attack on the spectra of the red stars with more
powerful instrumental means, which is now pro-
vided by the 100-inch telescope and its large stellar
spectrograph.
Other elements in the design of the 100-inch
Hooker telescope have the same general object in
view that of developing and applying in astronom-
ical practice the effective research methods sug-
gested by recent advances in physics. Fresh possi-
bilities of progress are constantly arising, and these
are utilized as rapidly as circumstances permit.
The policy of undertaking the interpretations of
celestial phenomena by laboratory experiments, an
important element in the initial organization of
Mount Wilson, has certainly been justified by its
results. Indeed, the development of many of the
chief solar investigations would have been impos-
sible without the aid of special laboratory studies,
going hand in hand with the astronomical observa-
tions. So indispensable are such researches, and so
great is the promise of their extension, that the
time has now come for advancing the laboratory
work from an accessory feature to full equality with
the major factors in the work of the observatory.
156 ASTRONOMY TO-DAY
Accordingly a new instrument now under installa-
tion is an extremely powerful electro-magnet, de-
signed by Anderson for the extension of researches
on the Zeeman effect, and for other related investi-
gations. Within the large and uniform field of this
magnet, which is built in the form of a solenoid, a
special electric furnace, designed for this purpose
by King, is used for the study of the inverse Zee-
man effect at various angles with the lines of force.
This will provide the means of interpreting cer-
tain remarkable anomalies in the magnetic phe-
nomena of sun spots.
The 100-inch telescope is now in regular use. All
the tests so far applied show that it greatly sur-
passes the 60-inch telescope in every class of work.
For many months most of the observations and pho-
tographs have been made with the Cassegrain com-
bination of mirrors, giving an equivalent focal
length of 134 feet and involving three reflections of
light. The 100-inch telescope is found to give nearly
2.8 times as much light as the 60-inch telescope, and
therefore extends the scope of the instrument to all
the stars an entire magnitude fainter. This is a
very important gain for research on the faint
globular clusters, as well as the small and faint
spiral and planetary nebulae, providing a much
larger scale for these objects and sufficient light at
the same time. Photographs of the moon and many
other less critical tests have been made with very
satisfactory results. Those of the moon appear to
be decidedly superior in definition to any previously
taken with other instruments.
Another investigation is of great importance in
the light of recent advances in theoretical dynamics.
Darwin, in his fundamental researches on the dy-
PROGRAM OF A GREAT OBSERVATORY 157
namics of rotating masses, dealt with incompres-
sible matter, which assumes the well-known pear-
shaped figure, and may ultimately separate into two
bodies. Roche on the other hand discussed the evo-
lution of a highly compressible mass, which finally
acquires a lens-shaped form and ejects matter at
its periphery. Both of these are extreme cases.
Jeans has recently dealt with intermediate cases,
such as are actually encountered in stars and
nebulae. He finds that when the density is less
than about one-fourth that of water, a lens-shaped
figure will be produced with sharp edges, as de-
picted by Roche. Matter thrown off at opposite
points on the periphery, under the influence of small
tidal forces from neighboring masses, may take the
form of two symmetric filaments, though it is not
yet entirely clear how these may attain the char-
acteristic configuration of spiral nebulae. The pre-
liminary results of Van Maanen indicate motion
outward along the arms, in harmony with Jeans's
views.
Jeans further discusses the evolution of the arms,
which will break up into nuclei (of the order of
mass of the sun) if they are sufficiently massive, but
will diffuse away if their gravitational attraction is
small. The mass of our solar system is apparently
not great enough, according to Jeans, to account for
its formation in this way. As is apparent, these in-
vestigations lead to conclusions very different from
those derived by Chamberlin and Moulton from the
planetesimal hypothesis.
This is a critical study of spiral nebulae for which
the 100-inch telescope is of all instruments in exis-
tence the best suited. The spectra of the spirals
must be studied, as well as the motions of the matter
158 ASTRONOMY TO-DAY
composing the arms. Their parallaxes, too, must be
ascertained. A photographic campaign including
spiral nebulae of various types will settle the ques-
tion of internal motions. The large scale of the
spiral nebulae at the principal focus of the Hooker
telescope, and the experience gained in the measure-
ment of nebular nuclei for parallax determination,
will help greatly in this research. A multiple-slit
spectrograph, already applied at Mount Wilson, will
be employed, not only on spiral nebulae whose plane
is directed toward us, but also on those whose plane
lies at an angle sufficient to permit both components
of motion to be measured by the two methods.
In dealing with problems of structure and motion
in the Galactic system, the 100-inch telescope offers
especial advantages, because of its vast light-gather-
ing power. Studies of radial velocities of the stars
have hitherto been necessarily confined to the
brighter stars, for the most part even to those visible
to the naked eye. While some of these are very
distant, most of the stars whose radial velocities are
known belong to a very limited group, perhaps con-
stituting a distinct cluster of which the sun is a
member, but in any event of insignificant propor-
tions when contrasted with the Galaxy. Current
spectrographic work with the 60-inch telescope in-
cludes stars of the eighth magnitude, and some even
fainter. But while the 60-inch has enabled Adams
to measure the distances of many remote stars by
his new spectroscopic method, and to double the
known extent (so far as spectroscopic evidence is
concerned) of the star streams of Kapteyn, a much
greater advance into space is necessary to find out
the community of motion among the stars compris-
ing the Galactic system. The Hooker telescope will
PROGRAM OF A GREAT OBSERVATORY 159
enable us to determine accurate radial velocities
to stars of the eleventh magnitude, which doubtless
truly represent the Galaxy.
In order to secure a maximum return within a
reasonable period of time, the stars in the selected
areas of Kapteyn will be given the preference, be-
cause of the vast amount of work already done, re-
lating to their positions, proper motions, and visual
and photographic magnitudes. Such consideration
as spectral type, the known directions of star-
streaming, and the position of the chosen regions
with reference to the plane of the Galaxy are given
adequate weight, and it is of fundamental impor-
tance that the method of spectroscopic parallaxes
will permit dwarf stars to be distinguished from
stars that are in the giant class, but rendered faint
by their much greater distance. In addition to these
problems, the stellar spectrograms will provide rich
material for study of the relationship between
stellar mass and speed, and the nature of giant
stars and dwarf stars.
Shapley's recent studies of globular clusters have
indicated the significance of these objects in both
evolutional and structural problems, and the possi-
bility of determining their parallaxes by a number
of independent methods is of prime importance, both
in its bearing on the structure of the universe and
because it permits a host of apparent magnitudes to
be at once transformed into absolute magnitudes.
Here the advantage of the Hooker telescope is two-
fold: at its 134-foot focus the increased scale of
the crowded clusters makes it possible to select
separate stars for spectrum photography (which
could not be done with the 60-inch where the images
were commingled) ; and the great gain in light is
160 ASTRONOMY TO-DAY
such that the spectra of stars to the 14th magnitude
have been photographed in loss than an hour.
Faint globular clusters, then, will comprise a large
part of the early program with the 100-inch tele-
scope : the faintest possible stars in them must be de-
tected and their magnitudes and colors measured;
spectral types must be determined, and the radial
velocities of individual stars and of clusters as a
whole; spectroscopic evidence of possible axial rota-
tion of globular clusters must be searched for; and
the method of spectroscopic parallaxes, as well as
other methods, must be applied to ascertaining the
distances of these clusters,
The possibility of dealing with many problems
relating to the distribution and evolution of the
faintest stars depends upon the establishment of
photographic and photovisual magnitude scales.
Below the twelfth magnitude, the only existing scale
of standard visual or photovisual magnitudes is the
Mount Wilson sequence, already extended by Scares
to magnitude 17.5 with the 60-inch telescope.
Extension of this scale to even fainter magni-
tudes, and its application to the faintest stars within
its range is an important task for this great tele-
scope, as it will doubtless bring within range hun-
dreds of millions of stars that are beyond the reach
of the 60-inch. The giants among them will form for
us the outer boundary of the Galactic system, while
the dwarfs will be of almost equal interest from the
evolutional standpoint. The photometric program
of the 100-inch, then, will deal with such questions
as the condensation of the fainter stars toward the
Galactic plane, the color of the most distant stars,
and the final settlement of the long inquiry regard-
ing the possible absorption of light in space.
O <""
TO O
THE SUN'S DISK. The view shows the "rice grain" structure of the
photosphere and brilliant calcium flocculi. (Photo, Ycrkes Observatory.)
THE LUNAR SURFACE VISIBLE DURING A TOTAL ECLIPSE OF THE MOON,
FEBRUARY 8, 1906. (Photo, Yerkes Observatory.)
PROGRAM OF A GREAT OBSERVATORY 161
Another research of exceptional promise will be
undertaken, which is of great importance in a gen-
eral study of stellar evolution; and that is the de-
termination of the spectral-energy curves of stars
of various classes, for the purpose of measuring
their surface temperatures. A very few of the
nebulse are found to be variable, and their peculiar-
ities need investigation, also special problems of
variable stars and temporary stars, and the spectra
of the components of close double stars which are
beyond the power of all other instruments to
photograph.
Such a program of research conveys an excel-
lent idea of many of the great problems that are
under investigation by astronomers to-day, and gives
some notion of the instrumental means requisite in
executing comprehensive plans of this character.
It will not escape notice that the climax of instru-
mental development attained at Mount Wilson has
only been made possible by an unbroken chain of
progress, link by link, each antecedent link being
necessary to the successful forging of its following
one. In very large part, and certainly indispensable
to these instrumental advances, has the art of work-
ing in glass and metals been the mainstay of re-
search. As we review the history of astronomical
progress, from Galileo's time to our own, the con-
summate genius of the artisan and his deft handi-
work compel our admiration almost equally with the
keen intelligence of the astronomer who uses these
powerful engines of his own devising to wrest the
secrets of nature from the heavens.
Sci. Vol. 26
CHAPTER XXIV
OUR SOLAR SYSTEM
NOW let us go upward in imagination, far, far be-
yond the tops of the highest mountains, beyond
the moon and sun, and outward in space until we
reach a point in the northern heavens millions and
millions of miles away, directly above and equally
distant from all points in the ecliptic, or path in
which our earth travels yearly round the sun. Then
we should have that sort of comprehensive view of
the solar system which is necessary if we are to
visualize as a whole the working of the vast machine,
and the motions, sizes, and distances of all the bodies
that comprise it. Of such stupendous mechanism our
earth is part.
Or in lieu of this, let us attempt to get in mind a
picture of the solar system by means of Sir William
Herschel's apt illustration : "Choose any well-leveled
field. On it place a globe two feet in diameter. This
will represent thejmn ; Mercury will be represented
by a grain of mustard seed on the circumference of
a circle 164 feet in diameter for its orbit; Venus,
a pea on a circle of 284 feet in diameter ; the Earth
also a pea, on a circle of 430 feet; Mars a rather
larger pin's head on a circle of 654 feet; the aster-
oids, grains of sand in orbits of 1,000 to 1,200 feet ;
Jupiter, a moderate sized orange in a circle of nearly
half a mile across; Saturn, a small orange on a
circle of four-fifths of a mile; Uranus, a full-sized
162
OUR SOLAR SYSTEM 163
cherry or small plum upon the circumference of a
circle more than a mile and a half; and finally Nep-
tune, a good-sized plum on a circle about two miles
and a half in diameter To imitate the motions of
the planets in the above mentioned orbits, Mercury
must describe its own diameter in 41 seconds ; Venus
in 4 minutes, 14 seconds; the Earth in 7 minutes;
Mars in 4 minutes 48 seconds ; Jupiter in 2 minutes
56 seconds ; Saturn in 3 minutes 13 seconds ; Uranus
in 2 minutes 16 seconds ; and Neptune in 3 minutes
30 seconds."
Now, let us look earthward from our imaginary
station near the north pole of the ecliptic. All
these planetary bodies would be seen to be traveling
eastward round the sun, that is, in a counter-clock-
wise direction, or contrary to the motions of the
hands of a timepiece. Their orbits or paths of motion
are very nearly circular, and the sun is practically
at the center of all of them except Mercury and
Mars ; of Venus and Neptune, almost at the absolute
center. The planes of all their orbits are very nearly
the same as that of the ecliptic, or plane in which
the earth moves. These and many other resem-
blances and characteristics suggest a uniformity of
origin which comports with the idea of a family, and
so the whole is spoken of as the solar system, or
the sun and his family of planets.
In addition to the nine bodies already specified,
the solar system comprises a great variety of other
and lesser bodies ; no less than twenty-six moons or
satellites tributary to the planets and traveling
round them in various periods as the moon does
round our earth. Then between the orbits of Mars
and Jupiter are many thousands of asteroids, so
called, or minor planets (about 1,000 of them have
164 ASTRONOMY TO-DAY
actually been discovered, and their paths accurately
calculated). And at all sorts of angles with the
planetary orbits are the paths of hundreds of comets,
delicate filmy bodies of a wholly different constitu-
tion from the planets, and which now and then blaze
forth in the sky, their tails appearing much like the
beam of a searchlight, and compelling for the time
the attention of everybody. Connected with the
comets and doubtless originally parts of them are
uncounted millions of millions of meteors, which
for the time become a part of the solar system, their
minute masses being attracted to the planets, upon
which they fall, those hitting the earth being visible
to us as familiar shooting stars.
We next follow the story of astronomy through
the solar system, beginning with the sun itself and
proceeding outward through his family of planets,
now much more numerous and vastly more extended
than it was to the ancient world, or indeed till
within a century and a half of our own day.
CHAPTER XXV
THE SUN AND OBSERVING IT
AS lord of day, king of the heavens, mankind
-*-Mn the ancient world adored the sun. By their
researches into the epoch of the Assyrians, Hittites,
Phoenicians and other early peoples now passed
from earth, archaeologists have unearthed many
monuments that evidence the veneration in which
the early peoples who inhabited Egypt and Asia
Minor many thousand years ago held the sun. A
striking example is found in the architecture of
early Egyptian temples, on the lintels of which are
carved representations of the winged globe or the
winged solar disk, and there is a bare possibility that
the wings of the globe were suggested by a type of
the solar corona as glimpsed by the ancients.
Little knew they about the distance and size of
the sun ; but the effects of his light and heat upon all
vegetal and animal life were obvious to them. Doubt-
less this formed the basis for their worship of the
sun. Occasional huge spots must have been visible
to the naked eye, and the sun's corona was seen at
rare intervals. Plutarch and Philostratus describe
it very much as we see it today.
How completely dependent mankind is upon the
sun and its powerful radiations, only the science
of the present day can tell us. By means of the sun's
heat the forests of early geologic ages were enabled
to wrest carbon from the atmosphere and store it in
165
166 ASTRONOMY TO-DAY
forms later converted by nature's chemistry into
peat and coal. Through processes but imperfectly
understood, the varying forms of vegetable life are
empowered to conserve, from air and soil, nitrogen
and other substances suitable for and essential to the
life maintenance of animal creatures. Breezes that
bring rain and purify the air ; the energy of water
held under storage in stream and dam and fall;
trade winds facilitating commerce between the con-
tinents; oceanic currents modifying coastal cli-
piates ; the violence of tornado, typhoon and water-
spout, together with other manifestations of natural
forces all can be traced back to their origin in the
tremendous heating power of the solar rays. In
everything material the sun is our constant and
bountiful benefactor. If his light and heat were
withdrawn, practically every form of human activ-
ity on this planet would come to an early end.
How far away is the sun ? What is the size of the
sun? These are questions that astronomers of the
present day can answer with accuracy.
So closely do they know the sun's distance that
it is employed as their yardstick of the sky, or
unit of celestial measurement. Many methods have
been utilized in ascertaining the distance of the sun,
and the remarkable agreement among them all is
very extraordinary. Some of them depend upon
pure geometry, and the basic measure which we
make from the earth is not the distance of the sun
directly; but we find out how far away Venus is
during a transit of Yenus, for example, or how far
away Mars is or some of the asteroids are at their
closer oppositions. Then it is possible to calculate
how far away the sun is, because one measurement
of distance in the solar system affords us the scale
THE SUN AND OBSERVING IT 167
on which the whole structure is built. But perhaps
the simplest method of getting the sun's distance is
by the velocity of light, 186,300 miles a second. From
eclipses of Jupiter's moons we know that light takes
8 minutes 20 seconds to pass from sun to earth.
So that the sun's distance is the simple product of
the two, or 93 millions of miles.
Once this fundamental unit is established, we have
a firm basis on which to build up our knowledge of
the distances, the sizes and motions of the heavenly
bodies, especially those that comprise the solar sys-
tem. We can at once ascertain the size of the sun,
which we do by measuring the angle which it fills,
that is, the sun's apparent diameter. Finding this
to be something over a half a degree in arc, the proc-
esses of elementary trigonometry tell us that the
sun's globe is 865,000 miles in diameter. For nearly
a century this has been accurately measured with
the greatest care, and diameters taken in every
direction are found to be equal and invariably the
same. So we conclude that the sun is a perfect
sphere, and so far as our instruments can inform
us, its actual diameter is not subject to appreciable
change.
The vastness of the sun's volume commands our
attention. As his diameter is 110 times that of the
earth, his mere size or volume is 110x110x110 or
1,300 thousand times that of the earth, because the
volumes of spheres are in proportion as the cubes
of their diameters. If the materials that compose
the sun were as heavy as those that make up the
earth, it would take 1,300 thousand earths to weigh
as much as the sun does. But by a method which we
need not detail here, the sun's actual weight or
mass is found to be only 300 thousand (more nearly
168 ASTRONOMY TO-DAY
330,000), times greater than the earth's. So we must
infer that, bulk for bulk, the component materials
of the sun are about one-fourth lighter than those
of the earth, that is, about one and one-half times
as dense as water.
To look at this in another way : it is known that
a body falling freely toward the earth from outer
space would acquire a speed of seven miles a second,
whereas if it were to fall toward the sun instead,
the velocity would be 383 miles a second on reach-
ing his surface. If all the other bodies of the solir
system, that is, the earth and moon, all the planets
and their satellites, the comets and all were to be
fused together in a single globe, it would weigh only
one-seven hundred and fiftieth as much as the sun
does.
At the surface, however, the disproportion of
gravity is not so great, because of the sun's vast
size : it is only about twenty-eight times greater on
the sun than on the earth; and instead of a body
falling 16 feet the first second as here, it would fall
444 feet there. Pendulums of clocks on the sun
would swing five times for every tick here, and an
athlete's running high jump would be scaled down
to three inches.
Let us next inquire into the amount of the sun's
light and heat, and the enormously high temperature
of a body whose heat is so intense even at the vast
distance at which we are from it. The intensity
of its brightness is such that we have no artificial
source of light that we can readily compare it with.
In the sky the next object in brightness is the full
moon, but that gives less than the half-millionth
part as much light as the sun. The standard candle
used in physics gives so little light in comparison
THE SUN AND OBSERVING IT 169
that we have to use an enormous number to express
the quantity of light that the sun gives.
A sperm candle burning 120 grains hourly is the
standard, and if we compare this with the sun when'
overhead, and allow for the light absorbed by the at-
mosphere, we get the number 1575 with twenty-four
ciphers following it, to express the candlepower of
the sun's light. If we interpose the intense calcium
light or an electric arc light between the eye and
the sun, these artificial sources will look like black
spots on the disk. Indeed, the sun is nearly four
times brighter than the "crater," or brightest part
of the electric arc. The late Professor Langley at
a steel works in Pennsylvania once compared direct
sunlight with the dazzling stream of molten metal
from a Bessemer converter ; but bright as it was, sun-
light was found to be five thousand times brighter.
Equally enormous is the heat of the sun. Our in-
tensest sources of artificial heat do not exceed 4,000
degrees Fahrenheit, but the temperature at the sun's
surface is probably not less than 16,000 degrees F.
One square meter of his surface radiates enough
heat to generate 100,000 horsepower continuously.
At our vast distance of 93 millions of miles, the sun's
heat received by the earth is still powerful enough to
melt annually a layer of ice on the earth more than
a hundred feet in thickness. If the solar heat that
strikes the deck of a tropical steamship could be
fully utilized in propelling it, the speed would reach
at least ten knots.
Many attempts have been made in tropical and
sub-tropical climates to utilize the sun's heat directly
for power, and Ericsson in Sweden, Mouchot in
France, and Shuman in Egypt have built successful
and efficient solar engines. Necessary intermission
170 ASTRONOMY TO-DAY
of their power at night, as well as on cloudy days,
will preclude their industrial introduction until
present fuels have advanced very greatly in cost.
All regions of the sun's disk radiate heat uniformly,
and the sun's own atmosphere absorbs so much that
we should receive 1.7 times more heat if it were re-
moved. So far as is known, solar light and heat are
radiated equally in all directions, so that only a very
minute fraction of the total amount ever reaches the
earth, that is, 1 2200 millionth part of the whole.
Indeed all the planets and other bodies of the solar
system together receive only one one hundred mil-
lionth part; the vast remainder is, so far as we
know, effectively wasted. It is transformed, but
what becomes of it, and whether it ever reappears in
any other form, we cannot say.
How is this inconceivably vast output of energy
maintained practically invariable throughout the
centuries? Many theories have been advanced, but
only one has received nearly universal assent, that
of secular contraction of the sun's huge mass upon
itself. Shrinkage means evolution of heat ; and it is
found by calculation that if the sun were to contract
its diameter by shrinking only two-hundred and fifty
feet per year, the entire output of solar heat might
thus be accounted for. So distant is the sun and so
slow this rate of contraction that centuries must
elapse before we could verify the theory by actual
measurements. Meanwhile, the progress of physical
research on, the structure and elemental properties
of matter has brought to light the existence of
highly active internal forces which are doubtless
intimately concerned in the enormous output of
radiant energy, though the mechanism of its main-
tenance is as yet known only in part.
THE SUN AND OBSERVING IT 171
Abbot, from many years' observations of the
solar constant, at Washington, on Mount Wilson,
and in Algeria, finds certain evidence of fluctuation
in the solar heat received by the earth. It cannot
be a local phenomenon due to disturbances in our
atmosphere, but must originate in causes entirely
extraneous to the earth. Interposition of meteoric
dust might conceivably account for it, but there is
sufficient evidence to show that the changes must be
attributed to the sun itself. The sun, then, is a vari-
able star; and it has not only a period connected
with the periodicity of the sun spots, but also an
irregular, nonperiodic variation during a cycle of a
week or ten days, though sometimes longer, and oc-
casioning irregular fluctuations of two to ten per
cent of the total radiation. Radiation is found to
increase with the spottedness.
Attempts have been made on the basis of the con-
traction theory to find out the past history of the sun
and to predict its future. Probably 20 to 50 millions
of years in the past represents the life of the sun
much as it is at present; and if solar radiation in
the future is maintained substantially as now, the
sun will have shrunk to one-half its present diameter
in the next five million years.
So far then as heat and light from the sun are
concerned, the sun may continue to support life on
the earth not to exceed ten million years in the
future. But the sun's own existence, independently
of the orbs of the system dependent upon it, .might
continue for indefinite millions of seons before it
would ever become a cold dead globe; indeed, in
the present state of science, we cannot be sure
that it is destined to reach that condition within
calculable time.
172 ASTRONOMY TO-DAY
A few words on observing the sun, an object much
neglected by amateurs. On account of the intense
light, a very slight degree of optical power is suffi-
cient. Indeed a piece of window glass, smoked in a
candle flame with uniform graduation from end to
end, will be found worth while in a beginner's daily
observation of the sun. The glass should be smoked
densely enough at one end so that the sunlight as
seen through it will not dazzle the eye on the clearest
days. At the other end of the glass, the degree of
smoke film should not be quite so dense, so that
the sun can be examined on hazy, foggy or partly
cloudy days. An occasional naked-eye spot will re-
ward the patient observer.
If a small spyglass, opera glass or field glass is
at hand, excellent views of the sun may be had by
mounting the glass so that it can be held steadily
pointed on the sun, and then viewing the disk by pro-
jection on a white card or sheet of paper. Care
must be taken to get a good focus on the projected
image, and then the faculse, or whitish spots, or
mottling nearer the sun's edge will usually be well
seen. By moving the card farther away from the
eyepiece, a larger disk may be obtained, in effect
a higher degree of magnification. But care must be
used not to increase it too much. Keep direct sun-
light outside the tube from falling on the card where
the image is being examined. This is conveniently
done by cutting a large hole, the size of the brass
cell of the object glass, through a sheet of corrugated
strawboard, and slipping this on over the cell. In
this way the spots on the sun can be examined with
ease and safety to the eye.
For large instruments a special type of eyepiece
is provided known as a helioscope, which disposes of
THE SUN AND OBSERVING IT 173
the intense heat rays that are harmful to the eye.
Frequent examination of the eyepiece should be
made and the eyepiece cooled if necessary. That
part of the sun's surface under observation is known
as the photosphere, that is, the part which radiates
light. If the atmosphere admits the use of high
magnifying powers, the structure of the photosphere
will be found more and more interesting the higher
the power employed. It is an irregularly mottled
surface showing a species of rice-grain structure
under fairly high magnification. These grains are
grouped irregularly and are about 500 miles across.
Under fine conditions of vision they may be sub-
divided into granules. The faculae, or white spots,
are sometimes elevations above the general solar
level; they have occassionally been seen projecting
outside the limb, or edge of the disk.
CHAPTER XXVI
SUN SPOTS AND PROMINENCES
DARK spots of a deep bluish black will often be
seen on the photosphere of the sun. Sometimes
single, though generally in groups, the larger ones
will have a dark center, called the umbra, sur-
rounded by the very irregular penumbra which is
darker near its outer edge and much brighter ap-
parently on its inner edge where it joins on the
umbra. The penumbra often shows a species of
thatch-work structure, and systematic sketches of
sun spots by observers skilled in drawing are greatly
to be desired, because photography has not yet
reached the stage where it is possible to compete
with visual observation in the matter of fine detail.
The spots themselves nearly always appear like de-
pressions in the photosphere, and on repeated
occasions they have been seen as actual notches
when on the edge of the sun.
Many spots, however, are not depressions: some
appear to be actual elevations, with the umbra per-
haps a central depression, like the crater in the
general elevation of a volcano. Spots are some-
times of enormous size. The largest on record was
seen in 1858 ; it was nearly 150,000 miles in breadth,
and covered a considerable proportion of the whole
visible hemisphere of the sun. A spot must be
nearly 30,000 miles across in order to be seen with
the naked eye.
174
SUN SPOTS AND PROMINENCES 175
III their beginning, development, and end, each
spot or group of spots appears to be a law unto
itself. Sometimes in a few hours they will form,
though generally it is a question of days and even
weeks. Very soon after their formation is com-
plete, tonguelike encroachments of the penumbra
appear to force their way across the umbra, and this
splitting up of the central spot usually goes on quite
rapidly. Sun spots in violent disturbance are rarely
observed. As the sun turns round on his axis, the
spots will often be carried across the disk from the
center to the edge, when they become very much
foreshortened. The sun's period of rotation is 28
days, so that if a spot lasts more than two weeks
without breaking up, it may reappear on the eastern
limb of the sun after having disappeared at the
western edge. Two or three months is an average
duration for a spot; the longest on record lasted
through 18 months in 1840-41.
The position of the sun's axis is well known, its
equator being tilted about 7 degrees to the ecliptic,
and the spots are distributed in zones north and
south of the equator, extending as far as 30 degrees
of solar latitude. In very high latitudes spots are
never seen; they are most abundant in about latitude
15 degrees both north and south, and rather more
numerous in the northern than in the southern
hemisphere of the sun. Recent research at Mount
Wilson makes the sun a great magnet; and its
magnetic axis is inclined at an angle of 6 degrees
to the axis of rotation, around which it revolves in
32 days.
There is a most interesting periodicity of the
spots on the sun, for months will sometimes elapse
with spots in abundance and visible every day, while
176 ASTRONOMY TO-DAY
at other periods, days and even weeks will elapse
without a single spot being seen. There is a well
recognized period of eleven and one-tenth years, the
reason underlying which is not, however, known.
After passing through the minimum of spottedness,
they begin to break out again first in latitudes of
25 degrees-30 degrees, rather suddenly, and on
both sides of the equator, and they move toward
the equator as their number and individual size
decrease.
The last observed epoch of maximum spot activity
on the sun was passed in 1917.
Many attempts have been made to ascertain the
cause of the periodicity of sun spots, but the real
cause is not yet known. If the spots are eruptional
in character, the forces held in check during seasons
of few spots may well break out in period. The
brighter streaks and mottlings known as f aculse are
probably elevations above the general photosphere,
and seem to be crusts of luminous matter, often in-
candescent calcium, protruding through from the
lower levels. Generally the faculae are numerous
around the dark spots, and absorption of the sun's
light by his own atmosphere affords a darker back-
ground for them, with better visibility nearer the
rim of the solar disk. The spectroheliograph re-
veals vast zones of faculse otherwise invisible,
related to the sun-spot zones proper on both sides
of the equator.
In some intimate way the magnetism of sun and
earth are so related that outbreaks of solar spots
are accompanied with disturbances of electrical and
other instruments on the earth; also the aurora
borealis is seen with greater frequency during
periods when many spots are visible.
SUN SPOTS AND PROMINENCES 177
Within very recent years the discovery of a mag-
netic field in sun spots has been made by Hale with
powerful instruments of his own design. Sun spots
had never been investigated before with adequate
instrumental means. He recognized the necessity
of having a spectroscope that would record the
widened lines of sun-spot spectra, and the strength-
ened and weakened lines on a large scale. Certain
changes in relative intensity were traced to a re-
duced temperature of the spot vapors by compari-
son with photographs of the spectrum of iron and
other metallic vapors in an electric arc at different
temperatures. Here the work of the laboratory was
essential. Sun spots were thus found to be regions
of reduced temperature in the solar atmosphere.
Chemical unions were thus possible, and thousands
of faint lines in spot-spectra were measured and
identified as band lines due to chemical compounds.
Thus the chemical changes at work in sun-spot
vapors were recognized.
Then followed the highly significant investiga-
tions of solar vortices and magnetic fields. Improve-
ments in photographic methods had revealed im-
mense vortices surrounding sun spots in the higher
part of the hydrogen atmosphere ; and this led to the
hypothesis that a sun spot is a solar storm, resem-
bling a terrestrial tornado, and in which the hot
vapors whirling at high velocity are cooled by ex-
pansion. This would account for the observed inten-
sity changes of the spectrum lines and the presence
of chemical compounds. The vortex hypothesis sug-
gested an explanation of the widening of many spot
lines, and the doubling or trebling of some of them.
As it is known that electrons are emitted by hot
bodies, they must be present in vast numbers in the
178 ASTRONOMY TO-DAY
sun; and positive or negative electrons, if caught
and whirled in a vortex, would produce a magnetic
field.
Zeeman in 1896 had discovered that the lines in
the spectrum of a luminous vapor in a magnetic
field are widened, or even split into several com-
ponents if the field is strong enough. Characteristic
effects of polarization appear also. The new ap-
paratus of the observatory in conjunction with ex-
periments in the laboratory immediately provided
evidence that proved the existence of magnetic fields
in sun spots, and strengthened the view that the
spots are caused by electric vortices.
Extended investigations have led Hale to the con-
clusion that the sun itself is a magnet, with its poles
situated at or near the poles of rotation. In this re-
spect the sun resembles the earth, which has long
been known to be a magnet. The sun's axial rota-
tion permits investigation of the magnetic phe-
nomena of all parts of its surface, so that ultimately
the exact position of the sun's magnetic poles and the
intensity of the field at different levels in the solar
atmosphere will be ascertained. Schuster is of the
opinion that not only the sun and earth, but every
star, and perhaps every rotating body, becomes a
magnet by virtue of its rotation. Hale is confident
that the 100-inch reflector will permit the test for
magnetism to be applied to a few of the stars.
The sun can be observed at Mount Wilson on at
least nine-tenths of all the days in the year, and a
daily record of the polarities of all spots with the
150-foot tower telescope is a part of the routine. A
method has been devised for classifying sun spots on
the basis of their magnetic properties, and more
than a thousand spots have already been so classi-
SUN SPOTS AND PROMINENCES 179
fled. About 60 per cent of all sun spots are found
to be binary groups, the single or multiple members
of which are of opposite magnetic polarity. Uni-
polar spots are very seldom observed without some
indication of the characteristics of bipolar groups.
These are usually exhibited in the form of flocculi
following the spot. The bipolar spot seems to be
the dominant type, and the unipolar type a variant
of it
Although devised for quite another purpose, that
of photographing the hydrogen prominences on the
limb of the sun, the spectroheliograph has con-
tributed very effectively to many departments of
solar research. The prominences are dull reddish
cloudlets that were first seen during total eclipses
of the sun. Probably Vassenius, a Swedish astrono-
mer, during the total eclipse of 1733, made the
earliest record of them, as pinkish clouds quite de-
tached from the edge of the moon ; and in that day,
when it had not yet been proved that the moon was
without atmosphere, he naturally thought they be-
longed to the moon, not the sun. Undoubtedly Ulloa,
a Spanish admiral, also saw the prominences in ob-
serving the total eclipse of 1778 ; but they seem to
have attracted little attention till 1842, when a very
important total eclipse was central throughout
Europe, and observed with great care by many of
the eminent astronomers of all countries.
So different did the prominences appear to dif-
ferent eyes, and so many were the theories as to
what they were, that no general consensus of opinion
was reached, and some thought them no part of
either sun or moon, but a mere mirage or optical
illusion. But at the return of this eclipse in 1860,
photography was employed so as to demonstrate be-
180 ASTRONOMY TO-DAY
yond a shadow of doubt the real existence and true
solar character of the prominences. By the slow
progress of the moon across the sun and the prom-
inences on the edge, a unique series of photographs
by De la Rue showed the moon's edge gradually
cutting off the prominences piecemeal on one side
of the sun, and equally gradually uncovering them
on the opposite side.
The prominences, then, were known to be real
phenomena of the sun, some of them disconnectedly
floating in his atmosphere, as if clouds. Their forms
did not vary rapidly, they were* very abundant, and
their light was so rich in rays of great photographic
intensity that many were caught on the plate which
the eye failed to see; they appeared at every part
of the sun's limb and their height above it indicated
that they must be many thousand miles in actual
dimension. What they were, however, remained an
entire mystery, and no one even thought it possible
to find out what their chemical constitution might
be or to measure the speed with which they moved.
A few years later came the great Indian eclipse
(August 28, 1868), at that date the longest total
eclipse ever observed. Janssen of France and many
others went out to India to witness it. Fortunately
the prominences were very brilliant and this led
Janssen to believe it would be possible for him to
see them the day after the eclipse was over. By
modifying the adjustment of his apparatus suitably
and changing its relation to the sun's edge, he found
that hydrogen is the main constituent in the light of
the prominences. In addition to this he was able
to trace out the shapes of the prominences, and even
measure their dimensions. His station in India was
at Guntoor, many weeks by post from home ; so that
SUN SPOTS AND PROMINENCES 181
his account of this important discovery reached the
Paris Academy of Sciences for communication with
another from the late Sir Norman Lockyer of Eng-
land, announcing a like discovery, wholly inde-
pendently.
The principle is simply this, and admirably stated
by Young : "Under ordinary circumstances the prom-
inences are invisible, for the same reason as the
stars in the daytime : they are hidden by the intense
light reflected from the particles of our own atmos-
phere near the sun's place in the sky; and if we
could only sufficiently weaken this aerial illumina-
tion, without at the same time weakening their
light, the end would be gained. And the spectro-
scope accomplishes this very thing. Since the air-
light is reflected sunshine, it of course presents the
same spectrum as sunlight, a continuous band of
color crossed by dark lines. Now, this sort of
spectrum is greatly weakened by every increase of
dispersive power, because the light is spread out into
a longer ribbon and made to cover a more extended
area. On the other hand, a spectrum of bright lines
undergoes no such weakening by an increase in the
dispersive power of the spectroscope. The bright
lines are only more widely separated not in the
least diffused or shorn of their brightness."
Simultaneous announcement of this great dis-
covery, by astronomers of different nations, work-
ing in widely separate regions of the earth, led to
the striking of a gold medal by the French Govern-
ment in honor of both astronomers and bearing their
united effigies. Ever since the famous Indian eclipse
of 1868, it has not been necessary to wait for a total
eclipse in order to observe the solar prominences,
but every observer provided with suitable apparatus
182 ASTRONOMY TO-DAY
has been able to observe them in full sunlight when-
ever desired, and the charting of them is part of the
daily routine at several observatories in different
parts of the world. So vast has been the accumula-
tion of data about them that we know their numbers
to fluctuate with the spots on the sun ; and their dis-
tribution over the sun's surface resembles in a way
that of the spots.
While the spots and protuberances are most nu-
merous around solar latitude 20 degrees both north
and south, the prominences do not disappear above
latitude 35 to 40 degrees, as the spots do, but from
latitude 60 degrees they increase in number to about
75 degrees, and are occasionally observed even at
the sun's poles. Faculse and prominences are more
closely related than the sun spots and prominences.
There are wide variations in both magnitude and
type of the prominences. Heights above the sun's
limb of a few thousand miles are very common, and
they rarely reach elevations as great as 100,000
miles, though a very occasional one reaches even
greater heights.
Classification of the prominences divides them
into two broad types, the quiescent and the eruptive.
The former are for the most part hydrogen, and
the latter metallic. The quiescent prominences re-
semble closely the stratus and cirrus type of terres-
trial clouds, and are frequently of enormous extent
along the sun's edge. They are relatively long-lived,
persisting sometimes for days without much change.
The eruptive prominences are more brilliant, chang-
ing their form and brightness rapidly. Often they
appear as brilliant spikes or jets, reaching altitudes
that average about 25,000 miles. Rarely seen near
the sun's poles, they are much more numerous nearer
SUN SPOTS AND PROMINENCES 183
the sun spots. Speed of motion of their filaments
sometimes exceeds one hundred miles a second, and
the changing variety of shapes of the eruptive prom-
inences is most interesting. Oftentimes they change
so rapidly that only photography can do them justice.
Prominence photography began with Young a
half century ago, who obtained the first successful
impression on a microscope slide with at sensitized
film of collodion; as was necessary in the earlier
wet-plate process of photography, which required
exposures so long that little progress was effected
for about twenty years. Then it was taken up by
Deslandres of Paris and Hale of Chicago independ-
ently, both of whom succeeded in devising a com-
plex type of apparatus known as the spectrohelio-
graph, by which all the prominences surrounding
the entire limb of the sun can be photographed at
any time by light of a single wave-length, together
with the disk of the sun on the same negative.
The prominences appear to be intimately con-
nected with a gaseous envelope surrounding the solar
photosphere, in which sodium and magnesium are
present as well as hydrogen. The depth of the
chromosphere is usually between 5,000 and 10,000
miles, and its existence was first made out during
the total solar eclipses of 1605 and 1706, when it ap-
peared as an irregular rose-tinted fringe, though not
at the time recognized as belonging to the sun.
The constitution of the sun and its envelopes are
still under discussion, and no complete theory of the
sun has yet been advanced which commands the
widest acceptance. Of the interior of the sun we can
only surmise that it is composed of gases which, be-
cause of intense heat and compression, are in a state
unfamiliar on earth and impossible to reproduce in
184 ASTRONOMY TO-DAY
our laboratories. Their consistency may be that of
melted pitch or tar.
Surrounding the main body of the sun are a series
of layers, shells, or atmospheres. Outside of all and
very irregular in structure, indeed probably not a
solar atmosphere at all, is the solar corona, parts
of which behave much as if it were an atmosphere,
but it appears to be bound up in some way with the
sun's radiation. It has streamers that vary with the
sunspot period, but its constitution and function are
very imperfectly known, because it has never been
seen or photographed except at rare intervals on
occasion of total eclipses of the sun.
Beneath the corona we meet the projecting prom-
inences, to which parts of the corona are certainly
related, and beneath them the first true layer or
atmosphere of the sun known as the chromosphere,
its average depth being about one-hundredth part of
the sun's diameter. Beneath the chromosphere is
the layer of the sun from which emanates the light
by which we see it, called the photosphere. It ap-
pears to be composed of filaments due to the con-
densation of metallic vapors, and it is the outer
extremities of these filaments which are seen as
the granular structures everywhere covering the
disk of the sun. Their light shines through the
chromosphere and the spots are ruptures in this
envelope.
Between photosphere and chromosphere is a very
thin envelope, probably not over 700 miles in thick-
ness, called the reversing layer. It is this relatively
thin shell that is responsible for the absorption
which produces the dark lines in the spectrum of
the sun. Under normal conditions the filaments of
the photosphere are radial, that is vertical on the
SUN SPOTS AND PROMINENCES 185
sun; but whenever eruptions take place, as during
the occurrence of spots, the adjacent filaments are
violently swept out of their normal vertical lines
and these displaced columns then form what we view
as the spot's penumbra. From the outer surface of
the sun's chromosphere rise in eruptive columns
vapors of hydrogen and the various metals of which
the sun is composed. These and the spots would
naturally occur in periods just as we see them.
We have said that the sun is composed of a mass
of highly heated or incandescent vapors or gases,
whose compression on account of gravity must
render their physical condition quite different from
any gaseous forms known on the earth or which we
can reproduce here. As the result of more than
half a century of studious observation of the sun
and mapping of its spectrum in every part, and dili-
gent comparison with the spectra of all known chem-
ical elements on the earth, we find that the sun con-
tains no elements not already found here, but that
a great preponderance of elements known to earth
are found in the sun.
The intensity of their spectral lines is one prom-
inent indication of the presence of elements in the
sun, and the number of coincidences of spectral
lines is another. Iron, nickel, calcium, manganese,
sodium, cobalt, and carbon are among the elements
most strongly identified. A few of the rarer ter-
restrial elements are of doubtful existence in the sun,
and a very few, as gold, bismuth, antimony, and
sulphur are not found there, and the existence of
oxygen in the sun is regarded by some experts as
doubtful. But if the whole earth were vaporized by
heat, probably its spectrum would resemble that of
the sun very closely.
186 ASTRONOMY TO-DAY
What are the effects of the sun, and sun spots in
particular, on our weather? Is the influence of their
periodicity potent or negligible? If we investigate
conditions pertaining to terrestrial magnetism, as
fluctuations of the magnetic needle, and the fre-
quency of aurorse, there is no occasion for doubt of
the sun's direct influence, although we are not able
to say just how that influence becomes potent. If,
however, we look into questions of temperature,
barometric pressure, rainfall, cyclones, crops, and
^consequent financial conditions, we find fully as much
evidence against solar influence as for it. The slight
variations of the sun's light and heat due to the
presence or absence of sun spots can scarcely be
sensible, and much longer periods of closer observa-
tion are necessary before such questions can be
finally decided. The slighter such influences are, if
they actually exist, and the more veiled they are by
other influences more or less powerful, the more dif-
ficult it is to discover their effects with certainty.
The importance of solar radiation in the predic-
tion of terrestrial weather has long been recognized,
but until very recently no practical application has
been made. The Smithsonian Astrophysical Ob-
servatory at Washington, under the direction of Dr.
Abbot, has for many years carried on at a number
of stations a series of determinations of the constant
of solar radiation by the spectro-bolometric method
originated by Langley. A new station in Calama,
Chile, has recently been inaugurated, at which the
solar constant is worked out each day, and tele-
graphed to the Argentine weather service, where
it is employed in forecasting for the day.
Abbot's new method of solar constant determina-
tion is based on the fact that atmospheric transpar-
SUN SPOTS AND PROMINENCES 187
ency varies oppositely to the variations of bright-
ness of the sky. Increase of haziness presents more
reflecting surface to scatter the solar rays indirectly
to the earth. Of course it presents also additional
surface to obstruct the direct rays from the sun. By
measuring the brightness of the sky near the sun,
it becomes possible to infer the coefficients of at-
mospheric transmission at all wave lengths. The
direct observations and the complete deduction of
the solar constant for the day can all be completed
within two or three hours.
Clayton of Buenos Aires has now employed these
results in the Argentine weather predictions for two
years, and the introduction of this new element in
forecasting has brought about a pronounced gain in
the value of the predictions. Its adoption by the
weather bureaus of other nations will doubtless
come in due time, and the new method take a firmly
established rank in practical meteorology.
Abbot's observations many years ago first called
attention to the variability of the solar constant
through a range of several per cent both from year
to year, and in irregular short periods of weeks or
even days. Abbot considers this the more likely
explanation than that atmospheric changes should
take place simultaneously all over the earth. The
sun is but a star, the stars that are irregularly
variable in light and heat are numerous, and the
sun itself appears to be one of these.
Especially important to the agricultural and vine-
yard interests of Argentina is the question of pre-
cipitation, and Clayton finds this very dependent on
solar radiation. At epochs of practically stationary
solar intensity, there is little or no precipitation;
but quite generally he finds that great decrease of
188 ASTRONOMY TO-DAY
solar radiation is followed in from three to five days
by heavy precipitation. Direct temperature effects
are also traced in Buenos Aires and other South
American cities, lagging from two to three days
behind the observed solar fluctuations.
The station at Calama yields about 250 determina-
tions of the solar constant each year, and the Mount
Wilson station about half that number. They are
the only stations of this character at present in ex-
istence, and others should be established in widely
separated and cloudless regions, as Egypt, southern
California and Australia. Uniformity in the
methods of observing would be highly desirable,
and the Smithsonian Institution has perfected the
details of common control of such stations which it
is expected may be established at an early day.
CHAPTER XXVII
THE INNER PLANETS
1
VULCAN
ABOUT the middle of the last century, Le Verrier,
a great French astronomer, having added the
planet Neptune beyond the outside confines of the
solar system, sought evidence of a lesser planet trav-
eling round the sun within the orbit of Mercury.
For many years close watch was kept on the sun in
the hope of discovering such a body in the act of
passing across the disk, or in transit, as it is
technically termed. Lescarbault, a French phy-
sician, announced that he had actually seen such a
planet, Vulcan it was called, passing over the sun
in 1859. Total eclipses of the sun would afford the
best opportunity for seeing such a body, and on
several such occasions astronomers thought they had
found it. But the signal advantages of photography
have been applied so often to this search, and always
unsuccessfully, that the existence of Vulcan, or the
intramercurian planet, is now regarded as mythical.
MERCURY
This planet is an elusive body that very few, even
astronomers, have ever seen. It is not very bright,
has a rapid motion and never retreats far from the
sun, so that it was a puzzle to the ancients who saw
it, sometimes in the twilight after sunset and again
in the twilight of dawn. When following the sun
180
190 ASTRONOMY TO-DAY
down in the west, in March or April, Mercury is
likely to be best seen ; twinkling rather violently and
nearly as bright as a star of the first magnitude.
Very little is to be seen on the minute disk of this
planet, except that it goes through all the phases of
the moon crescent, gibbous, full, gibbous, crescent.
Whether Mercury turns round on its axis or not,
cannot be said to be known, because the markings
that are suspected on its surface are too indefinite
to permit exact observation. More than likely the
planet presents always the same side or face to the
sun, so that it turns round on its axis once, while
traveling once around the sun in its orbit. Mercury's
day and year would therefore be equal in length.
Nor have we much evidence on the question of an
atmosphere surrounding Mercury; probably it is
very thin, if indeed there is any at all. When Mer-
cury comes directly between us and the sun, cross-
ing in transit, the edge of the planet as projected
against the sun is very sharply defined, and this
would indicate an absence of atmosphere on Mercury.
Transits of Mercury can occur in May and
November only : there was one on November 7, 1914,
and there will be one on May 7, 1924. The latter
will be nearly eight hours in length, which is almost
the limit. Mercury's distance from the sun averages
36 million miles, the diameter of the planet is 3,000
miles, and his orbital speed is 30 miles per second,
the swiftest of all the planets. No moon of Mercury
is known to exist, although many times diligently
searched for, especially during transits of the planet.
VENUS
Brightest of all the planets, and the most beauti-
ful of all is Venus. Its path is next outside the orbit
THE INNER PLANETS 191
of Mercury, but within that of the earth, so that it
partakes of all the phases of the moon. Like Mer-
cury it sometimes passes exactly between us and
the sun, a rare phenomenon which is known as a
transit of Venus.
Being without telescopes, the ancients knew noth-
ing about these occurrences, but they were puzzled
for centuries over the appearance of the planet in
the west after sunset, when they called it Hesperus,
and in early dawn in the east when they gave it the
name Phosphorus.
Venus is known to be girdled with an atmosphere
denser than ours, and it seems to be always filled
with dense clouds. It is the reflection of sunlight
from this perpetually cloudy exterior which gives
Venus her singular radiance. So brilliant is she
that even full daylight is not strong enough to over-
power her rays ; and she may often be seen glisten-
ing in the clear blue daytime sky, if one knows pretty
nearly in what direction to look for her.
Venus is 67 million miles from the sun, and as
our own distance is 93 million miles, this planet
can come within 26 million miles of the earth. It
is therefore at times our nearest known neighbor in
space, excepting only the Moon and Eros, one of the
erratic little planets that travel round the sun be-
tween Mars and Jupiter. Also possibly a comet
might come much nearer.
Astronomers always take advantage of this near-
ness of Venus to us, if a transit across the sun takes
place ; because it affords an excellent method of find-
ing out what the distance of the sun is from the
Earth. A pair of these transits happens about once
a century, there were transits in 1874 and 1882, and
the next pair occur in 2004 and 2012. In actual size,
192 ASTRONOMY TO-DAY
Venus is almost as large a planet as our own, being
7,700 miles in diameter, as compared with 7,920
for the earth. Her velocity in her orbit is twenty-
two miles per second, and she travels all the way
round the sun in seven and one half months or 225
days.
Venus from her striking brilliancy always leads
the novice to expect to see great things on applying
the telescope. But aside from a brilliant disk, now
a slender crescent, now half full like the moon at
quarter, and again gibbous as the moon is between
quarter and full, the telescope reveals but little.
There is pretty good evidence that the markings
thought to have been seen on the planet's surface
are illusory, and so it is wholly uncertain in what
direction the planet's axis lies; also there is great
uncertainty about the length of the day on Venus,
or the period of turning round on its axis. Probably
it is the same in length as the planet's year.
Once when Venus passed very close to the sun,
just barely escaping a transit, Lyman of Yale Uni-
versity caught sight of it by hiding the sun behind
a tall building or church spire. The dark side of
Venus was turned toward us and he could not of
course see that. But the planet was clearly there,
completely encircled by a narrow delicate luminous
ring, which was due to sunlight shining through the
atmosphere that surrounds the planet. Similar
ring effects were seen by observers of the transits
of Venus in 1874 and 1882 ; and from all their ob-
servations it is concluded that Venus has an at-
mosphere probably at least twice as dense and ex-
tensive as that which encircles the earth. Spurious
satellites of Venus are many, but no real moon is
known to attend this planet.
THE SURFACE OF THE MOON IN THE REGION OF COPERNICUS. Photograph
made with the Hooker 100-inch reflecting telescope. (Photo, Mt. Wilson
Solar Observatory.)
8SR5^< ' ^ l i*sl?A..i
A VIEW OP THE SOUTH CENTRAL PORTION OF THE MOON AT LAST
QUARTER. (Photo, Mt. Wilson Solar Observatory.)
CHAPTER XXVIII
THE MOON AND HER SURFACE
A S the sun has always reigned as king of day, so
XJL is the moon queen of night. Observation of her
phases, now waxing, now waning, with her stately
motion always eastward among the stars, began
with the earliest ages. Often when near the full
she must have been seen herself eclipsed, and much
more rarely the occurrence of total eclipses of the
sun are certain to have suggested the moon's inter-
vention between earth and sun, shutting off the sun-
light completely, because these eclipses never took
place except when the moon was in the same part
of the sky with the sun.
If we watch the nightly march of the moon, we
shall find that she travels over her own breadth in
about an hour's time. By using a telescope on the
stars just eastward or to the left of her, she will now
and then be seen to pass between us and a star on
very rare occasions a planet extinguishing its light
with great suddenness, the most nearly instantane-
ous of all phenomena in nature. Draw a line con-
necting the cusps, or horns of the lunar crescent, and
then a line eastward at right angles to this, and it
will show the direction of the moon's own motion in
its orbit round the earth quite accurately v
As the phase advances, note the inside edge of the
advancing crescent: this will be quite rough and
jagged, compared to the outside edge which is the
193 Sci. Vol. 27
194 ASTRONOMY TO-DAY
-moon's real contour and relatively very smooth. The
position of the inside curve will change from night
to night, and it marks the line of sunrise on the
moon during the fortnight elapsing between new
moon and full ; while from full through last quarter
and back to new moon, this advancing line marks
the region of sunset on the moon. The general shape
of this line is never a circle but always elliptical,
and astronomers call it the terminator. All along
the terminator, sunlight strikes the lunar surface
at a small angle, whether near sunrise or sunset;
so that owing to the mountains and other high
masses of the moon's surface, the terminator
is always a more or less jagged and irregular
line.
Onward from new moon toward full the horns of
the crescent are always turned upward or eastward.
When the general line of the terminator becomes
a straight line from cusp to cusp, the moon is said
to have reached first quarter or quadrature. On-
ward toward full the terminator will be seen to bend
the other way, and in about a week's time it will have
merged itself with the moon's limb. The moon is
then said to be full. Afterward the phase phenom-
ena recur in the reverse order, with third quarter
midway between full and new moon again ; the phase
of the moon being called gibbous all the way from
first quarter to third quarter, except when exactly
full.
As we know that the moon is, like the earth, a
nonluminous body, and shines only by virtue of the
sunlight falling upon it, clearly an entire half of the
moon's globe must be perpetually illumined by sun-
light. The varying phases then are due simply to
that part of the illuminated hemisphere which is
THE MOON AND HER SURFACE 195
turned toward us. New moon is entirely invisible
because the sunward hemisphere is turned wholly
away from us, while at full moon we see the lunar
disk complete because we are on the same side of
the moon that the sun is and practically in line with
both sun and moon.
If we could visit the moon, we should see the earth
in exactly complementary phase. At new moon here
we should be enjoying full earth there, and full moon
here would be coincident with new or dark earth
there. The narrow crescent of new moon here would
be the period of gibbous earth there; and it is the
reflection of sunlight from this gibbous earth which
illuminates the part of the moon but faintly seen
at this time, popularly known as the "old moon in
the new moon's arms." Its greater visibility at some
times than at others is due to greater prevalence of
clouded area in the reflecting regions of the earth
turned toward the moon, and the higher reflective
power of clouds than that possessed by mere land
and water.
As the moon goes all the way round the sky every
month, the same as the sun does in a year, and
travels in nearly the same path, clearly it must also
go north and south every month as the sun does.
So in midsummer when the sun runs high upon the
meridian, we expect to find full moons running low,
and likewise in midwinter the full moon always runs
high, as almost everyone has sometimes or other
noticed.
This eastward or true orbital motion of the moon
is responsible for another relation which soon comes
to light when we begin to observe the moon; and
that is the later hour of rising or setting each night.
Our clock time is regulated by the sun, which also
196 ASTRONOMY TO-DAY
is moving eastward about 1 daily, or twice its own
breadth. So the moon's eastward gain on the sun
amounts to about 12 degrees daily, and one degree
being equal to 4 minutes, the retarded time of moon-
rise or moonset each day amounts to very nearly 50
minutes on the average; though sometimes the de-
lay will be less than a half hour and at other times
it will exceed an hour and a quarter. The season
of least retardation of rising of the full moon is in
the autumn, and so the moon that falls in late Sep-
tember or October is known as the Harvest moon,
and the next succeeding full moon is called the
Hunter's moon.
Lunation is a term sometimes given to the moon's
period from any definite phase round to the same
phase again. Its length is the true period of the
moon's revolution once around the earth, from the
sun all the way round till it overtakes the sun again.
The synodic period is another name for lunation,
and its true length is 29 and one-half days, or very
accurately 29 d. 12 h. 44 m. 2.7 s. as calculated by
astronomers with great exactness from many thou-
sand revolutions of the moon. But if we want the
true period of the moon round the earth as referred
to a star, it is much shorter than this, amounting to
only 27 days and nearly one-third. This is called
the moon's sidereal period of revolution, because it
is the time elapsed while she is traveling eastward
from a given star around to coincidence with the
same star again.
If we study the moon's path in the sky more
critically, we shall find that it does not quite follow
the ecliptic, or the sun's path, but that twice each
month she deviates from the ecliptic, once to the
north and once to the south of it, by roughly ten
THE MOON AND HER SURFACE 197
times her own breadth. More accurately this angle
is 58'40", an almost invariable quantity, and it is
therefore known as an astronomical constant, or the
inclination of the moon's orbit to the ecliptic. So
the moon's orbit must intersect the ecliptic, and
as both are great circles in the sky, the points of in-
tersection are known as the moon's nodes, one as-
cending and the other descending, and the nodes are
180 degrees apart.
The figure of the moon's orbit is not circular, al-
though it deviates only slightly from that form. But
like the paths of all other satellites round their
primary planets, and of the planets themselves round
the sun, the moon's orbit is also an ellipse. The dis-
tance of the moon's center from the earth's center
is therefore perpetually changing; the point of
nearest approach is called perigee, and that of farth-
est recession, apogee.
The moon's distance from the earth is easier
and simpler to be ascertained than that of any other
heavenly body, because it is the nearest. An out-
line of the method of finding this distance is not
difficult to present ; and it resembles in every partic-
ular the method a surveyor uses to find the distance
of some inaccessible point which he cannot measure
directly. Up and down a stream, for example, he
measures the length of a line, and from each end of
it he measures the angle between the other end of
the line and the object on the opposite side of the
stream whose distance he wishes to find out. Then
he applies the science of trigonometry to these three
measures, two of angles and one the length of the
side or base included between them, and a few min-
utes' calculation gives the distance of the inacces-
sible object from either end of the base line.
ASTRONOMY TO-DAY
Now in like manner, to transfer the process to the
sky, let the two ends of the base be represented by
two astronomical observatories, for example, Green-
wich in the northern hemisphere and Cape Town in
the southern. The base line is the chord or straight
line through the earth connecting the two observa-
tories, and we know the length of this line pretty ac-
curately, because we know the size of the earth. The
angles measured are somewhat different from those
in the terrestrial example, but the process amounts
to the same thing because the astronomers at the
two observatories measure the angular distance of
the center of the moon from the zenith, each using
his own zenith at the same time; and the same
science of trigonometry enables them to figure out
the length of any side of the triangles involved. The
side which belongs to both triangles is the distance
from the center of the earth to the center of the
moon, and the average of many hundred measures
of this gives 238,800 miles, or about ten times the
distance round the equator of the earth.
We have said that the orbit in which the moon
travels round the earth is practically a circle, but
the earth's center is found not at the center of
this orbit, but set to one side, or eccentrically* so
that the distance spanning the centers of the two
bodies is sometimes as small as 221,610 miles at
perigee, and 252,970 miles at apogee. The moon's
speed in this orbit averages rather more than half
a mile every second of time more accurately 3,850
feet a second, or 2,290 miles per hour.
Once the moon's distance is known, its size or
diameter is easy to ascertain. An angular measure
is necessary, of course, that of the angle which
the disk of the moon fills as seen from the earth.
THE MOON AND HER SURFACE 199
There are many types of astronomical instruments
with which this angle can be measured, and its
value is something more than half a degree (31' 7").
The moon's actual diameter figures out from this
2,163 miles; and it would therefore require nearly
fifty moons merged in one to make a ball the size
of the earth.
Still, no other planet has a satellite as large in
proportion to its primary as the moon is in rela-
tion to the earth. But the materials that com-
pose the moon have less than two-thirds the
average density of those that make up the earth,
so that eighty-one moons fused together would be
necessary to equal the mass or weight of the earth.
If we figure out the force of attraction of the moon
for bodies on its surface, we find it equals about
one-sixth that of the earth. Athletes could per-
form some astounding feats there miracles of high
jump and hammer-throw.
Our interest in the moon's physical characteristics
never wanes. Her nearness to us has always fas-
cinated astronomer and layman alike. Early users
of the telescope were readily led into error regard-
ing the general characteristics of the lunar surface ;
and it is easy to see why they thought the smooth
level planes must be seas, and gave them names to
that effect which persist to-day, as Mare Crisium,
Mare Serenitatis and so on. We may be sure that
no water exists on the moon's surface, although
some astronomers think that solid water, as ice or
snow, may still exist there at a temperature too low
for appreciable evaporation.
Perhaps water, seas, and oceans were once there,
but their secular dissemination and loss as vapor
have gone on through the millions of millions of
200 ASTRONOMY TO-DAY
years till even the moon's atmosphere appears to
have vanished completely. At least there is much
better evidence of absence of atmosphere on the
moon than of its presence not enough at any rate
to equal a thousandth part of the barometric pres-
sure that we have at the earth's surface. Frequent
observations of stars passing behind the moon
in occultation have satisfied astronomers on this
point.
We often say of the brilliant full moon, it is as
bright as day. The photometer or instrument for
accurate comparison of lights, their amount and in-
tensity, tells a different story. Indeed, if the en-
tire dome of the sky were filled with full moons, we
should be receiving only one-eighth of the light the
sun gives us, and it would require more than 600,-
000 average full moons to equal the light radiation
of the sun. Heat from the moon, however, is quite
different. Early attempts to measure it detected
none at all, but with modern instruments there is
little trouble in detecting heat from the moon,
though measurement of it is not easy.
Much of the moon's heat is sun heat, directly re-
flected from the moon, as sunlight is, but most of it
is due to radiation of solar heat previously absorbed
by the materials of the lunar surface. The actual
temperature of the moon's surface suffers great
variation. A fortnight's perpetual shining of the
sun upon the lunar rocks would certainly heat them
above the temperature of boiling water, if the moon
had an atmosphere to conserve and store this heat;
but the entire absence of such an air blanket proba-
bly permits the sun's heat to be radiated away nearly
as fast as it is received, leaving the temperature at
the surface always very low.
THE MOON AND HER SURFACE 201
What physical influences the moon really has upon
the earth must be very slight, barring the tides.
But there is little hope of getting people generally
to take that view, because the moon appears to be
the planet of the people, and opinion that the moon
controls the weather, for instance, amounts with
them to practical certainty. More than likely all
these notions are but legitimate survivals of super-
stition and astrology. In addition to the tides, our
magnetic observatories reveal slight disturbances
with the swinging of the moon from apogee to peri-
gee and back; but long series of weather observa-
tions have been faithfully interrogated, with nega-
tive or contradictory results. If one believes that
the moon's changes affect the weather, it is easy to
remember coincidences, and pass over the many
times when no change has taken place. The moon
changes pretty frequently anyhow. As Young well
puts it: "A change of the moon necessarily occurs
about once a week All changes, of the weather
for instance, must therefore occur within three and
three-fourth days of a change of the moon, and fifty
per cent of them ought to occur within forty-six
hours of a change, even if there were no causal con-
nection whatever."
When we turn to the strongly diversified surface
of the moon itself, we find much to rivet the at-
tention, even with slender optical aid. Everyone
wants to know how near the telescope, the biggest
possible telescope, brings the moon to us. That will
depend on many things, first of all on the magnify-
ing power of the eyepiece employed on the telescope,
and eyepieces are changed on telescopes just as they
are on microscopes, though not for the same reasons.
The theoretical limit of the power of a telescope is
202 ASTROISTOMY TO-DAY
usually considered as 100 for each inch of diameter
or aperture of the object glass.
A 40-inch telescope, as that of the Yerkes
Observatory, the largest refracting telescope in
existence, should bear a magnifying power not to
exceed 4,000. But this limit is practically never
reached, one-half of it or fifty to the inch of aperture
being a good working limit of power, even under
exceptional conditions of steadiness of atmosphere.
If we reduce the effective distance of the moon from
240,000 miles to 100 miles, that is about the utmost
that can be expected. But even at that distance we
can make out only landscape details, nothing what-
ever like buildings or the works of intelligence.
The larger relations of light and shade, so
obvious to the naked eye on the moon, vanish on
looking at it with the telescope, but we are at once
captivated by the novel character of the surface
and the seemingly great variety of detail that is
clearly visible. As soon as the new moon comes
out in the west, one may begin to gaze with in-
terest and watch the terminator or sunrise line
gradually steal over the roughened surface, bring-
ing new and striking craters into view each night.
Around the time of quarter moon, or a little past
it, is one of the best times for telescopic views of
the moon, because the huge craters, Tycho and
Copernicus, are then in fine illumination. Close
to the phase of full moon is never a good time,
because there are no shadows of the rough surface
then, and its entire structure seems to be quite
flat and uninteresting, except for the streaks or
rills which radiate from Tycho in every direction,
and are the only lunar features that are best seen
near full.
THE MOON AND HER SURFACE 203
In a broad, general way, the moon's surface, if
compared with the earth's, differs in having no.
water. Our extensive oceans are replaced there by
smooth, level plains which were at first thought to
be seas and so named. There are ten or twelve of
them in all. Then we find mountain ranges, so
numerous on the earth, relatively few on the moon.
Those that exist are named, in part, for terrestrial
mountain ranges, as the Alps, Caucasus, and the
Apennines.
But the nearly circular crater, a relatively rare
formation on the earth, is seen dotted al} over the
moon in every size, from a fraction of a mile in
diameter up to sixty, seventy, and in extreme cases
a hundred miles. No mere description of plains
and mountains and craters affords an adequate
idea of the moon's surface as it actually is; a
telescopic view is necessary, or some of the modern
photographs which give an even better notion of
the moon than any telescopic view. Many of the
lunar craters are without doubt volcanic in origin,
others seem to be ruins of molten lakes* Many
thousands of the smaller ones appear as if formed
by a violent pelting of the surface when semi-
plastic, perhaps by enormous showers of meteoric
matter. More than 30,000 craters coyer the half
of the lunar surface visible from the earth, and
hundreds of them are named for philosophers an<J
astronomers.
Measurement of the height of lunar mountains
has been made in numerous instances, especially
when their shadows fall on plains or surfaces that
are nearly level, so that the length of t}ie shadow
can be measured. In general, the height of lunar
peaks is greater than that of terrestrial peaks,
204 ASTRONOMY TO-DAY
owing probably to the lesser surface gravity on
the moon. About forty lunar peaks are higher
than Mont Blanc.
Most astronomers regard it as certain that no
changes ever take place on the moon; probably no
very conspicuous changes ever do. Some, however,
have made out a fair case for comparatively recent
changes in surface detail. Extreme caution is neces-
sary in drawing conclusions, because the varying
changes of illumination from one phase to another
are themselves sufficient to cause the appearance
of change. At intervals of a double lunation, equal
to fifty-nine days, one and one-half hours, the
terminator goes very nearly through the same
objects, so that the circumstances of illumination
are comparable. In Mare Serenitatis the little
crater named Linne was announced to have disap-
peared about a half century ago; subsequently it
became visible again and other minor changes were
reported, perhaps due to falling in of the walls of
the crater.
If one were to visit the moon, he must needs
take air and water along with him, as well as
other sustenance. No atmosphere means no dif-
fused light; we could see nothing unless the sun's
direct rays were shining upon it. Anyone stepping
into the shadow of a lunar crag would become
wholly invisible. No sound, however loud, could be
heard; sound in fact would become impossible. A
rock might roll down the wall of a lunar crater,
but there would be no noise; though we should
know what had happened by the tremor produced.
So slight is gravity there that a good ball player
might bat a baseball half a mile or more. Looking
upward, all the stars would be appreciably brighter
THE MOON AND HER SURFACE 205
than here, and visible perpetually in the daytime
as well as at night.
If one were to go to the opposite side of the
moon, he would lose sight of the earth until he
came back to the side which is always turned to-
ward the earth. Even then the earth would never
rise and set at any given place, as the moon does
to us, but would remain all the time at about the
same height above the lunar horizon. The earth
would go through all the phases that the moon
shows to us here, full earth occurring there when
it is new moon here. Our globe would appear to
be nearly four times broader than the moon seems
to us. Its white polar caps of ice and snow, its
dark oceans, and the vast cloud areas would be
very conspicuous. Faint stars, the zodiacal light,
and the filmy solar corona would be visible, prob-
ably even close up to the sun's edge; but although
his rays might shine upon the lunar rocks without
intermission for a fortnight, probably they would
still be too cold to touch with safety. On the side
of the moon turned away from the sun, the tem-
perature of the moon's surface would fall to that
of space, or many hundred degrees below zero.
CHAPTER XXIX
ECLIPSES OF THE MOON
OF all the weird happenings of the nighttime sky,
eclipses of the moon are the most impressive.
Rarely is there a year without one. What is the
cause? Simply the earth getting in between sun and
moon, and thereby shutting off the sunlight which at
all other times enables us to see the moon. As the
earth is a dark body it must cast a black shadow on
the side away from the sun, and it is the moon's pass-
ing into this shadow or some part of it that causes a
lunar eclipse.
Sun and earth being so different in size, the earth's
shadow must stretch away from it into space, grow-
ing smaller and smaller, until at length it comes to
an end the apex of a cone 857,000 miles long.
If we cut off this shadow at the moon's distance from
the earth, we find it about 6,000 miles in diameter
at that point ; and this accounts for the fact that the
curvature on the side of the moon, when the eclipse
is coming on and where it is dropping into the
shadow, is always much less rapid than the curva-
ture of the moon's own disk is.
When an eclipse is approaching, the eastern limb
will be duskily darkened for half an hour or more,
because the moon must first pass through the outer
penumbra, or half-shadow which everywhere sur-
rounds the true shadow itself. If the moon hits
only the upper or lower part of the shadow, the
206
ECLIPSES OF THE MOON 207
eclipse will be only partial, and during the progress
of the eclipse it will seem as if the imeclipsed part
had swung or twisted around in the sky, from the
western limb of the moon to the eastern. But when
the moon passes through the middle regions of the
shadow, the eclipse is always total, and direct sun-
light is wholly cut off from every part of the moon's
face, for a greater or less length of time, according
to the part of the shadow through which it passes.
When passing centrally through the shadow, the
total eclipse will last about two hours, as the moon's
diameter is about one-third of the breadth of the
shadow; and the eclipse will be partial about two
hours longer, an hour at beginning and an hour at
the end, because the moon moves over her own
breadth in about an hour.
While the moon is wholly immersed in the shadow,
her body is nevertheless visible, as a dull tarnished
copper disk; and this is caused by the reddish sun-
light which grazes the earth all around and is re-
fracted or bent by our atmosphere into the shadow
itself. If this belt or ring of terrestrial atmosphere
happens to be everywhere filled with dense clouds,
as was the case in 1886, even the familiar copper
moon of a total lunar eclipse disappears completely
in the black sky.
Quite different from a solar eclipse, all the phases
of a lunar eclipse are visible at the same time on the
earth wherever the moon is above the horizon.
Eclipses of the moon are therefore seen with great
frequency at any given place as compared with solar
eclipses, which are restricted to relatively narrow
areas of the earth's surface. Nor are lunar eclipses
of very much significance to the astronomer, mainly
because of the slowness and indefiniteness of the
208 ASTRONOMY TO-DAY
phenomena. It is a good time to observe occulta-
tions of faint stars at the moon's edge or limb,
and several such programs have been carried out
by cooperation of observatories in widely separate
regions of the world : the object being improvement
in our knowledge of the distance of the moon, and
in the accuracy of the mathematical tables of her
motion. Search by photography for a possible satel-
lite, or moon of the moon, has been made on several
occasions, though without success.
A lunar eclipse was first observed and photo-
graphed from an aeroplane, May 2, 1920. At the
request of the writer, two aviators of the United
States navy ascended to a height of 15,000 feet above
Rockaway, and secured many advantages accruing
from great elevation in viewing a celestial phenom-
enon of this character.
CHAPTER XXX
TOTAL ECLIPSES OF THE SUN
PRIMITIVE peoples indulged in every variety of
explanation of mysterious happenings in the sky.
To the Chinese and all through India, a total eclipse
of the sun is caused by "a certain dragon with very
black claws," who, except for their frightening him
away by every conceivable sort of hideous noise,
would most certainly "eat up the sun." The eclipse
always goes off, the sun has never been eaten yet.
Can you convince a Chinaman that Rahu, the
Dragon, wouldn't have eaten up the sun, if his un-
earthly din hadn't frightened him away?
In Japan the eclipse drops poison from the sky
into wells, so the Japanese cover them up. Fon-
tenelle relates that in the middle of the seventeenth
century a multitude of people shut themselves up in
cellars in Paris during a total eclipse.
In the Shu-king, an ancient Chinese work, occurs
the earliest record of a total eclipse of the sun, in
the year B. c. 2158. The Nineveh eclipse of B. C. 763
is perhaps the first of the ancient eclipses of
which we possess a really clear description on the
Assyrian eponym tablets in the British Museum.
It is the eclipse possibly referred to in the Book
of Amos, viii.
But of all the ancient eclipses none perhaps ex-
ceeds in interest the famous eclipse of Thales, B. c.
585, May 28. It is the first eclipse to have been
predicted, probably by means of the saros, or 18-
209
210 ASTRONOMY TO-DAY
year period of eclipses, which is useful as an
approximate method even at the present day. But
the accident of a war between the Lydians and the
Medes has added greatly to the historic interest,
because the combatants were so terrified by the
sudden turning of day into night that they at once
concluded a peace cemented by two marriages.
Very many of the ancient eclipses have been of great
use to the historian in verifying dates, and mathe-
matical astronomers have employed them in correct-
ing the lunar tables, or intricate mathematical data
by which the motion of the moon is predicted.
Coming down to the middle of the sixth cen-
tury, we find the first eclipse recorded in England,
in the "Saxon Chronicle," A. D. 538. During the
epoch of the Arabian Nights several eclipses were
witnessed at Bagdad, A. D. 829 to 928, and many a
century later by Ibu-Jounis, court astronomer of
Hakem, the Caliph of Egypt. Nothing is more in-
teresting than to search the quaint records of these
ancient eclipses. One occurring in 1560, when
Tycho Brahe was but fourteen, had much to do
with turning his permanent interest toward mathe-
matics and astronomy. The eclipse of 1612 was the
first "seen through a tube," the telescope having
been invented only a few years before. "Paradise
Lost" was completed about 1665, and the censor-
ship was still in existence; and it is matter of
record that the oft-quoted passage,
"As when the Sun, new risen,
Looks through the horizontal misty air,
Shorn of his beams ; or from behind the Moon,
In dim eclipse, disastrous twilight sheds
On half the nations, and with fear of change
Perplexes monarchs."
P. L., i. 594
TOTAL ECLIPSES OF THE SUN 211
was strongly urged as sufficient reason for sup-
pressing the entire epic.
London was favored with the outflashing corona,
May 3, 1715, and a pamphlet was issued in pre-
diction, entitled "The Black Day, or a Prospect of
Doomsday."
The first American eclipse expedition was on
occasion of the totality of Oct. 27, 1780, sent out
by Harvard College and the American Academy df
Arts and Sciences under Professor Samuel Wil-
liams to Penobscot. There was a fine total eclipse
from Albany to Boston on June 16, 1806, and many
important observations of it were made in this
country.
But it was not till the European eclipse of 1842
that research got fully under way, because the germ
of the new astronomy, particularly as applied to
the sun, had begun its development ; and the signifi-
cance of the corona was obvious, if it could be
proved a true appendage of the sun. Photography
had not long been discovered, and the corona of
1851 was the first to be automatically registered
on a daguerreotype. In 1860 it was proved that
prominences and corona both belong to the sun and
not to the moon.
The great Indian eclipse of 1868 brought the im-
portant discovery that the prominences can be
observed at any time without an eclipse by means
of the spectroscope. In 1869 bright lines were
found in the spectrum of the corona, one line in
the green indicating the presence of an element not
then known on the earth and hence called coro-
nium. In 1870 the reversing layer or stratum of
the sun was discovered. In 1878 a vast ecliptic
extension of the streams of the corona many mil-
212 ASTRONOMY TO-DAY
lions of miles both east and west of the sun was
first seen. This is now known to be the type of
corona characteristic of minimum spots on the sun.
In 1882 the spectrum of the corona was first photo-
graphed and in 1889 excellent detail photographs
of the corona were taken. In 1893 it was shown
that the corona quite certainly rotates bodily with
the sun. In 1896 actual spectrum photographs of
the reversing layer established its existence beyond
doubt "flash spectrum" it is often called. In 1898
the long ecliptic streamers of the corona were suc-
cessfully photographed for the first time. In 1900
the depth of the reversing layer was found to aver-
age 500 miles, the heat of the corona was first
measured by the bolometer, and many observations
showed that the coronal streamers, in part at least,
partake of the nature of electric discharges.
All subsequent total eclipses have been carefully
observed, in whatever part of the world they may
happen, and each has added new results of signifi-
cance to our theories of the corona and its rela-
tion to the radiant energy of the sun. In very
recent eclipses the cinematograph has been brought
into action as an efficient adjunct of observation;
in 1914 the first successful "movie" of the eclipse
was secured in Sweden, and in 1918 Frost of the
Yerkes Observatory first applied the cinematograph
to registry of the "flash spectrum," and Stebbins
tested out his photo-electric cell on the corona, mak-
ing the brightness 0.5 that of the full moon. In
1914 (Russia) and again in 1919 (on the Atlantic)
the obvious advantages of the aeroplane in ecliptic
observation and photography were sought by the
writer, though unsuccessfully. The photographic
tests, however, conducted in preparation for these
TOTAL ECLIPSES OF THE SUN 213
expeditions proved the entire practicability of
securing eclipse results of much value, indepen-
dently of clouds below.
Eclipses in the near future will be total in Aus-
tralia about six minutes on September 21, 1922 ; in
California and Mexico about four minutes on Sep-
tember 10, 1923, and along a line from Toronto to
Nantucket about two minutes on the morning of
January 24, 1925.
To all spectators, savage or civilized, scientist
or layman, a total eclipse is wonderful and im-
pressive. Langley said: "The spectacle is one of
which, though the man of science may prosaically
state the facts, perhaps only the poet could render
the impression." Very gradually the moon steals
its way across the face of the sun, the lessened
light is hardly noticed. If one is near a tree through
whose foliage the sunlight filters, an extraordinary
sight is seen ; the ground all about is covered with
luminous crescents, instead of the overlapping
disks which were there before the eclipse came on;
in both cases they are images of the disk of the
sun at the time, and the narrowing crescents will
be watched with interest as totality approaches.
Then the shadow bands may be seen flitting across
the landscape, like "visible wind." They are prob-
ably related to our atmosphere and the very slen-
der crescent from which true sunlight still comes.
Then for a few seconds the moon's actual
shadow may be caught in its approach, very sud-
denly the darkness steals over the landscape and
totality is on. How lucky if there are no clouds !
Every eye is riveted on "the incomparable corona,
a silvery, soft, unearthly light, with radiant
streamers, stretching at times millions of uncom-
214 ASTRONOMY TO-DAY
prehended miles into space, while the rosy flam-
ing protuberances skirt the black rim of the moon
in ethereal splendor."
Then it is now or never with observer and photog-
rapher. Months of diligent preparations at home fol-
lowed by weeks of tedious journey abroad, with days
of strenuous preparation and rehearsals at the sta-
tionall go for naught unless the whole is tuned up
to perfect operation the instant totality begins. It
may last but a minute, or even less; in 1937, how-
ever, total eclipse will last 7 minutes 20 seconds, the
longest ever observed, and within half a minute of
the longest possible. All is over as suddenly as it
came on. The first thing is to complete records, de-
velop plates, and see if everything worked perfectly.
There is great utility back of all eclipse research,
on account of its wide bearing on meteorology and
terrestrial physics, and possibly the direct use of
solar energy for industrial purposes. With this
purpose in view the astronomer devotes himself un-
sparingly to the acquisition of every possible fact
about the sun and his corona.
Considering the earth as a whole, the number of
total eclipses will average nearly seventy to the
century. But at any given place, one may count him-
self very fortunate if he sees a single total eclipse,
although he may see several partial ones without
going from home. Then, too, there are annular or
ring eclipses, averaging seven in eight years. But
had one been born in Boston or New York in the
latter part of the eighteenth century, he might have
lived through the entire nineteenth century an4 a
long way into the twentieth without seeing mqre
than one total eclipse of the sun. In London in 1715
no total eclipse had been visible for six centuries.
TOTAL ECLIPSES OF THE SUN 215
However, taking general averages, and recalling the
comparatively narrow belt of total eclipse, every
part of the earth is likely to come within range
of the moon's shadow once in about three and a
half centuries.
The longest total eclipses always occur near the
equator ; this is because an observer on the equator
is carried eastward by the earth's rotation at a
velocity of about 1,000 miles per hour, so that he
remains longer in the moon's shadow which is pass-
ing over him in the same direction with a velocity
about twice as great.
The general circumstances of total eclipses are
readily foretold by means of the ancient Chaldean
period of eclipses known as the saros. It is 18
years and 10 or 11 days in length (according to
the number of leap years intervening). In one
complete saros, forty-one solar eclipses will gen-
erally happen, but only about one-fourth of them
will be total. The saros is a period at the end of
which the centers of sun and moon return very
nearly to their relative positions at the beginning
of the cycle. So, in general, the eclipse of any
year will be a repetition of one which took place 18
years before, and another very similar in circum-
stances will happen 18 years in the future. Three
periods of the saros, or 54 years and 1 month, will
usually bring about a return of any given eclipse
to any particular part of the earth, so far as longi-
tude is concerned, though the returning track will
lie about 600 miles to the north or south of the one
54 years earlier.
Paths of total eclipses frequently intersect, if
large areas like an entire country are considered;
Spain, for instance, where total eclipses have oc-
216 ASTKONOMY TO-DAY
curred in 1842, 1860, 1870, 1900 and 1905. Be-
sides crossing Spain, the tracks of totality on May
28, 1900, and August 30, 1905, were unique in inter-
secting exactly over a large city Tripoli in Bar-
bary, on both of which occasions the writer's ex-
peditions to that city were rewarded with perfect
observing conditions in that now Italian province
on the edge of the great desert.
Kepler was the first astronomer to calculate
eclipses with some approach to scientific form, as
exemplified in his Rudolphine Tables. His method
was of course geometrical. But La Grange, who
applied the methods of more refined analysis to the
problem, was the first to develop a method by which
an eclipse and all its circumstances could be accu-
rately predicted for any part of the earth. To many
minds, the prediction of an eclipse affords the best
illustration of the superior knowledge of the astron-
omer: it seems little short of the marvelous. But
recalling that the motion of the moon follows the
law of gravitation, and that its position in the sky
is predictable for years in advance with a high de-
gree of precision, it will readily be seen how the
arrival of the moon's shadow, and hence the total
eclipses of the sun, can be foretold for any place
over which the shadow passes.
All these data derived by the mathematician are
known as the elements of the eclipse, and they
are prepared many years in advance and published
in the nautical almanacs and astronomical ephe-
merides issued by the leading nations. Buchanan's
"Treatise on Eclipses" will supply all the technical
information regarding the prediction of eclipses
that anyone desirous of inquiring into this phase
of the problem may desire.
TOTAL ECLIPSES OF THE SUN 217
So important are total eclipses in the scheme of
modern solar research, and so necessary are clear
skies in order that expeditions may be favored with
success, that every effort is now made to ascertain
the weather chances at particular stations along
the line of eclipse many years in advance. This
method of securing preliminary cloud observations
for a series of years has proved especially useful
for the eclipses of 1893, 1896, 1900, and 1918 ; and
had it been employed in Russia for totality of
1914, many well-equipped expeditions might have
been spared disaster. The California and Mexico
totality of 1923 does not require this forethought,
as the regions visited are quite likely to be free
from cloud ; but observations are now in process of
accumulation for the total eclipse of 1925. The out-
look for clear skies on that occasion, the total
eclipse nearest New York for more than a century,
is not very promising. The path of totality passes
over Marquette, Michigan, Rochester and Pough-
keepsie, New York, Newport, Rhode Island, and
Nantucket about nine in the morning.
Everyone who saw it will remember the last
total eclipse in this part of the world on June 8,
1918, visible from Oregon to Florida. Many will
recall the last total eclipse that was visible before
that in the eastern part of the United States, on
May 28, 1900, visible in a narrow path from New
Orleans to Norfolk. One's father or grandfather
will perhaps remember the total eclipse of July 29,
1878, which passed over the United States from
Pike's Peak to Texas (it was the writer's maiden
eclipse), and another on August 7, 1869, which
passed southeasterly over Iowa and Kentucky. On
all these occasions the paths of total eclipse were
218 ASTRONOMY TO-DAY
dotted with numerous observing parties, many of
them equipped with elaborate apparatus for study-
ing and photographing the solar corona and prom-
inences, together with a multitude of other phe-
nomena which are seen only when total eclipses
take place.
Looking forward rather than backward, a strik-
ing series, or family, of eclipses happens in the
future: it is the series of May, 1901 and 1919, re-
curring again on June 8, 1937 (over the Pacific
Ocean), June 20, 1955 (through India, Siam, and
Luzon), and June 30, 1973 (visible in Sahara,
Abyssinia, and Somali). Already in 1919 this
totality was 6 minutes 50 seconds in duration; in
1937, as already mentioned, it will be 7 minutes 20
seconds, and at the subsequent returns even longer
yet, approaching the estimated maximum of 7
minutes 58 seconds which has never been observed.
This remarkable series of total eclipses is longer in
duration than any others during a thousand years.
Its next subsequent return is in 1991, occurring with
the eclipsed sun practically at noon in the zenith of
Mount Popocatepetl in Mexico.
Whatever may be the progress of solar research
during the intervening years, it is impossible to
imagine the alert astronomer of that remote day
without incentive for further investigation of the
sun's corona, in which are concealed no doubt many
secrets of the sun's evolution from nebula to star-
CHAPTER XXXI
THE SOLAR CORONA
AND what is the sun's corona?" mildly asked
*- a college professor of a student who might
better have answered "Not prepared."
"I did know, Professor, but I have forgotten,"
was his reply.
"What an incalculable loss to science," returned
the professor with a twinkle. "The only man who
ever knew what the sun's corona is, and he has
forgotten !"
Only in part has the mystery of the corona been
cleared by the research of the present day. Our
knowledge proceeds but slowly, because the corona
has never been seen except during total eclipses of
the sun ; and astronomers, as a matter of fact, have
never had a fair chance at it. Two total eclipses
happen on the average of every three years; their
average duration is only two or three minutes;
totality can be seen only in a narrow path about
a hundred miles wide, though it may be several
thousand miles long; there is usually about equal
chance of cloud with clear skies; and fully three-
fourths of the totality areas of the globe are un-
available because covered by water. So that even
if we imagine the tracks of eclipses quite thickly
populated with astronomers and telescopes, at least
one every hundred miles, how much solid watching
of the corona would this permit? Only a little more
than one week's time in a whole century*
219
220 ASTRONOMY TO-DAY
The true corona is at least a triple phenomenon
and a very complex one. The photographs reveal
it much as the eye sees it, with all its complexity
of interlacing streamers projected into a flat, or
plane, surrounding the disk of the dark moon which
hides the true sun completely. But we must keep
in mind the fact that the sun is a globe, not a
disk, and that the streamers of the corona radiate
more or less from all parts of the surface of the
solar sphere, much as quills from a porcupine.
From the sun's magnetic poles branch out the
polar rays, nearly straight throughout their visible
extent. Gradually as the coronal rays originate at
points around the solar disk farther and farther
removed from the poles, they are more and more
curved. Very probably they extend into the equa-
torial regions, but it is not easy to trace them there
because they are projected upon and confused with
the filaments having their origin remote from the
poles. Then there is the inner equatorial corona,
apparently connected intimately with truly solar
phenomena, quite as the polar rays are. The third
element in the composite is the outer ecliptic co-
rona, for the most part made up of long streamers.
This is most fully developed at the time of the few-
est spots on the sun. It is traceable much farther
against the black sky with the naked eye than by
photography. Without any doubt it is a solar ap-
pendage and possibly it may merge into the zodi-
acal light.
Naturally this superb spectacle must have been
an amazing sight to the beholders of antiquity who
were fortunate enough to see it. Historical refer-
ences are rare: perhaps the earliest was by Plu-
tarch about A. D. 100, who wrote of it, "A radiance
THE SOLAR CORONA 221
shone round the rim, and would not suffer darkness
to become deep and intense." Philostratus a century
later mentions the death of the emperor Domitian at
Ephesus as "announced" by a total eclipse.
Kepler thought the corona was evidence of a
lunar atmosphere; indeed, it was not until the
middle of the 19th century that its lack of relation
to the moon was finally demonstrated. Later ob-
servers, Wyberd in 1652 and Ulloa, got the im-
pression that the corona turned round the disk
catherine-wheel fashion, "like an ignited wheel
in fireworks, turning on its center." But no later
observer has reported anything of the sort. Quite
the contrary, there it stands against the black sky
in motionless magnificence a colorless pearly mass
of wisps and streamers for the most part nebulous
and ill-defined, fading out very irregularly into the
black sky beyond, but with a complex interlacing of
filaments, sometimes very sharply defined near the
solar poles. It defies the skill of artist and draughts-
man to sketch it before it is gone.
Photograph it? Yes, but there are troubles. Of
course the camera work is superior to sketches by
hand. As Langley used to say, "The camera has
no nerves, and what it sets down we may rely on."
Foremost among the photographic difficulties is the
wide variation in intensity of the coronal light in
different regions of the corona. If a plate is ex-
posed long enough to get the outer corona, the
exceeding brightness of the inner corona overex-
poses and burns out that part of the plate or film.
If the exposure is short, we get certain regions of
the inner corona excellently, but the outer regions
are a blank because they can be caught only by a
long exposure.
222 ASTRONOMY TO-DAY
So the only way is to take a series of pictures
with a wide range of exposures, and then by care-
ful and artistic handwork, combine them all into
a single drawing. Wesley of London has succeeded
eminently in work of this character, and his draw-
ings of the sun's corona, visible at total eclipses
from 1871 onward, in possession of the Royal
Astronomical Society, are the finest in existence.
They give a vastly better idea of the corona, as
the eye sees it, than any single photograph possi-
bly can.
The early observers apparently never thought of
the corona as being connected with the sun. It was
a halo merely, and so drawn. Its real structure was
neither known, depicted, or investigated. Sketches
were structureless, as any aureola formed by stray
sunlight grazing the moon might naturally be. That
the rays are curved and far from radial round the
sun was shown for the first time in the sketches
of 1842, and in 1860 Sir Francis Galton observed
that the long arms or streamers "do not radiate
strictly from the center."
The inner corona had first been recorded photo-
graphically on a daguerreotype plate during the
eclipse of 1851, but the lens belonged to a heliom-
eter, and was of course uncorrected for the photo-
graphic rays. The wet collodion plates of the
eclipse of 1860, by De la Rue, proved that not only
the prominences but the corona were truly solar,
because his series of technically perfect pictures
revealed the steady and unchanged character of
these phenomena while the moon's disk was pass-
ing over them as totality progressed. And at the
eclipse of 1869, Young put the solar theory of the
corona beyond the shadow of any further doubt
THE SOLAR CORONA 223
by examination of its light with the spectroscope
and discovering a green line in the spectrum due
to incandescent vapor of a substance not then
identified with anything terrestrial, and therefore
called coronium.
The total brilliance of the corona was very dif-
ferently estimated by the earlier observers, though
pretty carefully measured at later eclipses. The
standard full moon is used for reference, and at
one eclipse the corona falls short of, while at an-
other it will exceed the full moon in brightness.
Variations in brilliancy are quite marked: at one
eclipse it was nearly four times as bright as the
full moon. Much evidence has already accumulated
on this question; but whether the observed varia-
tions are real, or due mainly to the varying rela-
tive sizes of sun and moon at different eclipses, is
not yet known. The coronal light is largely bluish
in tint, and this is the region of the spectrum most
powerfully absorbed by our atmosphere. Eclipses
are observed by different expeditions located at
stations where the eclipsed sun stands at very dif-
ferent altitudes above the horizon; besides this the
localities of observation are at varied elevations
above sea level; so that the varying amount of
absorption of the coronal light renders the prob-
lem one of much difficulty.
The long ecliptic streamers of the corona were
first seen by Newcomb and Langley during the
totality of 1878. On one side of the sun there was
a stupendous extension of at least twelve solar
diameters, or nearly 11 millions of miles. Lang-
ley observed from the summit of Pike's Peak, over
14,000 feet high, and was sure that he was wit-
nessing a "real phenomenon heretofore unde-
224 ASTRONOMY TO-DAY
scribed." The vast advantage of elevation was ap-
parent also from the fact that he held the corona
for more than four minutes after true totality had
ended. These streamers are characteristic of the
epoch of minimum spots on the sun, as Ranyard
first suggested. It was found that this type of
corona had been recorded also in 1867 ; and it has
reappeared in 1889, 1900 and 1911, and will doubt-
less be visible again in 1922.
How rapidly the streamers of the corona vary
is not known. Occasionally an observer reports
having seen the filaments vibrate rapidly as in the
aurora borealis, but this is not verified by others
who saw the same corona perfectly unmoving. Com-
parisons of photographs taken at widely separate
stations during the same eclipse have shown that at
least the corona remained stationary for hours at
a time. Whether it may be unchanged at the end
of a day, or a week, or a month, is not known ; be-
cause no two total eclipses can ever happen nearer
each other than within an interval of 173 days, or
one-half of the eclipse year. And usually the inter-
val between total eclipses is twice or three times
this period.
Theories of what the solar corona may be are
very numerous. The extreme inner corona is per-
haps in part a sort of gaseous atmosphere of the
sun, due to matter ejected from the sun, and kept
in motion by forces of ejection, gravity, and repul-
sion of some sort. Meteoric matter is likely con-
cerned in it, and Huggins suggested the debris of
disintegrating comets. Schuster was in agreement
with Huggins that the brighter filaments of the
corona might be due to electric discharges, but it
seems very unlikely that any single hypothesis can
VENUS, SHOWING CRESCENT PHASE OF THE PLANET. Venus is the earth's
nearest neighbor on the side toward the sun. (Photo, Yerlces Observatory.)
MARS, THE PLANET NEXT BEYOND THE EARTH. The photograph shows
one of the white polar caps. The caps are thought to be snow or ice and
may indicate the existence of atmosphere. (Photo, Yerkes Observatory.}
THE SOLAR CORONA 225
completely account for the intricate tracery of so
complex a phenomenon.
Elaborate spectroscopic programs have been
carried out at recent eclipses, affording evidence
that certain regions are due to incandescent matter
of lower temperature than the sun's surface. A
small part of the light of the corona is sunlight re-
flected from dark particles possibly meteoric, but
more likely dust particles or fog of some sort. This
accounts for the weakened solar spectrum with
Fraunhofer absorption lines, and this part of the
light is polarized.
Many have been the attempts to see, or photo-
graph, the corona without an eclipse. None of
them has, however, suceeded as yet. Huggins got
very promising results nearly forty years ago, and
success was thought to have been reached ; but sub-
sequent experiments on the Riffelberg in 1884 and
later convinced him that his results related only to
a spurious corona. In 1887 the writer made an un-
successful attempt to visualize the corona from the
summit of Fujiyama, and Hale tried both optical
and photographic methods on Pikes Peak in 1893
without success. He devised later a promising
method by which the heat of the corona in different
regions can be measured by the bolometer, and an
outline corona afterward sketched from these
results.
Still another method of attacking the problem
occurred to the writer in 1919, which has not yet
been carried out. It would take advantage of re-
cent advances in aeronautics, and contemplates an
artificial eclipse in the upper air by means of a
black spherical balloon. This would be sent up to
an altitude of perhaps 40,000 feet, where it would
Sci. Vol. 28
226 ASTRONOMY TO-DAY
partake of the motion of the air current in which
it came to equilibrium. Then a snapshot camera
would be mounted on an areoplane, in which the
aviator would ascend to such a height that the bal-
loon just covered the sun, as the moon does in a
total eclipse. With the center of the balloon in line
with the sun's center, he would photograph the
regions of the sky immediately surrounding the sun,
against which the corona is projected. As the en-
tire apparatus would be above more than an entire
half of the earth's atmosphere, the experiment
would be well worth the attempt, as pretty much
everything else has been tried and found wanting.
Needless to say, the importance of seeing the co-
rona at regular intervals whenever desired, without
waiting for eclipses of the sun, remains as insistent
as ever.
CHAPTER XXXII
THE RUDDY PLANET
MARS is a planet next in order beyond the
earth, and its distance from the sun averages
141% million miles. It has a relatively rapid motion
among the stars, its color is reddish, and, when
nearest to us, it is perhaps the most conspicuous
object in the sky.
Mars appeared to the ancients just as it does to us
to-day. Aristotle recorded an observation of Mars,
356 B. c., when the moon passed over the planet, or
occulted it, as our expression is. Galileo made the
first observations of Mars with a telescope in 1610,
and his little instrument was powerful enough to
enable him to discover that the planet had phases,
though it did not pass through all the phases that
Mercury and Venus do. This was obvious from the
fact that Mars is always at a greater distance from
the sun than we are, and the phase can only be
gibbous, or about like the moon when midway be-
tween full and quarter.
Many observers in the seventeenth century fol-
lowed up the planet with such feeble optical power
as the telescopes of that epoch provided: Fontana
(who made the first sketch) , Riccioli and Bianchini
in Italy, Cassini in France, Huygens in Holland, and
later Sir William Herschel in England.
It was Cassini who first made out the whitish
spots or polar caps of Mars in 1666, but not until
227
228 ASTRONOMY TO-DAY
after Huygens had noted the fact that Mars turned
round on an axis in a period but little longer than
the earth's. Cassini followed it up later with a
more accurate value; and observations in our own
day, when combined with these early ones, enable
us to say that the Martian day is equal to 24
hours 37 minutes 22.67 seconds, accurate probably
to the hundredth part of a second.
When we know that a planet turns round on an
axis, we know that it has a day. When we know
the direction of the axis in space or in relation to the
plane of its path round the sun, we know that it
has seasons : we can tell their length and when they
begin and end. It did not take many years of ob-
servation to prove that the axis round which Mars
turns is tilted to the plane of its path round the
sun by an angle practically the same as that at
which the earth's axis is tilted. So there is the im-
mediate inference that on Mars the order and per-
haps the character of the seasons is much the same
as here on the earth.
At least two things, however, tend to modify
them. First, the year of Mars is not 365 days like
ours, but 687 days. Each of the four seasons on
Mars, therefore, is proportionally longer than our
seasons are. Then comes the question of atmos-
phere how much of an atmosphere does Mars
really possess in proportion to ours, and how would
its lesser amount modify the blending of the
seasons into one another?
All discussion of Mars and the problems of exis-
tence of life upon that planet hinge upon the char-
acter and extent of Martian atmosphere. The
planet seems never to be covered, as the earth
usually is, with extensive areas of cloud which to
THE RUDDY PLANET 229
an observer in space would completely mask its
oceans and continents. Nearly all the time Mars
in his equatorial and temperate zones is quite clear
of clouds. A few whitish spots are occasionally
seen to change their form and position in both
northern and southern latitudes, and they vary*
with the progress of the day on Mars, as clouds
naturally would. But Schiaparelli, perhaps the best
of all observers, thought them to be not low-lying
clouds of the nimbus type that would produce rains,
but rather a veil of fog, or perhaps a temporary
condensation of vapor, as dew or hoar frost. But
the strongest argument for an atmosphere is based
on the temporary darkening or obscuration of well
known and permanent markings on the surface of
Mars. These are more or less frequently observed
and clouds afford the best explanation of their
occurrence.
So much for evidence supplied by the telescope
alone. When, however, we employ the spectroscope
in conjunction with the telescope, another sort of
evidence is at hand. Several astronomers have
reached the conclusion that watery vapor exists in
the atmosphere of Mars, while other astronomers
equipped with equal or superior apparatus, and
under equally favorable or even better conditions,
have reached the remarkable conclusion that the
spectra of Mars and the moon are identical in every
particular. From this we should be led to infer
that Mars has perhaps no more atmosphere than
the moon has, that is to say, none whatever that
present instruments and methods of investigation
have enabled us to detect.
What then, shall we conclude? Simply that the
atmosphere of Mars is neither very dense nor exten-
230 ASTRONOMY TO-DAY
sive. Probably its lower strata close to the planet's
surface are about as dense as the earth's atmosphere
is at the summits of our highest mountains.
This conclusion is not unwelcome, if we keep a
few fundamental facts in clear and constant view.
Mars is a planet of intermediate size betwen the
earth and the moon: twice the moon's diameter
(2,160 miles) very nearly equals the diameter of
Mars (4,200 miles), and twice the diameter of
Mars does not greatly exceed the earth's diameter
(7,920 miles). As to the weights or masses of
these bodies, Mars is about one-ninth, and the moon
one-eightieth of the earth. The atmospheric en-
velope of the earth is abundant, the moon has none
as far as we can ascertain; so it seems safe to
infer that Mars has an atmosphere of slight den-
sity: not dense enough to be detected by spectro-
scopic methods, but yet dense enough to enable us
to explain the varying telescopic phenomena of the
planet's disk which we should not know how to ac-
count for, if there were no atmosphere whatever.
One astronomer has, indeed, gone so far as to cal-
culate that in comparison with our planet Mars is
entitled to one-twentieth as much atmosphere as
we have, and that the mercurial barometer at "sea
level*' would run about five and a half inches, as
against thirty inches on the earth.
In general, then, the climate of Mars is prob-
ably very much like that of a clear season on a very
high terrestrial table land or mountain a climate
of wide extremes, with great changes of tempera-
ture from day to night. The inequality of Martian
seasons is such that in his northern hemisphere the
winter lasts 381 days and the summer only 306
days.
THE RUDDY PLANET 231
Now, the polar caps of Mars, which are reasonably
assumed to be due to snow or hoar frost, attain
their maximum three or four months after the
winter solstice, and their minimum about the same
length of time after the summer solstice. This
lagging should be interpreted as an argument for a
Martian atmosphere with heat-storing qualities,
similar to that possessed by the earth.
Upon this characteristic, indeed, depends the
climate at the surface of Mars : whether it is at all
similar to our own, and whether fluid water is a
possibility on Mars or not. While the cosmic rela-
tions of the planet in its orbit are quite the same as
ours, nevertheless the greater distance of Mars
diminishes his supply of direct solar heat to about
half what we receive. On the other hand, his dis-
tance from the sun during his year of motion
around it varies much more widely than ours, so
that he receives when nearest the sun about one-half
more of solar heat than he does when farthest away.
Southern summers on Mars, therefore, must be
much hotter, and southern winters colder than the
corresponding seasons of his northern hemisphere.
Indeed, the length of the southern summer, nearly
twice that of the terrestrial season, sometimes
amply suffices to melt all the polar ice and snow,
as in October, 1894, when the southern polar cap
of Mars dwindled rapidly and finally vanished
completely.
Very interesting in this connection are the re-
searches of Stoney on the general conditions affect-
ing planetary atmospheres and their composition.
According to the kinetic theory, if the molecules of
gases which are continually in motion travel out-
ward from the center of a planet, as they fre-
232 ASTRONOMY TO-DAY
quently must, and with velocities surpassing the
limit that a planet's gravity is capable of control-
ling, these molecules will effect a permanent
escape from the planet, and travel through space in
orbits of their own.
So the moon is wholly without atmosphere
because the moon's gravity is not powerful enough
to retain the molecules of its component gases. So
also the earth's atmosphere contains no helium or
free hydrogen. So, too, Mars is possessed of insuf-
ficient force of gravity to retain water vapor, and
the Martian atmosphere may therefore consist
mainly of nitrogen, argon, and carbon dioxide.
As everyone knows, the axis of the earth if ex-
tended to the northern heavens would pass very
near the north polar star, which on that account is
known as Polaris. In a similar manner the axis
of Mars pierces the northern heavens about mid-
way between the two bright stars Alpha Cephei
and Alpha Cygni (Deneb). The direction of this
axis is pretty accurately known, because the meas-
urement of the polar caps of the planet as they turn
round from night to night, year in and year out, has
enabled astronomers to assign the inclination of the
axis with great precision.
These caps are a brilliant white, and they are
generally supposed to be snow and ice. They
wax and wane alternately with the seasons on
Mars, being largest at the end of the Martian win-
ter and smallest near the end of summer. The
existence of the polar caps together with their
seasonal fluctuations afford a most convincing
argument for the reality of a Martian atmosphere,
sufficiently dense to be capable of diffusing and
transporting vapor.
THE RUDDY PLANET 233
The northern cap is centered on the pole almost
with geometric exactness, and as far as the 85th
parallel of latitude. On the other hand, the south
polar cap is centered about 200 miles from the true
pole, and this distance has been observed to vary
from one season to another. No suggestion has beeni
made to account for this singular variation. On one
occasion it stretched down to Martian latitude 70
degrees and was over 1,200 miles in diameter.
Pickering watched the changing conditions of
shrinking of the south polar cap in 1892 with a
large telescope located in the Andes of Peru. Mars
was faithfully followed on every night but one from
July 13 to September 9, and the apparent altera-
tions in this cap were very marked, even from night
to night. As the snows began to decrease, a long
dark line made its appearance near the middle of
the cap, and gradually grew until it cut the cap in
two. This white polar area (and probably also the
northern one in similar fashion) becomes notched
on the edge with the progress of its summer sea-
son ; dark interior spots and fissures form, isolated
patches separate from the principal mass, and later
seem to dissolve and disappear. Possibly if one
were located on Mars and viewing our earth with a
big telescope, the seasonal variation of our north
and south polar caps might present somewhat simi-
lar phenomena. All the recent oppositions of Mars
have been critically observed by Pickering from an
excellent station in Jamaica.
Quite obviously the fluctuations of the polar caps
are the key to the physiographic situation on Mars,
and they are made the subject of the closest
scrutiny at every recurring opposition of the planet.
Several observers, Lowell in particular, record a
234 ASTRONOMY TO-DAY
bluish line or a sort of retreating polar sea, follow-
ing up the diminishing polar cap as it shrinks with
the advance of summer. It is said that no such line
is visible during the formation of the polar cap with
the approach of winter. All such results of critical
observation, just on the limit of visibility, have to
be repeated over and over again before they become
part of the body of accepted scientific fact. And in
many instances the only sure way is to fall back on
the photographic record, which all astronomers,
whether prejudiced or not, may have the opportunity
to examine and draw their individual conclusions.
Already the approaching opposition of 1924, the
most favorable since the invention of the telescope,
is beginning to attract attention, and preparations
are in progress, of new and more powerful instru-
ments, with new and more sensitive photographic
processes, by means of which many of the present
riddles of Mars may be solved.
CHAPTER XXXIII
THE CANALS OF MARS
npHEN there are the so-called canals of Mars,
-L about which so much is written and relatively
little known. Faint markings which resemble them
in character were first drawn in 1840 and later in
1864, but Schiaparelli, the famous Italian astrono-
mer, is probably their original discoverer, when
Mars was at its least distance from the earth in
1877. He made the first accurate detailed map of
Mars at this time, and most of the important or
more conspicuous canals (canali, he called them in
Italian, that is, channels merely, without any refer-
ence whatever to their being watercourses) were
accurately charted by him.
At all the subsequent close approaches of Mars,
the canals have been critically studied by a wide
range of astronomical observers, and their conclu-
sions as to the nature and visibility of the canals
have been equally wide and varied. The most
favorable oppositions have occurred in 1892 and
1894, also in 1907 and 1909. On these occasions
a close minimum distance of Mars was reached, that
is, about 35 millions of miles ; but in 1924 the planet
makes the closest approach in a period of nearly a
thousand years. Its distance will not much exceed
34 millions of miles.
But although this is a minimum distance for Mars,
it must not be forgotten that it is a really vast
235
236 ASTRONOMY TO-DAY
distance, absolutely speaking; it is something like
150 times greater than the distance of the moon.
With no telescopic power at our command could we
possibly see anything on the moon of the size of
the largest buildings or other works of human in-
telligence; so that we seem forever barred from
detecting anything of the sort on Mars.
Nevertheless, the closest scrutiny of the ruddy
planet by observers of great enthusiasm and intelli-
gence, coupled with imagination and persistence,
have built up a system of canals on Mars, covering
the surface of the planet like spider webs over
a printed page, crossing each other at intersect-
ing spots known as "lakes," and embodying a
wealth of detail which challenges criticism and
explanation.
To see the canals at all requires a favorable
presentation of Mars, a steady atmosphere and a
perfect telescope, with a trained eye behind it. Not
even then are they sure to be visible. The training
of the eye has no doubt much to do with it. So
photography has been called in, and very excellent
pictures of Mars have already been taken, some
nearly half as large as a dime, showing plainly the
lights and shades of the grander divisions of the
Martian surface, but only in a few instances reveal-
ing the actual canals more unmistakably than they
are seen at the eyepiece.
The appearance and degree of visibility of the
canals are variable: possibly clouds temporarily
obscure them. But there is a certain capriciousness
about their visibility that is little understood. In
consequence of the changing physical aspects, as to
season, on Mars and his orbital position with ref-
erence to the earth, some of the canals remain for
THE CANALS OF MARS 237
a long time invisible, adding to the intricacy of the
puzzle.
For the most part the canals are straight in their
course and do not swerve much from a great circle
on the planet. But their lengths are very different,
some as short as 250 miles, some as long as 4,000
miles; and they often join one another like spokes
in the hub of a wheel, though at various angles. As
depicted by Lowell and his corps of observers at
Flagstaff, Arizona, the canal system is a truly mar-
velous network of fine darkish stripes. Their color
is represented as a bluish green.
Each marking maintains its own breadth through-
out its entire length, but the breadth of all the canals
is by no means the same : the narrowest are perhaps
fifteen to twenty miles wide, and the broadest prob-
ably ten times that. At least that must be the
breadth of the Nilosyrtis, which is generally re-
garded as the most conspicuous of all the canals. The
Lowell Observatory has outstripped all others in the
number of canals seen and charted, now about 500.
What may be the true significance of this remark-
able system of markings it is impossible to conclude
at present. Schiaparelli from his long and critical
study of them, their changes of width and color, was
led to think that they may be a veritable hydro-
graphic system for distributing the liquid from the
melting polar snows. In this case it would be diffi-
cult to escape the conviction that the canals have, at
least in part, been designed and executed with a
definite end in view.
Lowell went even farther and built upon their
behavior an elaborate theory of life on the planet,
with intelligent beings constructing and opening
new canals on Mars at the present epoch. Pickering
238 ASTRONOMY TO-DAY
propounded the theory that the canals are not water-
bearing channels at all, but that they are due to
vegetation, starting in the spring when first seen
and vitalized by the progress of the season poleward,
the intensity of color of the vegetation coinciding
with the progress of the season as we observe it.
Extensive irrigation schemes for conducting
agricultural operations on a large scale seem a very
plausible explanation of the canals, especially if we
regard Mars as a world farther advanced in its life
history than our own. Erosion may have worn the
continents down to their minimum elevation, ren-
dering artificial waterways not difficult to build;
while with the vanishing Martian atmosphere and
absence of rains, the necessity of water for the sup-
port of animal and vegetal life could only be met
by conducting it in artificial channels from one re-
gion of the planet to another.
Interesting as this speculative interpretation is,
however, we cannot pass by the fact that many com-
petent astronomers with excellent instruments finely
located have been unable to see the canals, and there-
fore think the astronomers who do see them are
deceived in some way. Also many other astrono-
mers, perhaps on insufficient grounds, deny their
existence in toto.
Many patient years of labor would be required to
consult all the literature of investigation of the
planet Mars, but much of the detail has been
critically embodied in maps at different epochs, by
Kayser, Proctor, Green, and Dreyer. And Flam-
marion in two classic volumes on Mars has pre-
sented all the observations from the earliest time,
together with his own interpretation of them. Areo-
graphy is a term sometimes applied to a description
THE CANALS OF MARS 239
of the surface of Mars, and it is scarcely an ex-
aggeration to say that areography is now better
known than the geography of immense tracts of the
earth.
For some reason well recognized, though not at all
well understood, Mars although the nearest of all
the planets, Venus alone excepted, is an object by
no means easy to observe with the telescope. Pos-
sibly its unusual tint has something to do with this.
With an ordinary opera glass examine the moon
very closely, and try to settle precise markings,
colors, and the nature of objects on her surface;
Mars under the best conditions, scrutinized with
our largest and best telescope, presents a problem
of about the same order of difficulty. There are
delicate and changing local colors that add much
uncertainty. Nevertheless, the planet's leading
features are well made out, and their stability since
the time of the earliest observers leaves no room to
doubt their reality as parts of a permanent plane-
tary crust.
The border of the Martian disk is brighter than
the interior, but this brightness is far from uniform.
Variations in the color of the markings often depend
on the planet's turning round on its axis, and the
relation of the surface to our angle of vision. If
we keep in mind these obstacles to perfect vision in
our own day, it is easy to see why the early users of
very imperfect telescopes failed to see very much,
and were misled by much that they thought they
saw. Then, too, they had to contend, as we do, with
unsteadiness of atmosphere, which is least trouble-
some near the zenith.
As their telescopes were all located in the north-
ern hemisphere, the northern hemisphere of Mars
240 ASTRONOMY TO-DAY
is the one best circumstanced for their investiga-
tion; because at the remote oppositions of Mars,
which always happen in our northern winter with
the planet in high north declination, it is always the
north pole of Mars which is presented to our
view. Whereas the close oppositions of the planet
always come in our northern midsummer, with
Mars in south declination and therefore passing
through the zenith of places in corresponding south
latitude.
With Mars near opposition, high up from the
horizon, a fairly steady atmosphere, and a magnify-
ing power of at least 200 diameters, even the most
casual observer could not fail to notice the striking
difference in brightness of the two hemispheres:
the northern chiefly bright and the southern
markedly dark. Formerly this was thought to indi-
cate that the southern hemisphere of Mars was
chiefly water and the northern land, much as is the
case on the earth: with this difference, however,
that water and land on the earth are proportioned
about as eleven to four.
But Mars in its general topography presents no
analogy with the present relation of land and water
on the earth. There seems no reason to doubt that
the northern regions with their prevailing orange
tint, in some places a dark red and in others fading
to yellow and white, are really continental in charac-
ter. Other vast regions of the Martian surface are
possibly marshy, the varying depth of water causing
the diversity of color. If we could ever catch a
reflection of sunlight from any part of the surface
of Mars, we might conclude that deep water exists
on the planet; but the farther research progresses,
the more complete becomes the evidence that per-
THE CANALS OF MARS 241
manent water areas on Mars, if they exist at all,
are extremely limited.
Since 1877 Mars has been known to possess two
satellites, which were discovered in August of that
year by Hall at Washington. Moons of this planet
had long been suspected to exist and on one or two
previous occasions critically looked for, though
without success. In the writings of Dean Swift
there is a fanciful allusion to the two moons of
Mars; and if astronomers had chanced to give
serious attention to this, Phobos and Deimos, as
Hall named them, might have been discovered long
before.
They are very small bodies, not only faint in the
telescope, but actually of only ten or twenty miles
diameter; and from the strange relation that
Phobos, the inner moon, moves round Mars three
times while the planet itself is turning round only
once on its axis, some astronomers incline to the
hypothesis that this moon at least was never part of
Mars itself, but that it was originally an inner or
very eccentric member of the asteroid group, which
ventured within the sphere of gravitation of Mars,
was captured by that planet, and has ever since
been tributary to it as a secondary body or satellite.
CHAPTER XXXIV
LIFE IN OTHER WORLDS
T)OPULAR interest in astronomy is exceedingly
-L wide, but it is very largely confined to the idea of
resemblances and differences between our earth
and the bodies of the sky. The question most
frequently asked the astronomer is, "Have any of
the stars got people on them ?" Or more specifically,
"Is Mars inhabited?" The average questioner will
not readily be turned off with yes or no for an
answer. He may or may not know that it is quite
impossible for astronomers to ascertain anything
definite in this matter, most interesting as it is.
What he wants to find out is the view of the in-
dividual astronomer on this absorbing and ever re-
curring inquiry.
We ought first to understand what is meant by
the manifestation here on the earth called life, and
agree concerning the conditions that render it
possible. Apparently they are very simple. We
may or may not agree that a counterpart of life, or
life of a wholly different type from ours, may exist
on other planets under conditions wholly diverse
from those recognized as essential to its existence
here. The problem of the origin of life is, in the
present state of knowledge, highly speculative and
hardly within the domain of science. Here on
earth, life is intimately associated with certain
chemical compounds, in which carbon is the common
242
LIFE IN OTHER WORLDS 243
element without which life would not exist. Also
hydrogen, oxygen, and nitrogen are present, with
iron, sulphur, phosphorus, magnesium and a few
less important elements besides. But carbon is the
only substance absolutely essential. Protoplasm
cannot be built without it, and protoplasm makes up
the most of the living cell. Closely related to car-
bon is silica also, as a substitution in certain organic
compounds. Protoplasm is able to stand very low
temperatures, but its properties as a living cell cease
when the temperature reaches 150 Fahrenheit.
Animal life as it exists on the earth to-day ap-
pears to have been here many million years. The
palaeontologists agree that all life originated in the
waters of the earth. It has passed through evolu-
tionary stages from the lowest to the highest.
Throughout this vast period the astronomer is able
to say that the conditions of the earth which
appear to be essential to the maintenance of life
have been pretty constantly what they are to-day.
The higher the type of life, the narrower the range
of conditions under which it thrives. Man can
exist at the frigid poles even if the temperature is
75 degrees below Fahrenheit zero; and in the
deserts and the tropics, he swelters under temper-
atures of 115 degrees, but he still lives. At these
extremes, however, he can scarcely be said to thrive.
We have, then, a relatively narrow range of tem-
peratures which seems to be essential to his com-
fortable existence and development : we may call it
150 degrees in extent. Had not the surface temper-
ature of the earth been maintained within this range
for indefinite ages, in the regions where the human
race has developed, quite certainly man would not be
here. How this equability of temperature has been
244 ASTRONOMY TO-DAY
maintained does not now matter. Clearly the earth
must have existed through indefinite ages in the
process of cooling down from temperatures of at
least 6,000 degrees.
During this stage the temperature of the surface
was earth-controlled. Then this period merged
very gradually into the stage where life became
possible, and the temperature of the surface became,
as it now is, sun-controlled. How many years are
embraced in this span of periods, or ages, we have
no means of knowing. But of the sequence of
periods and the secular diminution of temperature,
we may be certain.
Then there is the equally important considera-
tion of water necessary for the origination, support,
and development of life. We cannot conceive of
life existing without it. On the earth water is
superabundant, and has been for indefinite ages in,
the past. There is little evidence that the oceans
are drying up; although the commonly accepted
view is that the waters of the earth will very gradu-
ally disappear. Water can exist in the fluid state,
which is essential to life, at all temperatures be-
tween 32 degrees and 680 degrees F.
Air to breathe is essential to life also. The atmos-
phere which envelops the earth is at least 100 miles
in depth, and its own weight compresses it to a
tension of nearly 15 pounds to the square inch at
sea level. This atmosphere and its physical prop-
erties have had everything to do with the develop-
ment of animal life on the planet. Without it and
its remarkable property of selective absorption,
which imprisons and diffuses the solar heat, it is in-
conceivable that the necessary equability of surface
temperature could be maintained. This appears to
LIFE IN OTHER WORLDS 245
be quite independent of the chemical constituents of
the atmosphere, and is perhaps the most important
single consideration affecting the existence of life
on a planet. If the surface of a planet is partly
covered with water, it will possess also an atmos-
phere containing aqueous vapor.
Heat, water, and air : these three essentials deter-
mine whether there is life on a planet or not. Of
course there must be nutrition suitable to the or-
ganism; mineral for the vegetal, and vegetal for
the animal. But the narrow range of variation
appears to be the striking thing: relatively but a
few degrees of temperature, and a narrow margin of
atmospheric pressure. If this pressure is doubled
or trebled, as in submarine caissons, life becomes in-
supportable. If, on the other hand, it is reduced
even one-third, as on mountains even 13,000 feet
high, the human mechanism fails to function, partly
from lack of oxygen necessary in vitalizing the
blood, but mainly because of simple reduction of
mechanical pressure.
If, then, we conceive of life in other worlds and
it is agreed that life there must manifest itself much
as it does here, our answer to the question of habit-
ability of the planets must follow upon an investiga-
tion of what we know, or can reasonably surmise,
about the surface temperatures of these bodies,
whether they have water, and what are the probable
physical characteristics of their atmospheres.
We may inquire about each planet, then, concern-
ing each of these details.
The case of Mercury is not difficult. At an
average distance of only 36 million miles from the
sun, and with a large eccentricity of orbit which
brings it a fifth part nearer, conditions of tempera-
246 ASTRONOMY TO-DAY
ture alone must be such as to forbid the existence of
life. The solar heat received is seven times greater
than at the earth, and this is perhaps sufficient
reason for a minimum of atmosphere, as indicated
by observation. If no air, then quite certainly no
water, as evaporation would supply a slight atmos-
phere. But according to the kinetic theory of gases,
the mass of Mercury, only a very small fraction of
that of the sun, is inadequate to retain an atmos-
pheric envelope. If, however, the planet's day and
year are equal, so that it turns a constant face to
the sun, surface conditions would be greatly com-
plicated, so that we cannot regard the planet as
absolutely uninhabitable on the hemisphere that is
always turned away from the sun.
Venus at 67 millions of miles from the sun
presents conditions that are quite different. She
receives double the solar heat that we do, but pos-
sessing an atmosphere perhaps threefold denser
than ours, as reliably indicated by observations of
transits of Venus, the intensity of the heat and its
diffusion may be greatly modified. What the selec-
tive absorption of the atmosphere of Venus may be,
we do not know. Nor is the rotation time of the
planet definitely ascertained: if equal to her year,
as many observations show and as indicated by the
theory of tidal evolution, there may well be certain
regions on the hemisphere perpetually turned away
from the sun where temperature conditions are
identical with those on the tropical earth, and where
every condition for the origin and development of
life is more fully met than anywhere else in the
solar system. Whether Venus has water distributed
as on the earth we do not know, as her surface is
never seen, owing to dense clouds under which she
LIFE IN OTHER WORLDS 247
is always enshrouded. Her cloudy condition pos-
sibly indicates an overplus of water.
Is the moon inhabited? Quite certainly not: no
appreciable air, no water, and a surface tempera-
ture unmodified by atmosphere rising perhaps to
100 degrees F. during the day, which is a fortnight
in length, and falling at night to 300 degrees below
zero, if not lower.
Is Mars inhabited? The probable surface tem-
perature is much lower than the earth's, because
Mars receives only half as much solar heat as we do ;
and more important still, the atmosphere of Mars is
neither so dense nor so extensive as our own.
Seasons on Mars are established, much the same as
here, except that they are nearly twice as long as
ours ; and alternate shrinking and enlarging of the
polar caps keeps even pace with the seasons, there-
by indicating a certainty of atmosphere whose
equatorial and polar circulation transports the
moisture poleward to form the snow and ice of
which the polar caps no doubt consist.
There is a variety of evidence pointing to an at-
mosphere on Mars of one-third to one-half the den-
sity of our own: an atmosphere in which free
hydrogen could not exist, although other gases
might. The spectroscopic evidence of water vapor
in the Martian atmosphere is not very strong. It
is very doubtful whether water exists on Mars in
large bodies: quite certainly not as oceans, though
the evidence of many small "lakes" is pretty well
made out. With very little water, a thin atmos-
phere and a zero temperature, is Mars likely to be
inhabited at the present time? The chances are
rather against it. If, however, the past develop-
ment of the planet has progressed in the way usually
248 ASTRONOMY TO-DAY
considered as probable, we may be practically cer-
tain that Mars has been inhabited in the past, when
water was more abundant, and the atmosphere more
dense so as to retain and diffuse the solar heat.
Biologists tell me that they hardly know enough
regarding the extreme adaptability of organisms
to environment to enable them to say whether life
on such a planet as Mars would or would not keep
on functioning with secular changes of moisture and
temperature. The survival of a race might be in-
sured against extremely low temperatures by dwell-
ing in sub-Martian caves, and sufficient water might
be preserved by conceivable engineering and me-
chanical schemes; but the secular reduction of the
quantity and pressure of atmosphere it is not easy
to see how a race even more advanced than ourselves
could maintain itself alive under serious lack of an
element so vital to existence. Both Wallace, the
great biologist, and Arrhenius, the eminent chemist
(but biologist, astronomer, and physicist as well),
both reject the habitation theory of Mars, regard-
ing the so-called canals as quite like the luminous
streaks on the moon ; that is, cracks in the volcanic
crust caused by internal strains due to the heated
interior. Wallace, indeed, argues that the planet is
absolutely uninhabitable.
The asteroids, or minor planets? We may dis-
miss them with the simple consideration that their
individual masses are so insignificant and their
gravity so slight that no atmosphere can possibly
surround them. Their temperatures must be ex-
ceedingly low, and water, if present at all, can only
exist in the form of ice.
Jupiter, the giant planet, presents the opposite
extreme. His mass is nearly a thousandth "part of
LIFE IN OTHER WORLDS 249
the sun's, and is sufficient to retain a very high
temperature, probably approximating to the condi-
tion we call red-hot. This precludes the possibility
of life at the outset, although the indications of a
very dense atmosphere many thousand miles in
depth are unmistakable.
Of Saturn, one thirty-five hundredth the mass of
the sun, practically the same may be said. Proctor
thought it quite likely that Saturn might be habit-
able for living creatures of some sort, but he re-
garded the planet as on many accounts unsuitable
as a habitation for beings constituted like ourselves.
Mere consideration of surface temperature pre-
cludes the possibility of life in the present stage of
Saturn's development ; but the consensus of opinion
is to the effect that life may make its appearance on
these great planets at some inconceivably remote
epoch in the future when the surface temperature is
sufficiently reduced for life processes to begin. Dis-
coveries of algse flourishing in hot springs approach-
ing 200 degrees Fahrenheit make it possible that
these beginnings may take place earlier and at
much higher temperatures than have hitherto been
thought possible.
A century ago, when the ring of Saturn was
believed to be a continuous plane, this was a favor-
ite corner of the solar system for speculation as to
habitability ; but now that we know the true con-
stitution of the rings, no one would for a moment
consider any such possibility. Conditions may,
however, be quite different with Saturn's huge
satellite Titan, the giant moon of the solar system.
Its diameter makes it approximately the size of the
planet Mars; and although it is much farther re-
moved from the sun, its relative nearness to the
250 ASTRONOMY TO-DAY
highly heated globe of Saturn may provide that
equability of temperature which is essential to life
processes.
Also the three inner Galilean moons of Jupiter,
especially III which is about the size of Titan, are
excellently placed for life possibilities, as far as
probable temperature is concerned, but we have of
course no basis for surmising what their conditions
may be as to air and water, except that their small
mass would indicate a probable deficiency of those
elements.
Uranus and Neptune are planets so remote, and
their apparent disks are so small, that very little is
known about their physical condition. They are
each about one-third the diameter of Jupiter, and
the spectrum of Uranus shows broad diffused bands,
indicating strong absorption by a dense atmosphere
very different from that of the earth. Indications
are that Neptune has a similar atmosphere.
It is possible that the denser atmospheres of these
remote planets may be so conditioned as to selective
absorption that the relatively slender supply of solar
heat may be conserved, and thus insure a relatively
high surface temperature when the sun comes into
control. If our theories of origin of the planets are
to be trusted, we may rather suppose that Uranus
and Neptune are still in a highly heated condition ;
that life has not yet made its appearance on them,
but that it will begin its development ages before
Saturn and Jupiter have cooled to the requisite tem-
perature.
Comets^ In his Lettres Cosmologiques (1765)
Lambert considers the question of habitability of
the comets, naturally enough in his day, because he
thought them solid bodies surrounded by atmos-
LIFE IN OTHER WORLDS 251
phere, and related to the planets. The extremes of
temperature at perihelia and aphelia to which comets
are subjected did not bother him particularly.
After calculating that the comet of 1680, "being
160 times nearer to the sun than we are ourselves,
must have been subjected to a degree of heat
25,600 times as great as we are," Lambert goes on
to say : "Whether this comet was of a more compact
substance than our globe, or was protected in some
other way, it made its perihelion passage in safety,
and we may suppose all its inhabitants also passed
safely. No doubt they would have to be of a more
vigorous temperament and of a constitution very
different from our own. But why should all living
beings necessarily be constituted like ourselves? Is
it not infinitely more probable that amongst the
different globes of the universe a variety of organ-
izations exist, adapted to the wants of the people
who inhabit them, and fitting them for the places
in which they dwell, and the temperatures to which
they will be subjected? Is man the only inhabitant
of the earth itself? And if we had never seen
either bird or fish, should we not believe that the
air and water were uninhabitable? Are we sure
that fire has not its invisible inhabitants, whose
bodies, made of asbestos, are impenetrable to flame?
Let us admit that the nature of the beings who
inhabit comets is unknown to us; but let us not
deny their existence, and still less the possibility
of it."
Little enough is really known about the physical
nature of comets even now, but what we do know in-
dicates incessant transformation and instability of
conditions that would render life of any type ex-
ceedingly difficult of maintenance.
252 ASTRONOMY TO-DAY
A word about Sir William Herschel's theory of
the sun and its habitability. He thought the core of
the sun a dark, solid body, quite cold, and sur-
rounded by a double layer, the inner one of which
he conceived to act as a sort of fire screen to shield
the sun proper against the intense heat of the outer
layer, or photosphere by which we see it. Viewed
in this light, the sun, he says, "appears to be nothing
else than a very eminent, large and lucid planet,
evidently the first, or, in strictness of speaking, the
only primary one of our system .... It is most prob-
ably also inhabited, like the rest of the planets, by
beings whose organs are adapted to the peculiar
circumstances of that vast globe." But physics and
biology were undeveloped sciences in Herschel's days.
Herschel knew, however, that the stars are all
suns, so that he must have conceived that they are
inhabited also, quite independently of the question
whether they possess retinues of planets, after the
manner of our solar system.
This again is a question to which the astronomer
of the present day can give no certain answer. So
immensely distant are even the nearest of these
multitudinous bodies that no telescope can ever be
built large enough or powerful enough to reveal a
dark planet as large as Jupiter, alongside even the
nearest fixed star. Whatever may be the process
of stellar evolution, there doubtless is an era of
many hundreds of millions of years in the life of a
star when it is passing through a planet-maintain-
ing stage. This would likely depend upon spectral
type, or to be indicated by it ; and as about half of
the stars are of the solar type, it would be a reason-
able inference that at least half of the stars may
have planets tributary to them.
LIFE IN OTHER WORLDS 253
In such a case, the chances must be overwhelm-
ingly in favor of vast numbers of the planets of
other stellar systems being favorably circumstanced
as to heat and moisture for the maintenance of life
at the present time. That is, they are habitable,
and if habitable, then thousands of them are no
doubt inhabited now. But astronomers know abso-
lutely nothing about this question, nor are they able
to conceive at present any way that may lead them
to any definite knowledge of it. There is, indeed,
one piece of quasi-evidence which might reasonably
be interpreted as implying that it is more likely
that the stars are not attended by families of planets
than that they are.
CHAPTER XXXV
THE LITTLE PLANETS
AjONG toward the end of the eighteenth century
and the beginning of the nineteenth, astrono-
mers were leading a quiet unexcited life. Sir Wil-
liam Herschel had been knighted by King George for
his discovery of the outer planet Uranus, and practi-
cally everything seemed to be known and discovered
in the solar system with a single exception. Be-
tween Mars and Jupiter there existed an obvious
gap in the planetary brotherhood.
Could it be possible that some time in the remote
cosmic past a planet had actually existed there, and
that some celestial cataclasm had blown it to frag-
ments? If so, would they still be traveling round
the sun as individual small planets? And might it
not be possible to discover some of them among the
faint stars that make up the belt of the zodiac in
which all the other planets travel?
So interesting was this question that the first in-
ternational association of astronomers banded them-
selves together to carry on a systematic search
round the entire zodiacal heavens in the faint hope
of detecting possible fragments of the original
planet of mere hypothesis.
The astronomers of that day placed much reliance
on what is known as Bode's law not a law at all,
but a mere arithmetical succession of numbers
which represented very well the relative distances
254
THE LITTLE PLANETS 255
of all the planets from the sun. And the distance
of the newly found Uranus fitted in so well with
this law that the utter absence of a planet in the gap
between Mars and Jupiter became very strongly
marked.
Quite by accident a discovery of one of the
guessed-at small planetary bodies was made, on
January 1, 1801, in Palermo, Sicily, by Piazzi, who
was regularly occupied in making an extensive cata-
logue of the stars. His observations soon showed
that the new object he had seen could not be a fixed
star, because it moved from night to night among
the stars. He concluded that it was a planet, and
named it Ceres (1), for the tutelary goddess of
Sicily.
Other astronomers kept up the search, and an-
other companion planet, Pallas (2) was found in
the following year. Juno (3) was found in 1804,
and Vesta (4), the largest and brightest of all the
minor planets, in 1807. Vesta is sometimes bright
enough when nearest the earth to be seen with the
naked eye; but it was the last of the brighter ones,
and no more discoveries of the kind were made till
the fifth was found in 1845. Since then discoveries
have been made in great abundance, more and more
with every year till the number of little planets at
present known is very near 1,000.
The early asteroid hunters found the search
rather tedious, and the labor increased as it became
necessary to examine the increasing thousands of
fainter and fainter stars that must be observed in
order to detect the undiscovered planets, which
naturally grow fainter and fainter as the chase is
prolonged. First a chart of the ecliptic sky had to
be prepared containing all the stars that the tele-
256 ASTRONOMY TO-DAY
scope employed in the search would show. Some of
the most detailed charts of the sky in existence were
prepared in connection with this work, particularly
by the late Dr. Peters of Hamilton College. Once
such charts are complete, they are compared with
the sky, night after night when the moon is absent.
Thousands upon thousands of tedious hours are
spent in this comparison, with no result whatever
except that chart and sky are found to correspond
exactly.
But now and then the planet hunter is rewarded
by finding a new object in the sky that does not
appear on his chart. Almost certainly this is a
small planet, and only a few night's observation will
be necessary to enable the discoverer to find out
approximately the orbit it is traveling in, and
whether it is out-and-out a new planet or only one
that had been previously recognized, and then lost
track of.
Nearly all the minor planets so far found have
had names assigned to them principally legendary
and mythological, and a nearly complete catalogue
of them, containing the elements of their orbits
(that is, all the mathematical data that tell us about
their distance from the sun and the circumstances
of their motion around him) is published each year in
the "Annuaire du Bureau des Longitudes" at Paris.
But these little planets require a great deal of care
and attention, for some astronomers must accurately
observe them every few years, and other astrono-
mers must conduct intricate mathematical computa-
tions based on these observations; otherwise they
get lost and have to be discovered all over again.
Professor Watson, of the University of Michigan
and later of the University of Wisconsin, endowed
JUPITER, LARGEST OF THE PLANETS. The irregular belts change their
mutual relation and shapes because they do not represent land, but
are part of the atmosphere. (Photo, Yerkes Observatory.)
THE PLANET NEPTUNE AND ITS SATELLITE. The photograph required
an exposure of the plate for one hour. (Photo, Yerkes Observatory.)
SATURN, AS SEEN THROUGH THE 40-iNCH REFRACTOR, at the time
when only the edge of the rings is visible, showing condensations.
(Photo, YerJces Observatory.)
SATURN, PHOTOGRAPHED THROUGH THE 40-INCH REFRACTOR. The
rings appear opened to the fullest extent they can be seen from the
earth. The picture was made July 7, 1898. (Photo, Yerkes Observa-
tory. )
THE LITTLE PLANETS 257
the 22 asteroids of his own discovery, leaving to the
National Academy of Sciences a fund for prosecut-
ing this work perpetually, and Leuschner is now
ably conducting it.
While the number of the asteroids is gratifyingly
large, their individual size is so small and their total
mass so slight that, even if there are a hundred
thousand of them (as is wholly possible) , they would
not be comparable in magnitude with any one of the
great planets. Vesta, the largest, is perhaps 400
miles in diameter, and if composed of substances
similar to those which make up the earth, its mass
may be perhaps one twenty-thousandth of the earth's
mass. If we calculate the surface gravity on such a
body, we find it about one-thirtieth of what it is
here; so that a rifle ball, if fired on Vesta with a
muzzle velocity of only 2,000 feet a second, might
overmaster the gravity of the little planet entirely
and be projected in space never to return.
If, as is likely, some of the smallest asteroids are
not more than ten miles in diameter, their gravity
must be so feeble a force that it might be overcome
by a stone thrown from the hand. There is no re-
liable evidence that any of the asteroids are sur-
rounded by atmospheric gases of any sort. Probably
they are for the most part spherical in form,
although there is very reliable evidence that a few
of the asteroids, being variable in the amount of
sunlight that they reflect, are irregular in form,
mere angular masses perhaps.
The network of orbits of the asteroids is incon-
ceivable complicated. Nevertheless, there is a wide
variation in their average distance from the sun,
and their periods of traveling round him vary in a
similar manner, the shortest being only about three
Sci. Vol. 29
258 ASTRONOMY TO-DAY
years. While the longest is nearly nine years in
duration, the average of all their periods is a little
over four years. The gap in the zone of asteroids,
at a distance from the sun equal to about five-eighths
that of Jupiter, is due to the excessive disturbing
action of Jupiter, whose periodic time is just twice
as long as that of a theoretical planet at this distance.
The average inclination of their orbits to the
plane of the ecliptic is not far from 8 degrees. But
the orbit of Pallas, for example, is inclined 35 de-
grees, and the eccentricities of the asteroid orbits
are equally erratic and excessive. Both eccentricity
and inclination of orbit at times suggest a possible
relation to cometary orbits, but nothing has ever
been definitely made out connecting asteroids and
comets in a related origin.
No comprehensive theory of the origin of the as-
teroid group has yet been propounded that has met
with universal acceptance. According to the nebular
hypothesis the original gaseous material, which
should have been so concentrated as to form a planet
of ordinary type, has in the case of the asteroids col-
lected into a multitude of small masses instead of
simply one. That there is a sound physical reason
for this can hardly be denied. According to the
Laplacian hypothesis, the nearness of the huge
planetary mass of Jupiter just beyond their orbits
produced violent perturbations which caused the
original ring of gaseous material to collect into
fragmentary masses instead of one considerable
planet. The theory of a century ago that an original
great planet was shattered by internal explosive
forces is no longer regarded as tenable.
To astronomers engaged upon investigation of
distances in the solar system, the asteroid group has
THE LITTLE PLANETS 259
proved very useful. The late Sir David Gill em-
ployed a number of them in a geometrical research
for finding the sun's distance, and more recently the
discovery of Eros (433) has made it possible to
apply a similar method for a like purpose when it
approaches nearest to the earth in 1924 and 1931.
Then the distance of Eros will be less than half that
of Mars or even Venus at their nearest.
When the total number of asteroids discovered
has reached 1,000, with accurate determination of
all their orbits, we shall have sufficient material for
a statistical investigation of the group which ought
to elucidate the question of its origin, and bear on
other problems of the cosmogony yet unsolved.
Present methods of discovery of the asteroids by
photography replace entirely the old method by
visual observation alone, with the result that dis-
coveries are made with relatively great ease and
rapidity.
CHAPTER XXXVI
THE GIANT PLANET
I CAN never forget as a young boy my first
glimpse of the planet Jupiter and his moons ; it
was through a bit of a telescope that I had put
together with my own hands ; a tube of pasteboard,
and a pair of old spectacle lenses that chanced to
be lying about the house.
In the field of view I saw five objects; four of
them looking quite alike, and as if they were stars
merely (they were Jupiter's moons), while the
fifth was vastly larger and brighter. It was cir-
cular in shape, and I thought I could see a faint
darkish line across the middle of it.
This experience encouraged me immensely, and
I availed myself eagerly of the first chance to see
Jupiter through a bigger and better glass. Then
I saw at once that I had observed nothing wrongly,
but that I had seen only the merest fraction of
what there was to see.
In the first place, the planet's disk was not per-
fectly circular, but slightly oval. Inquiring into
the cause of this, we must remember that Jupiter
is actually not a flat disk but a huge ball or globe,
more than ten times the diameter of the earth,
which turns swiftly round on its axis once every
ten hours as against the earth's turning round in
twenty-four hours. Then it is easy to see how the
centrifugal force bulges outward the equatorial
260
THE GIANT PLANET 261
regions of Jupiter, so that the polar regions are
correspondingly drawn inward, thereby making the
polar diameter shorter than the equatorial one,
which is in line with the moons or satellites. The
difference between the two diameters is very
marked, as much as one part in fifteen. All the
planets are slightly flattened in this way, but Ju-
piter is the most so of all except Saturn.
The little darkish line across the planet's middle
region or equator was found to be replaced by sev-
eral such lines or irregular belts and spots, often
seen highly colored, especially with reflecting tele-
scopes; and they are perpetually changing their
mutual relation and shapes, because they are not
solid territory or land on Jupiter, but merely the
outer shapes of atmospheric strata, blown and torn
and twisted by atmospheric circulation on this
planet, quite the same as clouds in the atmosphere
on the earth are.
Besides this the axial turning of Jupiter brings
an entirely different part of the planet into view
every two or three hours ; so that in making a map
or chart of the planet, an arbitrary meridian must
be selected. Even then the process is not an easy
one, and it is found that spots on Ju'piter's equator
turn round in 9 hours 50 minutes, while other
regions take a few minutes longer, the nearer the
poles are approached. The Great Red Spot, about
30,000 miles long and a quarter as much in breadth
has been visible for about half a century. Bolton,
an English observer, has made interesting studies of
it very recently.
The four moons, or satellites, which a small
telescope reveals, are exceedingly interesting on
many accounts. They were the first heavenly
262 ASTRONOMY TO-DAY
bodies seen by the aid of the telescope, Galileo hav-
ing discovered them in 1610. They travel round
Jupiter much the same as the moon does round the
earth, but faster, the innermost moon about four
times per week, the second moon about twice a
week, the third or largest moon (larger than the
planet Mercury) once a week, and the outermost
in about sixteen days. The innermost is about
260,000 miles from Jupiter, and the outermost more
than a million miles. From their nearness to the
huge and excessively hot globe of Jupiter, some
astronomers, Proctor especially, have inclined to the
view that these little bodies may be inhabited.
Jupiter has other moons; a very small one, close
to the planet, which goes round in less than twelve
hours, discovered by Barnard in 1892. Four others
are known, very small and faint and remote from
the planet, which travel slowly round it in orbits of
great magnitude. The ninth, or outermost, is at a
distance of fifteen and one-half million miles from
Jupiter, and requires nearly three years in going
round the planet. It was discovered by Nicholson
at the Lick Observatory in 1914. The eighth was
discovered by Melotte at Greenwich in 1908, and
is peculiar in the great angle of 28 degrees, at
which its orbit is inclined to the equator of Jupiter.
The sixth and seventh satellites revolve round Ju-
piter inside the eighth satellite, but outside the
orbit of IV; and they were discovered by photog-
raphy at the Lick Observatory in 1905 by Perrine,
now director of the Argentine National Observ-
atory at Cordoba.
The ever-changing positions of the Medicean
moons, as Galileo called the four satellites that he
discovered their passing into the shadow in
THE GIANT PLANET 263
eclipse, their transit in front of the disk, and their
occultation behind it form a succession of phe-
nomena which the telescopist always views with
delight. The times when all these events take
place are predicted in the "Nautical Almanac," many
thousand of them each year, and the predictions
cover two or three years in advance.
Jupiter, as the naked eye sees him high up in
the midnight sky, is the brightest of all the planets
except Venus ; indeed, he is five times brighter than
Sirius, the brightest of all the fixed st'ars. His
stately motion among the stars will usually be
visible by close observation from day to day, and
his distance from the earth, at times when he. is
best seen, is usually about 400 million miles.
Jupiter travels all the way round the sun in twelve
years; his motion in orbit is about eight miles
a second.
The eclipses of Jupiter's moons, caused by pass-
ing into the shadow of the planet, would take place
at almost perfectly regular intervals, if our dis-
tance from Jupiter were invariable. But it was
early found out that while the earth is approach-
ing Jupiter the eclipses take place earlier and
earlier, but later and later when the earth is mov-
ing away. The acceleration of the earliest eclipse
added to the retardation of the latest makes 1,000
seconds, which is the time that light takes in cross-
ing a diameter of the earth's orbit round the sun.
Now the velocity of light is well known to be 186,-
300 miles per second, so we calculate at once and
very simply that the sun's distance from the earth,
which is half the diameter of the orbit, equals 500
times 186,300, or 93,000,000 miles.
CHAPTER XXXVII
THE RINGED PLANET
QJATURN is the most remote of all the planets
^ that the ancient peoples knew anything about.
These anciently known planets are sometimes
called the lucid or naked-eye planets five in num-
ber: Mercury, Venus, Mars, Jupiter, and Saturn.
Saturn shines as a first-magnitude star, with a
steady straw-colored light, and is at a distance of
about 800 million miles from the earth when best
seen. Saturn travels completely round the sun in
a little short of thirty years, and the telescope,
when turned to Saturn, reveals a unique and
astonishing object; a vast globe somewhat similar
to Jupiter, but surrounded by a system of rings
wholly unlike anything else in the universe, as far
as at present known; the whole encircled by a
family of ten moons or satellites. The Saturnian
system, therefore, is regarded by many as the most
wonderful and most interesting of all the objects
that the telescope reveals.
At first the flattening of the disk of Saturn is
not easily made out, but every fifteen years (as
1921 and 1936) the earth comes into a position
where we look directly at the thin edge of the
rings, causing them to completely disappear. Then
the remarkable flattening of the poles of Saturn is
strikingly visible, amounting to as much as one-
tenth of the entire diameter. The atmospheric belt
system is also best seen at these times.
264
THE RINGED PLANET 265
But the rings of Saturn are easily the most fas-
cinating features of the system. They can never
be seen as if we were directly above or beneath
the planet so they never appear circular, as they
really are in space, but always oval or elliptical in
shape. The minor axis or greatest breadth is
about one-half the major axis or length. The lat-
ter is the outer ring's actual diameter, and it
amounts to 170,000 miles, or two and one-half times
the diameter of Saturn's globe.
There are in fact no less than four rings; an
outer ring, sometimes seen to be divided near its
middle; an inner, broader and brighter ring; and
an innermost dusky, or crape ring, as it is often
called. This comes within about 10,000 miles of
the planet itself. After the form and size of the
rings were well made out, their thickness, or rather
lack of thickness, was a great puzzle.
If a model about a foot in diameter were cut out
of tissue paper, the relative proportion of size and
thickness would be about right. In space the thick-
ness is very nearly 100 miles, so that, when we look
at the ring system edge-on, it becomes all but in-
visible except in very large telescopes. Clearly a
ring so thin cannot be a continuous solid object
and recent observations have proved beyond a
doubt that Saturn's rings are made up of millions
of separate particles moving round the planet, each
as if it were an individual satellite.
Ever since 1857 the true theory of the constitu-
tion of the Saturnian ring has been recognized on
theoretic grounds, because Clerke-Maxwell founded
the dynamical demonstration that the rings could
be neither fluid nor solid, so that they must be
made up of a vast multitude of particles traveling
268 ASTRONOMY TO-DAY
round the planet independently. But the physical
demonstration that absolutely verified this conclu-
sion did not come until 1895, when, as we have
said in a preceding chapter, Keeler, by radial veloc-
ity measures on different regions of the ring by
means of the spectroscope, proved that the inner
parts of the ring travel more swiftly round the
planet than the outer regions do. And he further
showed that the rates of revolution in different
parts of the ring exactly correspond to the periods
of revolution which satellites of Saturn would have,
if at the same distance from the center of the
planet. The innermost particles of the dusky ring,
for example, travel round Saturn in about five
hours, while the outermost particles of the outer
bright ring take 137 hours to make their revolu-
tion. For many years it was thought that the Sa-
turnian ring system was a new satellite in process
of formation, but this view is no longer enter-
tained ; and the system is regarded as a permanent
feature of the planet, although astronomers are
not in entire agreement as to the evolutionary
process by which it came into existence whether
by some cosmic cataclysm, or by gradual develop-
ment throughout indefinite aeons, as the rest of the
solar system is thought to have come to its present
state of existence. Possibly the planetesimal hy-
pothesis of Chamberlin and Moulton affords the true
explanation, as the result of a rupture due to exces-
sive tidal strain.
CHAPTER XXXVIII
THE FARTHEST PLANETS
ON the 13th of March, 1781, between 10 and 11
P. M., as Sir William Herschel was sweeping
the constellation Gemini with one of his great re-
flecting telescopes, one star among all that passed
through the field of view attracted his attention.
Removing the eyepiece and applying another with
a higher magnifying power, he found that, unlike
all the other stars, this one had a small disk and was
not a mere point of light, as all the fixed stars seem
to be.
A few nights' observation showed that the
stranger was moving among the stars, so he thought
it must be a comet ; but a week's observation follow-
ing showed that he had discovered a new member
of the planetary system, far out beyond Saturn,
which from time immemorial had been assumed to
be the outermost planet of all. This, then, was the
first real discovery of a planet, as the finding of the
satellites of Jupiter had been the first of all astro-
nomical discoveries. Herschel's discovery occasioned
great excitement, and he named the new planet
Georgium Sidus or the Georgian, after his King.
The King created him a knight and gave him a
pension, besides providing the means for building a
huge telescope, 40 feet long, with which he subse-
quently made many other astronomical discoveries.
The planet that Herschel discovered is now called
Uranus.
267
268 ASTRONOMY TO-DAY
Uranus is an object not wholly impossible to see
with the naked eye, if the sky background is clear
and black, and one knows exactly where to look for
it. Its brightness is about that of a sixth magnitude
star or a little fainter. Its average distance from
the sun is about 1,800 million miles and it takes
eighty-four years to complete its journey round the
sun, traveling only a little more than four miles a
second. When we examine Uranus closely with a
large telescope, we find a small disk slightly greenish
in tint, very slightly flattened, and at times faint
bands or belts are apparently seen. Uranus is about
30,000 miles in diameter, and is probably surrounded
by a dense atmosphere. Its rotation time is 10 h.
50m.
Uranus is attended by four moons or satellites,
named Ariel, Umbriel, Titania, and Oberon, the last
being the most remote from the planet. This system
of satellites has a remarkable peculiarity : the plane
of the orbits in which they travel round Uranus is
inclined about 80 degrees to the plane of the ecliptic,
so that the satellites travel backward, or in a retro-
grade direction; or we might regard their motion
as forward, or direct, if we considered the planes
of their orbits inclined at 100 degrees.
For many years after the discovery of Uranus
it was thought that all the great bodies of the solar
system had surely been found. Least of all was any
planet suspected beyond Uranus until the mathe-
matical tables of the motion of Uranus, although
built up and revised with the greatest care and thor-
oughness, began to show that some outside influence
was disturbing it in accordance with Newton's
law of gravitation. The attraction of a still more
distant planet would account for the disturbance,
THE FARTHEST PLANETS 269
and since no such planet was visible anywhere a
mathematical search for it was begun.
NEPTUNE
Wholly independently of each other, two young
astronomers, Adams of England and LeVerrier of
France, undertook to solve the unique problem of
finding out the position in the sky where a planet
might be found that would exactly account for the
irregular motion of Uranus. Both reached practi-
cally identical results. Adams was first in point of
time, and his announcement led to the earliest ob-
servation, without recognition of the new planet
(July 30, 1846) , although it was Le Verrier's work
that led directly to the new planet's being first seen
and recognized as such (September 23, 1846). Fig-
uring backward, it was found that the planet had
been accidentally observed in Paris in 1795 , but its
planetary character had been overlooked.
Neptune is the name finally assigned to this his-
torical planet. It is thirty times farther from the
sun than the earth, or 2,800 million miles; its
velocity in orbit is a little over three miles per
second, and it consumes 164 years in going once
completely round the sun. So faint is it that a
telescope of large size is necessary to show it plainly.
The brightness equals that of a star of the eighth
magnitude, and with a telescope of sufficient mag-
nifying power, the tiny disk can be seen and meas-
ured. The planet is about 30,000 miles in diameter,
and is not known to possess more than one moon or
satellite. If there are others, they are probably too
faint to be seen by any telescope at present in
existence.
CHAPTER XXXIX
THE TRANS-NEPTUNIAN PLANET
TNVESTIGATION of the question of a possible
JL trans-Neptunian planet was undertaken by the
writer in 1877. As Neptune requires 164 years to
travel completely round the sun, and the period
during which it has been carefully observed em-
braces only half that interval, clearly its orbit can-
not be regarded as very well known. Any possible
deviations from the mathematical orbit could not
therefore be traced to the action of a possible un-
known planet outside. But the case was different
with Uranus, which showed very slight disturb-
ances, and these were assumed to be due to a pos-
sible planet exterior to both Uranus and Neptune.
As a position for this body in the heavens was indi-
cated by the writer's investigation, that region of
the sky was searched by him with great care in
1877-1878 with the twenty-six-inch telescope at
Washington; and photographs of the same region
were afterward taken by others, though only with
negative results.
In 1880, Forbes of Edinburgh published his in-
vestigation of the problem from an entirely inde-
pendent angle. Families of comets have long been
recognized whose aphelion distances correspond so
nearly with the distances of the planets that these
comet families are now recognized as having been
created by the several planets, which have reduced
270
THE TRANS-NEPTUNIAN PLANET 271
the high original velocities possessed by the comets
on first entering the solar system.
Their orbits have ever since been ellipses with
their aphelia in groups corresponding to the dis-
tances of the planets concerned. Jupiter has a large
group of such comets, also Saturn. Uranus and
Neptune likewise have their families of comets, and
Forbes found two groups with average distances
far outside of Neptune ; from which he drew the in-
ference that there are two trans-Neptunian planets.
The position he assigned to the inner one agreed
fairly well with the writer's planet as indicated by
unexplained deviations of Uranus.
The theoretical problem of a trans-Neptunian
planet has since been taken up by Gaillot and Lau
of Paris, the late Percival Lowell, and W. H. Picker-
ing of Harvard. The photographic method of search
will, it is expected, ultimately lead to its discovery.
On account of the probable faintness of the planet,
at least the twelfth or thirteenth magnitude, Met-
calf 's method of search is well adapted to this prac-
tical problem. When near its opposition the motion
of Neptune retrograding among the stars amounts
to five seconds of arc in an hour; while the trans-
Neptunian planet would move but three seconds.
By shifting the plate this amount hourly during ex-
posure, the suspected object would readily be de-
tected on the photographic plate as a minute and
nearly circular disk, all the adjacent stars being
represented by short trails.
Interest in a possible planet or planets outside the
orbit of Neptune is likely to increase rather than
diminish. To the ancients seven was the perfect
number, there were seven heavenly bodies already
known, so there could be no use whatever in looking
272 ASTRONOMY TO-DAY
for an eighth. The discovery of Uranus in 1781
proved the futility of such logic, and Neptune fol-
lowed in 1846 with further demonstration, if need
be. The cosmogony of the present day sets no outer
limit to the solar system, and some astronomers ad-
vocate the existence of many trans-Neptunian
planets.
CHAPTER XL
COMETS THE HAIRY STARS
/COMETS hairy stars, as the origin of the name
vJ would indicate are the freaks of the heavens.
Of great variety in shape, some with heads and some
without, some with tails and some without, moving
very slowly at one time and with exceedingly high
velocity at another, in orbits at all possible angles
of inclination to the general plane of the planetary
paths round the sun, their antics and irregularities
were the wonder and terror of the ancient world,
and they are keenly dreaded by superstitious people
even to the present day.
Down through the Middle Ages the advent of a
comet was regarded as:
Threatening the world with famine, plague and war;
To princes, death; to kingdoms, many curses;
To all estates, inevitable losses;
To herdsmen, rot; to plowmen, hapless seasons;
To sailors, storms; to cities, civil treasons.
Comets appeared to be marvelous objects, as well
as sinister, chiefly because they bid apparent de-
fiance to all law. Kepler had shown that the moon
and the planets travel in regular paths slightly
elliptical to be sure, but nevertheless unvarying.
None of the comets were known to follow regular
paths till the time of Halley late in the seventeenth
century, when, as we have before told, a fine comet
273
274 ASTRONOMY TO-DAY
made its appearance, and Halley calculated its orbit
with much precision. Comparing this with the
orbits of comets that had previously been seen, he
found its path about the sun practically identical
with that of at least two comets previously observed
in 1531 and 1607.
So Halley ventured to think all these comets were
one and the same body, and that it traveled round the
sun in a long ellipse in a period of about seventy-five
or seventy-six years. We have seen how his pre-
diction of its return in 1758 was verified in every
particular. On the comet's return in 1910, Crowell
and Crommelin of Greenwich made a thorough
mathematical investigation of the orbit, indicating
that the year 1986 will witness its next return to the
sun.
There is a class of astronomers known as comet-
hunters, and they pass hours upon hours of clear,
sparkling, moonless nights in search for comets.
They are equipped with a peculiar sort of telescope
called a comet-seeker, which has an object glass
usually about four or five inches in diameter, and a
relatively short length of focus, so that a larger
field of view may be included. Regions near the
poles of the heavens are perhaps the most fruitful
fields for search, and thence toward the sun till its
light renders the sky too bright for the finding of
such a faint object as a new comet usually is at the
time of discovery. Generally when first seen it
resembles a small circular patch of faint luminous
cloud.
When a suspect is found, the first thing to do is
to observe its position accurately with relation to the
surrounding stars. Then, if on the next occasion
when it is seen the object has moved, the chances
COMETS THE HAIRY STARS 275
are that it is a comet; and a few days' observation
will provide material from which the path of the
comet in space can be calculated. By comparing this
with the complete lists of comets, now about 700 in
number, it is possible to tell whether the comet is
a new one, or an old one returning. The total number
of comets in the heavens must be very great, and
thousands are doubtless passing continually unde-
tected, because their light is wholly overpowered
by that of the sun. Of those that are known, per-
haps one in twelve develops into a naked-eye comet,
and in some years six or seven will be discovered.
With sufficiently powerful telescopes, there are as
a rule not many weeks in the year when no comet
is visible. Brilliant naked-eye comets are, however,
infrequent.
Comets, except Halley's, generally bear the name
of their discoverer, as Donati (1858), and Pons-
B rooks (1893) . Pons was a very active discoverer of
comets in France early in the nineteenth century : he
was a doorkeeper at the observatory of Marseilles,
and his name is now more famous in astronomy than
that of Thulis, then the director of the Observatory,
who taught and encouraged him. Messier was
another very successful discoverer of comets in]
France, and in America we have had many : Swift,
Brooks, and Barnard the most successful.
How bright a comet will be and how long it will
be visible depends upon many conditions. So the
comets vary much in these respects. The first comet
of 1811 was under observation for nearly a year and
a half, the longest on record till Halley's in 1910.
In case a comet eludes discovery and observation
until it has passed its perihelion, or nearest point to
the sun, its period of visibility may be reduced to a
276 ASTRONOMY TO-DAY
few weeks only. The brightest comets on record
were visible in 1843 and 1882 : so brilliant were they
that even the effulgence of full daylight did not over-
power them. In particular the comet of 1843 was
not only excessively bright, but at its nearest ap-
proach to the earth its tail swept all the way across
the sky from one horizon to the other. It must have
looked very much like the straight beam of an
enormous searchlight, though very much brighter.
The tails of comets are to the naked eye the most
compelling thing about them, and to the ancient
peoples they were naturally most terrifying. Their
tails are not only curved, but sometimes curved with
varying degrees of curvature, and this circumstance
adds to their weirdness of appearance. If we ex-
amine the tail of a comet with a telescope, it vanishes
as if there were nothing to it: as indeed one may
almost say there is not. Ordinarily, only the head
of the comet is of interest in the telescope. When
first seen there is usually nothing but the head
visible, and that is made up of portions which
develop more or less rapidly, presenting a suc-
cession of phenomena quite different in different
comets.
When first discovered a comet is usually at a great
distance from the sun, about the distance of Jupiter ;
and we see it, not as we do the planets, by sunlight
reflected from them, but by the comet's own light.
This is at that time very faint, and nearly all comets
at such a distance look alike: small roundish hazy
patches of faint, cloudlike light, with very often a
concentration toward the center called the nucleus,
on the average about 4,000 miles in diameter. Ap-
proach toward the sun brightens up the comet more
and more, and the nucleus usually becomes very
COMETSTHE HAIRY STARS 277
much brighter and more starlike. Then on the
sunward side of the nucleus, jetlike streamers or
envelopes appear to be thrown off, often as if in
parallel curved strata, or concentrically. As they
expand and move outward from the nucleus, these
envelopes grow fainter and are finally merged in
the general nebulosity known as the comet's head,
which is anywhere from 30,000 to 100,000 miles in
diameter. As a rule, this is an orderly development
which can be watched in the telescope from hour to
hour and from night to night; but occasionally a
cometary visitor is quite a law to itself in develop-
ment, presenting a fascinating succession of unpre-
dictable surprises.
Then follows the development of the comet's tail,
perhaps more striking than anything that has pre-
ceded it. Here a genuine repulsion from the sun
appears to come into play. It may be an electrical
repulsion. Much of the material projected from the
comet's nucleus, seems to be driven backward or re-
pelled by the sun, and it is this that goes to form
the tail. The particles which form the tail then
travel in modified paths which nevertheless can be
calculated. The tail is made up of these luminous
particles and it expands in space much in the form
of a hollow, horn-shaped cone, the nucleus being
near the tip of the horn.
Some comets possess multiple tails with different
degrees of curvature, Donati's for example. Usually
there is a nearly straight central dark space, mark-
ing the axis of the comet, and following the nucleus.
But occasionally this is replaced by a thin light
streak very much less in breadth than the diameter
of the head. Cometary tails are sometimes 100 mil-
lion miles in length.
278 ASTRONOMY TO-DAY
Three different types of cometary tails are recog-
nized. First, the long straight ones, apparently made
up of matter repelled by the sun twelve to fifteen
times more powerfully than gravitation attracts it.
Such particles must be brushed away from the
comet's head with a velocity of perhaps five miles
a second, and their speed is continually increasing.
Probably these straight tails are due to hydrogen.
The second type tails are somewhat curved, or plume-
like, and they form the most common type of comet-
ary tail. In them the sun's repulsion is perhaps
twice its gravitational attraction, and hydrocarbons
in some form appear to be responsible for tails of
this character. Then there is a third type, much less
often seen, short and quickly curving, probably due
to heavier vapors, as of chlorine, or iron, or sodium,
in which the repulsive force is only a small fraction
of that of gravitation.
Many features of this theory of cometary tails
are borne out by examination of their light with the
spectroscope, although the investigation is as yet
fragmentary. It is evident that the tail of a comet
is formed at the expense of the substance of the
nucleus and head; so that the matter repelled is
forever dissipated through the regions of space
which the comet has traveled. Comets must lose
much of their original substance every time they
return to perihelion. Comets actually age, there-
fore, and grow less and less in magnitude of material
as well as brightness, until they are at last opaque,
nonluminous bodies which it becomes impossible to
follow with the telescope.
CHAPTER XLI
WHERE DO COMETS COME FROM?
WHERE do comets come from? The answer to
this question is not yet fully made out. Most
likely they have not all had a similar origin, and
theories are abundant. Apparently they come into
the solar system from outer space, from any direc-
tion whatsoever. The depths of interstellar space
seem to be responsible for most, if not all, of the new
ones. Whether they have come from other stars or
stellar systems we cannot say.
While comets are tremendous in size or volume,
their mass or the amount of real substance in them
is relatively very slight. We know this by the effect
they produce on planets that they pass near, or
rather by the effect that they fail to produce.
The earth's atmosphere weighs about one two hun-
dred and fifty thousandth as much as the earth it-
self, but a comet's entire mass must be vastly less
than this. Even if a comet were to collide with the
earth head on, there is little reason to believe that
dire catastrophe would ensue. At least twice the
earth is known to have passed through the tail of
a comet, and the only effect noticed was upon the
comet itself; its orbit had been modified somewhat
by the attraction of the earth. If the comet were
a small one, collision with any of the planets would
result in absorption and dissipation of the comet
into vapor.
279
280 ASTRONOMY TO-DAY
The whole of a large comet has perhaps as much
mass or weight as a sphere of iron a hundred
miles in diameter. Even this could not wreck the
earth, but the effect would depend upon what part
of the earth was hit. A comet is very thin and
tenuous, because its relatively small mass is dis-
tributed through a volume so enormous. So it is
probable that the earth's atmosphere could scatter
and burn up the invading comet, and we should
have only a shower of meteors on an unprecedented
scale. Diffusion of noxious gases through the at-
mosphere might vitiate it to some extent, though
probably not enough to cause the extinction of
animal life.
Every comet has an interesting history of its
own, almost indeed unique. One of the smallest
comets and the briefest in its period round the sun
is known as Encke's comet. It is a telescopic comet
with a very short tail, its time of revolution is
about three and a half years, and it exhibits a
remarkable contraction of volume on approach to
the sun.
Biela's comet has a period about twice as long.
At one time it passed within about 15 million miles
of the earth, and somewhere about the year 1840
this comet divided into two distinct comets, which
traveled for months side by side, but later sepa-
rated and both have since completely disappeared.
Perhaps the most beautiful of all comets is that dis-
covered by Donati of Florence in 1858. Its coma
presented the development of jets and envelopes in
remarkable perfection, and its tail was of the sec-
ondary or hydrocarbon type, but accompanied by
two faint streamer tails, nearly tangential to the
main tail and of the hydrogen type. Donati's
WHERE DO COMETS COME FROM? 281
comet moves in an ellipse of extraordinary length,
and it will not return to the sun for nearly 2,000
years.
The most brilliant comet of the last half century
is known as the great comet of 1882. In a clear
sky it could readily be seen at midday. On Septem-
ber 17 it passed across the disk of the sun and was
practically as bright as the surface of the sun itself.
The comet had a multiple nucleus and a hydro-
carbon tail of the second type, nearly a hundred
million miles in length. Doubtless this great comet
is a member of what is known as a cometary group,
which consists of comets having the same orbit
and traveling tandem round the sun. The comets
of 1668, 1843, 1880, 1882 and 1887 belong to this
particular group, and they all pass within 300,000
miles of the sun's surface, at a maximum velocity
exceeding 300 miles a second. They must there-
fore invade the regions of the solar corona, the
inference being that the corona as well as the comet
is composed of exceedingly rare matter;
Photography of comets has developed remark-
ably within recent years, especially under the deft
manipulation of Barnard, whose plates, in par-
ticular during his residence at the Lick Observ-
atory on Mount Hamilton, California, show the
features of cometary heads and tails in excellent
definition. Halley's comet, at the 1910 appari-
tion, was particularly well photographed at many
observatories.
The question is often asked, When will the next
comet come? If a large bright comet is meant,
astronomers cannot tell. At almost any time one
may blaze into prominence within only a few days.
During the latter half of the last century, bright
282 ASTRONOMY TQ-DAY
comets appeared at perihelion at intervals of eight
years on the average. Several of the lesser and
fainter periodic comets return nearly every year,
but they are mostly telescopic, and are rarely seen
except by astronomers who are particularly inter-
ested in observing them.
CHAPTER XLII
METEORS AND SHOOTING STABS
TALLING STARS," or "shooting stars," have
been familiar sights in all ages of the world, but
the ancient philosophers thought them scarcely
worthy of notice. According to Aristotle they were
mere nothings of the upper atmosphere, of no more
account than the general happenings of the weather.
But about the end of the eighteenth century and
the beginning of the nineteenth the insufficiency of
this view began to be fully recognized, and inter-
planetary space was conceived as tenanted by shoals
of moving bodies exceedingly small in mass and
dimension as compared with the planets.
Millions of these bodies are all the time in col-
lision with the outlying regions of our atmosphere ;
and by their impact upon it and their friction in
passing swiftly through it, they become heated to
incandescence, thus creating the luminous appear-
ances commonly known as shooting stars. For the
most part they are consumed or dissipated in vapor
before reaching the solid surface of the earth; but
occasionally a luminous cloud or streak is left glow-
ing in the wake of a large meteor, which sometimes
remains visible for half an hour after the passage
of the meteor itself. These mistlike clouds pro-
jected upon the dark sky have been especially studied
by Trowbridge of Columbia University.
Many more meteors are seen during the morning
hours, say from four to six, than at any other nightly
283
284 ASTRONOMY TO-DAY
period of equal length, because the visible sky is at
that time nearly centered around the general di-
rection toward which the earth is moving in its
orbit round the sun ; so that the number of meteors
that would fall upon the earth if at rest is increased
by those which the earth overtakes by its own
motion. Also from January to July while the earth
is traveling from perihelion to aphelion, fewer
meteors are seen than in the last half of the year;
but this is chiefly because of the rich showers en-
countered in August and November.
Although the descent of meteoric bodies from the
sky was pretty generally discredited until early in
the nineteenth century, such falls had nevertheless
been recorded from very early times. They were
usually regarded as prodigies or miracles, and such
stones were commonly objects of worship among
ancient peoples. For example, the Phrygian Stone,
known as the "Diana of the Ephesians which fell
down from Jupiter," was a famous stone built into
the Kaaba at Mecca, and even to-day it is revered by
Mohammedans as a holy relic. Perhaps the earliest
known meteoric fall is that historically recorded in
the Parian Chronicle as having occurred in the
island of Crete, B. C. 1478. Also in the imperial
museum of Petrograd is the Pallas or Krasnoiarsk
iron, perhaps three-quarters of a ton in weight,
found in 1772 by Pallas, the famous traveler, at
Krasnoiarsk, Siberia.
But a fall of meteoric stones that chanced upon
the department of Orne, France, in 1805, led to a
critical investigation by Biot, the distinguished
physicist and academician. According to his report
a violent explosion in the neighborhood of L'Aigle
had been heard for a distance of seventy-five miles
METEORS AND SHOOTING STARS 285
around, and lasting five or six minutes, about 1 P.M.
on Tuesday, April 26. From several adjoining
towns a rapidly moving fireball had been seen in a
sky generally clear, and there was absolutely no
room for doubt that on the same day many stones
fell in the neighborhood of L'Aigle. Biot estimated
their number between two and three thousand, and
they were scattered over an elliptical area more
than six miles long, and two and a half miles broad.
Thenceforward the descent of meteoric matter from
outer space upon the earth has been recognized as
an unquestioned fact.
The origin of these bodies being cosmic, meteors
may be expected to fall upon the earth without ref-
erence to latitude, or season, or day and night, or
weather. On entering our upper atmosphere their
temperature must be that of space, many hundred
degrees below zero ; and their velocities range from
ten miles per second upward. But atmospheric re-
sistance to their flight is so great that their velocity
is quickly reduced : at ground impact it does not ex-
ceed a few hundred feet per second. On January 1,
1869, several meteoric stones fell on ice only a few
inches thick in Sweden, rebounding without either
breaking through the ice or being themselves
fractured.
Naturally the flight of a meteor through the at-
mosphere will be only a few seconds in duration,
and owing to the sudden reduction of velocity, it will
continue to be luminuous throughout only the upper
part of its course. Visibility generally begins at an
elevation of about seventy miles, and ends at
perhaps half that altitude.
What is the origin of meteors? Theories there
are in great abundance: that they come from the
286 ASTRONOMY TO-DAY
sun, that they come from the moon, that they come
from the earth in past ages as a result of volcanic
action, and so on. But there are many difficulties in
the way of acceptance of these and several other
theories. That all meteors were originally parts of
cometary masses is however a theory that may be
accepted without much hesitation.
Comets have been known to disintegrate. Biela's
comet even disappeared entirely, so that during a
shower of Biela meteors in November, 1885, an
actual fragment of the lost comet fell upon the earth,
at Mazapil, Mexico. And as the Bielid meteors en-
counter the earth with the relatively low velocity of
ten miles a second, we may expect to capture other
fragments in the future. Numerous observers saw
the weird disintegration of the nucleus of the great
comet of 1882, well recognized as a member of the
family of the comet of 1843. As these comets are
fellow voyagers through space along the same orbit,
probably all five members of the family, with per-
haps others, were originally a single comet of un-
paralleled magnitude.
The Brooks comet of 1890 affords another instance
of fragmentary nucleus. The oft-repeated action of
solar forces tending to disrupt the mass of a comet
more and more, and scatter its material throughout
space, the secular dismemberment of all comets be-
comes an obvious conclusion. During the hundreds
of millions of years that these forces are known to
have been operant, the original comets have been
broken up in great numbers, so that elliptical rings
of opaque meteoric bodies now travel round the sun
in place of the comets.
These bodies in vast numbers are everywhere
through space, each too small to reflect an appreci-
METEORS AND SHOOTING STARS 287
able amount of sunlight, and becoming visible only
when they come into collision with our outer at-
mosphere. The practical identity of several such
meteor streams and cometary orbits has already
been established, and there is every reason for as-
signing a similar origin to all meteoric bodies.
Meteors, then, were originally parts of comets, which
have trailed themselves out to such extent that
particles of the primal masses are liable to be picked
up anywhere along the original cometary paths. The
historic records of all countries contain trustworthy
accounts of meteoric showers. Making due allow-
ances for the flowery imagery of the oriental, it
is evident that all have at one time or another seen
much the same thing. In A. D. 472, for instance, the
Constantinople sky was reported alive with flying
stars. In October, 1202, "stars appeared like waves
upon the sky; and they flew about like grasshop-
pers." During the reign of King William II occurred
a very remarkable shower in which "stars seemed
to fall like rain from heaven."
But the showers of November, 1799 and 1833, are
easily the most striking of all. The sky was filled
with innumerable fiery trails and there was not a
space in the heavens a few times the size of the moon
that was not ablaze with celestial fireworks. Fre-
quently huge meteors blended their dazzling bril-
liancy with the long and seemingly phosphorescent
trails of the shooting stars.
The interval of thirty-four years between 1799
and 1833 appeared to indicate the possibility of a
return of the shower in November of 1866 or 1867,
and all the people of that day were aroused on this
subject and made every preparation to witness the
spectacle. Extemporized observatories were estab-
288 ASTRONOMY TO-DAY
lished, watchmen were everywhere on the lookout,
and bells were to be rung the minute the shower
began. The newspapers of the day did little to allay
the fears of the multitude, but the critical days of
November, 1866, passed with disappointment in
America. In Europe, however, a fine shower was
seen, though it was not equal to that of 1833. The
astronomers at Greenwich counted many thousand
meteors. In November of 1867, however, American
astronomers were gratified by a grand display,
which, although failing to match the general expec-
tation, nevertheless was a most striking spectacle,
and the careful preparation for observing it afforded
data of observation which were of the greatest scien-
tific value. The actual orbits of these bodies in space
became known with great exactitude, and it was
found that their general path was identical with that
of the first comet of 1866, which travels outward
somewhat beyond the planet Uranus. When the
visible paths of these meteors are traced backward,
all appear as if they originated from the constella-
tion Leo. So they are known as Leonids, and a re-
turn of the shower was confidently predicted for
November, 1900-1901, which for unknown reasons
failed to appear.
During the last half century meteors have been
pretty systematically observed, especially by the as-
tronomers of Italy and Denning of England, so that
several hundred distinct showers are now known,
their radiant points fall in every part of the heavens,
and there is scarcely a clear moonless night when
careful watching for meteors will be unrewarded.
Besides November, the months of August (Per-
seids), April (Lyrids), and December (Geminids)
are favorable. Following in tabular form is a fairly
Two VIEWS OF HALLEY'S COMET. Taken with the same camera from
the same position, one on May 12, and the other on May 15, 1910.
(Photo, Mt. Wilson Solar Observatory.)
SWIFT'S COMET OF 1892. This comet showed extraordinary and rapid
transformations, one day having a dozen streamers in its tail, another
only two. (Photo by Prof. E. E. Barnard.)
METEORS AND SHOOTING STARS 289
comprehensive list of the meteoric showers of the
year, with the positions of the radiant points and
the epochs of the showers according to Denning :
RADIANT POINT
Name of Shower
R. A.
Decl.
Date of Shower
230
+ 53
Jan. 2-4
331
+ 56
Jan. 25
155
4-14
Feb. 19-March 1
Tau Leonids
166
-j-4
March 1-4
161
+ 58
March 13-24
271
+33
April 20-22
Gamma Aquarids
338
2
May 1-6
Zeta Herculids
246
4-29
May 18-26
Eta Pegasids
330
+28
May 30-June 4
Theta Bootids
213
4-53
June 27-28
Alpha Capricornids . ......
304
12
July 15-28
Delta Aquarids
339
11
July 25-30
45
4-57
Aug. 10-12
Omicron Draconids
291
4-60
Aug. 15-25
Zeta Draconids
262
4-63
Aug. 21-Sept. 2
Piscids
348
+2
Sept. 4-14
Alpha Andromedids. . . .
4
4-28
Sept. 27
Epsilon Arietids
40
4-20
Oct. 11-24
Orionids
92
4-15
Oct. 17-24
Epsilon Perseids
61
4-35
Nov. 5
Leonids
150
4-23
Nov. 13-15
Epsilon Taurids
64
4-22
Nov. 14-25
Andromedids
25
4-43
Nov. 17-23
Beta Geminids
119
4-31
Dec. 1-12
108
+33
Dec. 1-14
Alpha Ursae Majorids. . . .
Kappa Draconids
161
194
4-58
4-68
Dec. 18-21
Dec. 18-28
The year 1916 was exceptional in providing an
abundant and previously unknown shower on
June 28, and its stream has nearly the same orbit
as that of the Pons-Winnecke periodic comet. Use-
ful observations of meteors are not difficult to make,
and they are of service to professional astronomers
investigating the orbits of these bodies, among whom
are Mitchell and Olivier of the University of Virginia.
Sci. Vol. 210
CHAPTER XLIII
METEORITES
TV TETEORITES, the name for meteors which have
iVJL actually gone all the way through our atmos-
phere, are never regular in form or spherical. As a
rule the iron meteorites are covered with pittings or
thumb marks, due probably to the resistance and im-
pact of the little columns of air which impede its
progress, together with the unequal condition and
fusibility of their surface material. The work done
by the atmosphere in suddenly checking the meteor's
velocity appears in considerable part as heat, fusing
the exterior to incandescence. This thin liquid shell
is quickly brushed off, making oftentimes a luminous
train.
But notwithstanding the exceedingly high temper-
ature of the exterior, enforced upon it for the brief
time of transit through the atmosphere, it is prob-
able that all large meteorites, if they could be reached
at once on striking the earth, would be found to
be cold, because the smooth, black, varnishlike
crust which always incases them as a result of
intense heat is never thick. On one occasion a
meteor which was seen to fall in India was dug out
of the ground as quickly as possible, and found to
be, not hot as was expected, but coated thickly over
with ice frozen on it from the moisture in the sur-
rounding soil.
290
METEORITES 291
As to the composition of shooting stars, and their
probable mass, and its effect upon the earth, our
data are quite insufficient. The lines of sodium and
magnesium have been hurriedly caught in the spec-
troscope, and, estimating on the basis of the light
emitted by them, the largest meteors must weigh
ounces rather than pounds. Nevertheless, it is in-
teresting to inquire what addition the continual fall
of many millions daily upon the earth makes to its
weight: somewhere between thirty and fifty thou-
sand tons annually is perhaps a conservative esti-
mate, but even this would not accumulate a layer
one inch in thickness over the entire surface of the
earth in less than a thousand million years.
Many hundreds of the meteors actually seen to
fall, together with those picked up accidentally, are
recovered and prized as specimens of great value in
our collections, the richest of which are now in New
York, Paris, and London. The detailed investiga-
tion of them is rather the province of the chemist,
the crystallographer and the mineralogist than of
the astronomer whose interest is more keen in their
life history before they reach the earth. To dis-
tinguish a stony meteorite from terrestrial rock sub-
stances is not always easy, but there is usually little
difficulty in pronouncing upon an iron meteorite.
These are most frequently found in deserts, because
the dryness of the climate renders their oxidation
and gradual disappearance very slow.
The surface of a suspected iron meteorite is
polished to a high luster and nitric acid is poured
upon it. If it quickly becomes etched with a char-
acteristic series of lines, or a sort of cross-hatching,
it is almost certain to be a meteorite. Occasionally
carbon has been found in meteorites, and the ex-
292 ASTRONOMY TO-DAY
istence of diamond has been suspected. The miner-
als composing meteorites are not unlike terrestrial
materials of volcanic origin, though many of them
are peculiar to meteorites only. More than one-
third of all the known chemical elements have been
found by analysis in meteorites, but not any new
ones.
Meteoric iron is a rich alloy containing about ten
per cent of nickel, also cobalt, tin, and copper in much
smaller amount. Calcium, chlorine, sodium, and
sulphur likewise are found in meteoric irons. At
very high temperatures iron will absorb gases and
retain them until again heated to red heat. Car-
bonic oxide, helium, hydrogen, and nitrogen are
thus imprisoned, or occluded, in meteoric irons in
very small quantities ; and in 1867, during a London
lecture by Graham, a room in the Royal Institution
was for a brief space illuminated by gas brought to
earth in a meteorite from interplanetary space.
Meteorites, too, have been most critically investi-
gated by the biologist, but no trace of germs of
organic life of any type has so far been found.
Farrington of Chicago has published a full
descriptive catalogue of all the North American
meteorites.
Recent investigations of the radioactivity of me-
teorites show that the average stone meteorite is
much less radioactive than the average rock, and
probably less than one-fourth as radioactive as
in average granite. The metallic meteorites ex-
amined were found about wholly free from radio-
activity.
From shooting stars, perhaps the chips of the
celestial workshop, or more possibly related to the
planetesimals which the processes of growth of the
METEORITES 293
universe have swept up into the vastly greater
bodies of the universe, transition is natural to the
stars themselves, the most numerous of the heavenly
bodies, all shining by their own light, and all in-
conceivably remote from the solar system, which
nevertheless appears to be not far removed from
the center of the stellar universe.
CHAPTER XLIV
THE UNIVERSE OF STARS
OUR consideration of the solar system hitherto
has kept us quite at home in the universe. The
outer known planets, Uranus and Neptune, are in-
deed far removed from the sun, and a few of the
comets that belong to our family travel to even
greater distances before they begin to retrace their
steps sunward. When we come to consider the vast
majority of the glistening points on the celestial
sphere all in fact except the five great planets,
Mercury, Venus, Mars, Jupiter, and Saturn we are
dealing with bodies that are self-luminous like the
sun, but that vary in size quite as the bodies of the
solar system do, some stars being smaller than the
sun and others many hundred fold larger than
he is; some being "giants," and others "dwarfs."
But the overwhelming remoteness of all these
bodies arrests our attention and even taxes our
credulity regarding the methods that astronomers
have depended on to ascertain their distances
from us.
Their seeming countlessness, too, is as bewilder-
ing as are the distances ; though, if we make actual
counts of those visible to the naked eye within a
certain area, in the body of the "Great Bear/' for
example, the great surprise will be that there are so
few. And if the entire dome of the sky is counted,
at any one time, a clear, moonless sky would reveal
294
THE UNIVERSE OF STARS 295
perhaps 2,500, so that in the entire sky, northern
and southern, we might expect to find 5,000 to 6,000
lucid stars, or stars visible to the naked eye.
But when the telescope is applied, every accession
of power increases the myriads of fainter and
fainter stars, until the number within optical reach
of present instruments is somewhere between 400
and 500 millions. But if we were to push the 100-
inch reflector on Mount Wilson to its limit by pho-
tography with plates of the highest sensitiveness,
millions upon millions of excessively faint stars
would be plainly visible on the plates which the
human eye can never hope to see directly with any
telescope present or future, and which would doubt-
less swell the total number of stars to a thousand
millions. Recent counts of stars by Chapman and
Melotte of Greenwich tend to substantiate this
estimate.
What have astronomers done to classify or cata-
logue this vast array of bodies in the sky? Even
before making any attempt to estimate their num-
ber, there is a system of classification simply by the
amount of light they send us, or by their apparent
stellar magnitudes not their actual magnitudes, for
of those we know as yet very little. We speak of
stars of the "first magnitude," of which there are
about 20, Sirius being the brightest and Regulus the
faintest. Then there are about 65 of the second, or
next fainter, magnitude, stars like Polaris, for ex-
ample, which give an amount of light two and a half
times less than the average first magnitude star.
Stars of the third magnitude are fainter than those
of the second in the same ratio, but their number in-
creases to 200 ; fourth magnitude, 500 ; fifth magni-
tude, 1,400; sixth magnitude, 5,000, and these are so
296 ASTRONOMY TO-DAY
faint that they are just visible on the best nights
without telescopic aid.
Decimals express all intermediate graduations of
magnitude. Astronomers carry the telescopic mag-
nitudes much farther, till a magnitude beyond the
twentieth is reached, preserving in every case the
ratio of two and one-half for each magnitude in re-
lation to that numerically next to it. Even Jupiter
and Venus, and the sun and moon, are sometimes
calculated on this scale of stellar magnitude, numer-
ically negative, of course, Venus sometimes being as
bright as magnitude 4.3, and the sun 26.7.
Knowing thus the relation of sun, moon, and
stars, and the number of the stars of different mag-
nitudes, it is possible to estimate the total light from
the stars. This interesting relation comes out this
way: that the stars we cannot see with the naked
eye give a greater total of light than those we can
because of their vastly greater numbers. And if
we calculate the total light of all the brighter stars
down to magnitude nine and one-half, we find it
equal to l/80th of the light of the average full moon.
Many stars show marked differences in color, and
strictly speaking the stars are now classified by
their colors. The atmosphere affects star colors
very considerably, low altitudes, or greater thick-
ness of air, absorbing the bluish rays more strongly
and making the stars appear redder than they really
are. Aldebaran, Betelgeuse and Antares are well-
known red stars, Capella and Alpha Ceti yellowish,
Vega and Sirius blue, and Procyon and Polaris
white. Among the telescopic stars are many of a
deep blood-red tint, variable stars being numerous
among them. Double stars, too, are often comple-
mentary in color. There is evidence indicating
THE UNIVERSE OF STARS 297
change of color of a very few stars in long periods
of time; Sirius, for example, two thousand years ago
was a red star, now it is blue or bluish white. But the
meaning of color, or change of color in a star is as
yet only incompletely ascertained. It may be con-
nected with the radiative intensity of the star, or
its age, or both.
The late Professor Edward C. Pickering was
famous for his life-long study and determination of
the magnitudes of the stars. Standards of com-
parison have been many, and have led to much un-
necessary work. Pickering chose Polaris as a
standard and devised the meridian photometer, an
ingenious instrument of high accuracy, in which the
light of a star is compared directly with that of the
pole star by reflection. All the bright stars of both
the northern and the southern skies are worked
into a standard system of magnitudes known as
HP, or the Harvard Photometry.
Astronomers make use of several different kinds
of magnitude for the stars : the apparent magnitude,
as the eye sees it, often called the visual magnitude ;
the photographic magnitude, as the photographic
plate records it, and these are now determined with
the highest accuracy; the photovisual magnitude,
quite the same as the visual, but determined photo-
graphically on an isochromatic plate with a yellow
screen or filter, so that the intensity is nearly the
same as it appears to the eye. The difference be-
tween the star's visual or photovisual magnitude
and its photographic magnitude is called its color-
index, and is often used as a measure of the star's
color. Light of the shorter wavelengths, as blue
and violet, affects the photographic plate more
rapidly than the reds and yellows of longer wave
298 ASTRONOMY TO-DAY
length by which the eye mainly sees; so that red
stars will appear much fainter and blue stars much
brighter on the ordinary photographic plate than
the eye sees them.
So great are the differences of color in the stars
that well-known asterisms, with which the eye is
perfectly familiar, are sometimes quite unrecogniz-
able on the photographic plate, except by relative
positions of the stars composing them. White stars
affect the eye and the plate about equally, so that
their visual or photovisual and photographic magni-
tudes are about equal. The studies of the colors
of the stars, the different methods of determining
them, and the relations of color to constitution have
been made the subject of especial investigation by
Seares of Mount Wilson and many other astrono-
mers.
Centuries of the work of astronomers have been
faithfully devoted to mapping or charting the stars
and cataloguing them. Just as we have geographical
maps of countries, so the heavens are parceled out
in sections, and the stars set down in their true
relative positions just as cities are on the map. Re-
cent years have added photographic charts, espe-
cially of detailed regions of the sky ; but owing to
spectral differences of the stars, their photographic
magnitudes are often quite different from their visual
magnitudes. From these maps and charts the
positions of the stars can be found with much pre-
cision; but if we want the utmost accuracy, we
must go to the star catalogues huge volumes often-
times, with stellar positions set down therein with
the last degree of precision.
First there will be the star's name, and in the next
column its magnitude, and in a third the star's
THE UNIVERSE OF STARS 299
right ascension. This is its angular distance east-
ward around the celestial sphere starting from the
vernal equinox, and it corresponds quite closely to
the longitude of a place which we should get from a
gazetteer, if we wished to locate it on the earth.
Then another column of the catalogue will give the
star's declination, north or south of the equator,
just as the gazetteer will locate a city by its north
or south latitude.
CHAPTER XLV
STAR CHARTS AND CATALOGUES
WHO made the first star chart or catalogue?
There is little doubt that Eudoxus (B. c. 200)
was the first to set down the positions of all the
brighter stars on a celestial globe, and he did this
from observations with a gnomon and an armillary
sphere. Later Hipparchus (B. C. 130) constructed
the first known catalogue of stars, so that astrono-
mers of a later day might discover what changes
are in progress among the stars, either in their re-
lative positions or caused by old stars disappearing
or new stars appearing at times in the heavens.
Hipparchus was an accurate observer, and he dis-
covered an apparent and perpetual shifting of the
vernal equinox westward, by which the right as-
censions of the stars are all the time increasing.
He determined the amount of it pretty accurately,
too. His catalogue contained 1,080 stars, and is
printed in the "Almagest" of Ptolemy.
Centuries elapsed before a second star catalogue
was made, by Ulugh-Beg, an Arabian astronomer,
A. D. 1420, who was a son of Tamerlane, the Tartar
monarch of Samarcand, where the observations for
the catalogue were made. The stars were mainly
those of Ptolemy, and much the same stars were re-
observed by Tycho Brahe (A. D. 1580) with his
greatly improved instruments, thus forming the
300
STAR CHARTS AND CATALOGUES 301
third and last star catalogue of importance before
the invention of the telescope.
From the end of the seventeenth century onward,
the application of the telescope to all the types of in-
struments for making observations of star places
has increased the accuracy manyfold. The entire
heavens has been covered by Argelander in the
northern hemisphere, and Gould in the southern-
over 700,000 stars in all. Many government observ-
atories are still at work cataloguing the stars. The
Carnegie Institution of Washington maintains a
department of astrometry under Boss of Albany,
which has already issued a preliminary catalogue
of more than 6,000 stars, and has a great general
catalogue in progress, together with investigations
of stellar motions and parallaxes. This catalogue
of star positions will include proper motions of
stars to the seventh magnitude.
In 1887 on proposal of the late Sir David Gill,
an international congress of astronomers met at
Paris and arranged for the construction of a photo-
graphic chart of the entire heavens, allotting the
work to eighteen observatories, equipped with photo-
graphic telescopes essentially alike. The total num-
ber of plates exceeds 25,000. Stars of the fourteenth
magnitude are recorded, but only those including
the eleventh magnitude will be catalogued, perhaps
2,000,000 in all. The expense of this comprehensive
map of the stars has already exceeded $2,000,000,
and the work is now nearly complete. Turner of
Oxford has conducted many special investigations
that have greatly enhanced the progress of this in-
ternational enterprise.
Other great photographic star charts have been
carried through by the Harvard Observatory, with
302 ASTRONOMY TO-DAY
the annex at Arequipa, Peru, employing the Bruce
photographic telescope, a doublet with 24-inch
lenses; also Kapteyn of Groningen has catalogued
about 300,000 stars on plates taken at Cape Town.
Charting and cataloguing the stars, both visually
and photographically, is a work that will never be
entirely finished. Improvements in processes will
be such that it can be better done in the future than
it is now, and the detection of changes in the fainter
stars and investigation of their motions will neces-
sitate repetition of the entire work from century
to century.
The origin of the names of individual stars is a
question of much interest. The constellation figures
form the basis of the method, and the earliest names
were given according to location in the especial
figure; as for instance, Cor Scorpii, the heart of the
Scorpion, later known as Antares or Alpha Scorpii.
The Arabians adopted many star names from the
Greeks, and gave about a hundred special names
to other stars. Some of these are in common use
to-day, by navigators, observers of meteors and of
variable stars. Sirius, Vega, Arcturus, and a few
other first magnitude stars, are instances.
But this method is quite insufficient for the
fainter stars whose numbers increase so rapidly.
Bayer, a contemporary of Galileo, originated our
present system, which also employs the names of the
constellations, the Latin genitive in each case, pre-
fixed by the small letters of the Greek alphabet,
from alpha to omega, in order of decreasing bright-
ness ; and followed by the Roman letters when the
Greek alphabet is exhausted.
If there were still stars left in a constellation un-
named, numbers were used, first by Flamsteed,
STAR CHARTS AND CATALOGUES 303
Astronomer Royal; and numbers in the order of
right ascension in various catalogues are used to
designate hundreds of other stars. The vast bulk
of the stars are, however, nameless; but about one
million are identifiable by their positions (right
ascension and declination) on the celestial sphere.
CHAPTER XLVI
THE SUN'S MOTION TOWARD LYRA
IF Hipparchus or Galileo should return to earth
to-night and look at the stars and constellations
as we see them, there would be no change whatever
discernible in either the brightness of the stars or in
their relative positions. So the name fixed stars
would appear to have been well chosen. Halley in the
seventeenth century was the first to detect that slow
relative change of position of a few stars which is
known as proper motion, and all the modern cata-
logues give the proper motions in both right as-
cension and declination. These are simply the
small annual changes in position athwart the line
of vision ; and, as a whole, the proper motions of the
brighter stars exceed the corresponding motions of
the fainter ones because they are nearer to us. The
average proper motion of the brightest stars is
0".25, and of stars of the sixth magnitude only one-
sixth as great.
A few extreme cases of proper motion have been
detected, one as large as 9", of an orange yellow
star of the eighth magnitude in the southern con-
stellation Pictor, and Barnard has recently dis-
covered a star with a proper motion exceeding 10" ;
several determinations of its parallax give 0".52,
corresponding to a distance of 6.27 light years.
Nevertheless, two centuries would elapse before
these stars would be displaced as much as the
304
SUN'S MOTION TOWARD LYRA 305
breadth of the moon among their neighbors in the
sky. The proper motions of stars are along per-
fectly straight lines, so far as yet observed. Ulti-
mately we may find a few moving in curved paths
or orbits, but this is hardly likely.
As for a central sun hypothesis, that pointing out
Alcyone in particular, there is no reliable evidence
whatever. Analysis of the proper motions of stars
in considerable numbers, first by Sir William Her-
schel, showed that they were moving radially from
the constellation Hercules, and in great numbers
also toward the opposite side of the stellar sphere.
Later investigation places this point, called the
sun's goal, or apex of the sun's way, over in the
adjacent constellation Lyra; and the opposite point,
or the sun's quit, is about halfway between Sirius
and Canopus. By means of the radial velocities of
stars in these antipodal regions of the sky, it is
found that the sun's motion toward Lyra, carrying
all his planetary family along with him, is taking
place at the rate of about 12 miles in every second.
While the right ascensions of the solar apex as
given by the different investigations have been
pretty uniform, the declination of this 'point has
shown a rather wide variation not yet explained.
For example, there is a difference of nearly ten
degrees between the declination (-h34.3) of the
apex as determined by Boss from the proper mo-
tions of more than 6,000 stars, and the declination
(H-25.3) found by Campbell from the radial velo-
cities of nearly 1,200 stars. Several investigations
tend to show that the fainter the stars are, the
greater is the declination of the solar apex. More
remarkable is the evidence that this declination
varies with the spectral type of the stars, the later
306 ASTRONOMY TO-DAY
types, especially G and K, giving much more north-
erly values. On the whole the great amount of re-
search that has been devoted to the solar motion
relative to the system of the stars for the past
hundred years may be said to indicate a point in
right ascension 18 h (270) and declination 34 K.
as the direction toward which the sun is moving.
This is not very far from the bright star Alpha
Lyrae, and the antipodal point from which the sun
is traveling is quite near to Beta Columbse.
So swift is this motion (nearly twenty kilometers
per second) that it has provided a base line of excep-
tional length, and very great service in determining
the average distance of stars in groups or classes.
After thousands of years the sun's own motion com-
bined with the proper motions of the stars will dis-
place many stars appreciably from their familiar
places. The constellations as we know them will
suffer slight distortions, particularly Orion, Cassio-
peia and Ursa Major. Identity or otherwise of
spectra often indicates what stars are associated
together in groups, and their community of motion
is known as star drift. Recent investigation of vast
numbers of stars by both these methods have led to
the epochal discovery of star streaming, which in-
dicates that the stars of our system are drifting by,
or rather through, each other, in two stately and
interpenetrating streams. The grand primary
cause underlying this motion is as yet only surmised.
CHAPTER XLVII
STARS AND THEIR SPECTRAL TYPE
WHEN in 1872 Dr. Henry Draper placed a very
small wet plate in the camera of his spectro-
scope and, by careful following, on account of the
necessarily long exposure, secured the first photo-
graphic spectrum of a star ever taken, he could
hardly have anticipated the wealth of the new field
of research which he was opening. His wife, Anna
Palmer Draper, was his enthusiastic assistant in
both laboratory and observatory, and on his death in
1882, she began to devote her resources very con-
siderably to the amplification of stellar spectrum
photography. At first with the cooperation of
Professor Young of Princeton, and later through
extension of the facilities of Harvard College
Observatory, whose director, the late Professor
Edward C. Pickering, devoted his energies in very
large part to this matter, all the preliminaries of
the great enterprise were worked out, and a compre-
hensive program was embarked upon, which cul-
minated in the "Henry Draper Memorial," a cata-
logue and classification of the spectra of all the stars
brighter than the ninth magnitude, in both the
northern and southern hemispheres.
One very remarkable result from the investiga-
tion of large numbers of stars according to their
type is the close correlation between a star's lumi-
nosity and its spectral type. But even more remark-
307
308 ASTRONOMY TO-DAY
able is the connection between spectral type and
speed of motion. As early as 1892 Monck of Dublin,
later Kapteyn, and still later Dyson, directed atten-
tion to the fact that stars of the Secchi type II had
on the average larger proper motions than those of
type I. In 1903 Frost and Adams brought out the
exceptional character of the Orion stars, the radial
velocities of twenty of which averaged only seven
kilometers per second.
Soon after, with the introduction of the two-
stream hypothesis, a wider generalization was
reached by Campbell and Kapteyn, whose radial ve-
locities showed that the average linear velocity in-
creases continually through the entire series B, A,
F, G, K, M, from the earliest types of evolution to
the latest. The younger stars of early type have
velocities of perhaps five or six kilometers per sec-
ond, while the older stars of later type have velocities
nearly fourfold greater.
The great question that occurs at once is : How do
the individual stars get their motions? The farther
back we go in a star's life history, the smaller we
find its velocity to be. When a star reaches the
Orion stage of development, its velocity is only one-
third of what it may be expected to have finally.
Apparently, then, the stars at birth have no motion,
but gradually acquire it in passing through their
several types or stages of development.
More striking still is the motion of the planetary
nebulae, in excess of 25 kilometers per second, while
type A stars move 11 kilometers, type G 15 kilome-
ters, and type M 17 kilometers per second. Can the
law connecting speed of motion and spectral type
be so general that the planetary nebula is to be re-
garded as the final evolutionary stage? Stars have
STARS AND THEIR SPECTRAL TYPE 309
been seen to become nebulse, and one astronomer at
least is strongly of the opinion that a single such
instance ought to outweigh all speculation to the
contrary, as that stars originate from nebulae.
In his discussion of stellar proper motions, Boss
has reached a striking confirmation of the relation
of speed to type, finding for the cross linear motion
of the different types a series of velocities closely
paralleling those of Kapteyn and Campbell.
Concerning the marked relation of the luminosi-
ties of the stars to their spectral types, there is a
pronounced tendency toward equality of brightness
among stars of a given type; also the brightness
diminishes very markedly with advance in the stage
of evolution. There has been much discussion as to
the order < of evolution as related to the type of
spectrum, and Russell of Princeton has put forward
the hypothesis of giant stars and dwarf stars, each
spectral type having these two divisions, though not
closely related. One class embraces intensely lumi-
nous stars, the other stars only feebly luminous.
When a star is in process of contraction from a
diffused gaseous mass, its temperature rises, accord-
ing to Lane's law, until that density is reached where
the loss of heat by radiation exceeds the rise in tem-
perature due to conversion of gravitational energy
into heat. Then the star begins to cool again. So
that if the spectrum of a star depends mainly on the
effective temperature of the body, clearly the classi-
fication of the Draper catalogue would group stars
together which are nearly alike in temperature,
taking no note as to whether their present temper-
ature is rising or falling.
Another classification of stars by Lockyer divides
them according to ascending and descending tern-
310 ASTRONOMY TO-DAY
peratures. Russell's theory would assign the suc-
cession of evolutionary types in the order, Mi, Ki,
Gi, Fi, Ai, B, A>, F2, G*, IG, Ms, the subscript 1 re-
ferring to the "giants," and 2 to the dwarf stars.
In large part the weight of evidence would appear
to favor the order of the Harvard classification, in-
dependently confirmed as it is by studies of stellar
velocities, Galactic distribution, and periods of bi-
nary stars both spectroscopic and visual, where
Campbell and Aiken find a marked increase in
length of period with advance in spectral type. At
the same time, a vast amount of evidence is accumu-
lating in support of Russell's theory. Investigations
in progress will doubtless reveal the ground on
which both may be harmonized.
The publication of the new Henry Draper Cata-
logue of Stellar Spectra is in progress, a work of
vast magnitude. The great catalogue of thirty
years ago embraced the spectra of more than ten
thousand stars, and was a huge work for that day ;
but the new catalogue utterly dwarfs it, with a
classification much more detailed than in the
earlier work, and with the number of stars increased
more than twentyfold. This work, projected by the
late director of the Harvard Observatory, has been
brought to a conclusion by the energy and enthu-
siasm of Miss Annie J. Cannon through six years of
close application, aided by many assistants. The
catalogue ranges over the stars of both hemispheres,
and is a monument to masterly organization and
completed execution which will be of the highest
importance and usefulness in all future researches
on the bodies of the stellar universe.
CHAPTER XLVIII
STAR DISTANCES
SO vast are the distances of the stars that all at-
tempts of the early astronomers to ascertain
them necessarily proved futile. This led many as-
tronomers after Copernicus to reject his doctrine
of the earth's motion round the sun, so that they
clung rather to the Ptolemaic view that the earth
was without motion and was the center about which
all the celestial motions took place. The geometry
of stellar distances was perfectly understood, and
many were the attempts made to find the parallaxes
and distances of the stars ; but the art of instrument
making had not yet advanced to a stage where
astronomers had the mechanisms that were abso-
lutely necessary to measure very small angles.
About 1835, Bessel undertook the work of deter-
mining stellar parallax in earnest. His instrument
was the heliometer, originally designed for measur-
ing the sun's diameter ; but as modified for parallax
work it is the most accurate of all angle-measuring
instruments that the astronomers employ. The star
that he selected was 61 Cygni, not a bright star,
of the sixth magnitude only, but its large proper
motion suggested that it might be one of those
nearest to us. He measured with the heliometer,
at opposite seasons of the year, the distance of 61
Cygni from another and very small star in the
same field of view, and thus determined the relative
311
312 ASTRONOMY TO-DAY
parallax of the two stars. The assumption was
made that the very faint star was very much more
distant than the bright one, and this assumption
will usually turn out to be sound. Bessel got 0".35
for his parallax of 61 Cygni, and Struve by apply-
ing the same method to Alpha Lyrse, about the
same time, got 0".25 for the parallax of that star.
These classic researches of Bessel and Struve
are the most important in the history of star dis-
tances, because they were the first to prove that
stellar parallax, although minute, could neverthe-
less be actually measured. About the same time
success was achieved in another quarter, and Hen-
derson, the British astronomer at the Cape of Good
Hope, found a parallax of nearly a whole second
for the bright star Alpha Centauri.
Although the parallaxes of many hundreds of
stars have been measured since, and the parallaxes
of other thousands of stars estimated, the measured
parallax of Alpha Centauri, as later investigated by
Elkin and Sir David Gill, and found to be 0".75,
is the largest known parallax, and therefore Alpha
Centauri is our nearest neighbor among the stars,
so far as we yet know. This star is a binary system
and the light of the two components together is
about the same as that of Capella (Alpha Aurigse) .
But it is never visible from this part of the world,
being in 60 degrees of south declination : one might
just glimpse it near the southern horizon from Key
West.
How the distances of the stars are found is not
difficult to explain, although the method of doing it
involves a good deal of complication, interesting to
the practical astronomer only. Recall the method of
getting the moon's distance from the earth: it was
STAR DISTANCES 313
done by measuring her displacement among the stars
as seen from two widely separated observatories,
as near the ends of a diameter of the earth as con-
venient. This is the base line, and the angle
which a radius of the earth as seen from the
center of the moon fills, or subtends, is the moon's
parallax.
So near is the moon that this angle is almost
an entire degree, and therefore not at all difficult
to measure. But if we go to the distance of even
Alpha Centauri, the nearest of the stars, our earth
shrinks to invisibility; so that we must seek a
longer base line. Fortunately there is one, but
although its length is 25,000 times the earth's diam-
eter, it is only just long enough to make the star
distances measurable. We found that the sun's dis-
tance from the earth was 93 million miles; the
diameter of the earth's orbit is therefore double
that amount. Now conceive the diameter of the
earth replaced by the diameter of the earth's
orbit: by our motion round the sun we are trans-
ported from one extremity of this diameter to the
opposite one in six month's time; so we may mea-
sure the displacement of a star from these two ex-
tremities, and half this displacement will be the
star's parallax, often called the annual parallax
because a year is consumed in traversing its period.
And it is this very minute angle which Bessel and
Struve were the first to measure with certainty,
and which Henderson found to be in the case of
Alpha Centauri the largest yet known.
Evidently the earth by its motion round the sun
makes every star describe ,a little parallactic ellipse ;
the nearer the star is the larger this ellipse will
be, and the farther the star the smaller : if the star
314 ASTRONOMY TO-DAY
were at an infinite distance, its ellipse would be-
come a point, that is, if we imagine ourselves
occupying the position of the star, even the vast
orbit of the earth, 186 million miles across, would
shrink to invisibility or become a mathematical
point.
Measurement of stellar parallax is one of many
problems of exceeding difficulty that confront the
practical astronomer. But the actual research now-
adays is greatly simplified by photography, which
enables the astronomer to select times when the air
is not only clear, but very steady for making the
exposures. Development and measurement of the
plates can then be done at any time. Pritchard of
Oxford, England, was among the earliest to appre-
ciate the advantages of photography in parallax
work, and Schlesinger, Mitchell, Miller, Slocum
and Van Maanen, with many others in this country,
have zealously prosecuted it.
How shall we intelligently express the vast dis-
tances at which the stars are removed from us?
Of course we can use miles, and pile up the millions
upon millions by adding on ciphers, but that fails to
give much notion of the star's distance. Let us try
with Alpha Centauri: its parallax of 0".75 means
that it is 275,000 times farther from the sun than
the earth is. Multiplying this out, we get 25 tril-
lion miles, that is, 25 millions of million miles
an inconceivable number, and an unthinkable
distance.
Suppose the entire solar system to shrink so that
the orbit of Neptune, sixty times 93 million miles
in diameter, would be a circle the size of the dot
over this letter i. On the same scale the sun itself,
although nearly a million miles in diameter, could
STAR DISTANCES 315
not be seen with the most powerful microscope in
existence; and on the same scale also we should
have to have a circle ten feet in diameter, if the
solar system were imagined at its center and Alpha
Centauri in its circumference.
So astronomers do not often use the mile as a
yardstick of stellar distance, any more than we
state the distance from London to San Francisco
in feet or inches. By convention of astronomers,
the average distance between the centers of sun
and earth, or 93 million miles, is the accepted
unit of measure in the solar system. So the adopted
unit of stellar distance is the distance traveled by
a wave of light in a year's time: and this unit is
technically called the light-year. This unit of dis-
tance, or stellar yardstick, as we may call it, is
nearly 6 millions of million miles in length. Alpha
Centauri, then, is four and one-third light-years
distant, and 61 Cygni seven and one-fifth light-
years away.
For convenience in their calculations most as-
tronomers now use a longer unit called the parsec,
first suggested by Turner. Its length is equal to the
distance of a star whose parallax is one second of
arc; that is, one parsec is equal to about three
and a quarter light-years. Or the light-year is
equal to 0.31 parsec. Also the parsec is equal to
206,000 astronomical units, or about 19 millions of
million miles.
We have, then four distinct methods of stating
the distance of a star: Sirius, for example, has a
parallax of 0".38 or its distance is two and two-
thirds parsecs, or eight and a half light-years, or
50 millions of million miles. It is the angle of
parallax which is always found first by actual meas-
316 ASTRONOMY TO-DAY
urement and from this the three other estimates
of distance are calculated.
So difficult and delicate is the determination of
a stellar distance that only a few hundred paral-
laxes have been ascertained in the past century.
The distance of the same star has been many times
measured by different astronomers, with much
seeming duplication of effort. Comprehensive cam-
paigns for determining star parallaxes in large
numbers have been undertaken in a few instances,
particularly at the suggestion of Kapteyn, the em-
inent astronomer of Groningen, Holland. His cata-
logue of star parallaxes is the most complete and
accurate yet published, and is the standard in all
statistical investigations of the stars.
That we find relatively large parallaxes for
some of the fainter stars, and almost no measur-
able parallax for some of the very bright stars is
one of the riddles of the stellar universe. We may
instance Arcturus, in the northern hemisphere and
Canopus in the southern; the latter almost as
bright as Sirius. Dr. Elkin and the late Sir David
Gill determined exhaustively the parallax of Cano-
pus, and found it very minute, only 0".03, making
its distance in excess of a hundred light-years. The
stupendous brilliancy of this star is apparent if we
remember that the intensity of its light must vary
inversely as the square of the distance; so that if
Canopus were to be brought as near us as even
61 Cygni is, it would be a hundredfold brighter
than Sirius, the brightest of all the stars of the
firmament.
In researches upon the distribution of the more
distant stars, the method of measuring parallaxes
of individual stars fails completely, and the secular
STAR DISTANCES 317
parallax, or parallactic motion of the stars is em-
ployed instead. By parallactic motion is meant the
apparent displacement in consequence of the solar
motion which is now known with great accuracy,
and amounts to 19.5 kilometers per second. Even
in a single year, then, the sun's motion is twice the
diameter of the earth's orbit, so that in a hundred
or more years, a much longer base line is available
than in the usual type of observations for stellar
parallax. If we ascertain the parallactic motion of
a group of stars, then we can find their average
distance. It is found, for example, that the mean
parallax of stars of the sixth magnitude is 0".014.
Also the mean distances of stars thrown into
classes according to their spectral type have been
investigated by Boss, Kapteyn, Campbell and
others. The complete intermingling of the two
great star streams has been proved, too, by using
the magnitude of the proper motions to measure
the average distances of both streams. These come
out essentially the same, so that the streaming can-
not be due to mere chance relation in the line of
sight.
Most unexpected and highly important is the dis-
covery that the peculiar behavior of certain lines
in the spectrum leads to a fixed relation between a
star's spectrum and its absolute magnitude, which
provides a new and very effective method of ascer-
taining stellar distances. By absolute magnitudes
are meant the magnitudes the stars would appear to
have if they were all at the same standard distance
from the earth.
Very satisfactory estimates of the distance of
exceedingly remote objects have been made within
recent years by this indirect method, which is espe-
318 ASTEONOMY TO-DAY
cially applicable to spiral nebulae and globular clus-
ters. The absolute magnitude of a star is inferred
from the relative intensities of certain lines in its
spectrum, so that the observed apparent magnitude
at once enables us to calculate the distance of the
star. Adams and Joy have recently determined
the luminosities and parallaxes of 500 stars by this
spectroscopic method. Of these stars 360 have had
their parallaxes previously measured; and the
average difference between the spectroscopic and
the trigonometric values of the parallax is only
the very small angle 0".0037, a highly satisfactory
verification.
An indirect method, but a very -simple one, and
of the greatest value because it provides the key
to stellar distances with the least possible calcula-
tion, and we can ascertain also the distances of
whole classes of stars too remote to be ascertained
in any other way at present known.
The problem of spectroscopic determinations of
luminosity and parallax has been investigated at
Mount Wilson with great thoroughness from all
sides, the separate investigations checking each
other. A definitive scale for the spectroscopic de-
termination of absolute magnitudes has now been
established, and the parallaxes and absolute magni-
tudes have already been derived for about 1,800
stars.
CHAPTER XLIX
THE NEAREST STARS
OF especial interest are the few stars that we
know are the nearest to us, and the following
table includes all those whose parallax is 0".20 or
greater. There are nineteen in all and nearly half
of them are binary systems. The radial motions
given are relative to the sun. The transverse veloc-
ities are formed by using the measured parallaxes
to transform proper motions into linear measures.
They are given by Eddington in his "Stellar Move-
ments" :
Star's Name
to
1
Parallax
in Seconds
of Arc
Proper Motion
in Seconds
of Arc
Linear
Velocity
Km. per sec.
Radial
Velocity
Km. per sec. I
y
CO
Luminosity
(Sun=l)
a
I
02
W
Groombridge 34.
Eta Cassiop ....
Tau Ceti
8.2
8.6
3 6
0.28
0.20
033
2.S5
1.25
1.93
48
30
28
+10
16
Ma
F8
K
0.010
1.4
0.50
I
I
11
Epsilon Erid. . . .
CZ 5h 243
Sirius
3.3
8.3
1 6
0.31
0.32
38
1.00
8.70
1 32
15
129
16
+16
+242
_7
K
G-K
A
0.79
0.007
48.0
11
11
11
Procyon
5
0.32
1 25
19
3
F5
9.7
I?
Lai. 21185
Lai. 21258
OA (N) 11677. .
7.6
8.9
9 2
0.40
0.20
20
4.77
4.46
3 03
57
106
72
Ma
Ma
0.009
0.011
008
11
I
I
Alpha Centauri.
OA (N) 17415..
Pos. Med. 2164. .
Sigma Draco. . . .
Alpha Aquilas. . .
61 Cygni
Epsilon Tndi . . . .
Kriiger 60
Lacaille 9352
0.3
9.3
8.8
4.8
0.9
5.6
4.7
9.2
7.4
0.76
0.27
0.29
0.20
0.24
0.31
0.28
0.26
0.29
3.66
1.31
2.28
1.84
0.65
5.25
4.67
0.92
7.02
23
23
37
43
13
80
79
17
115
-22
+25
-33
-39
-62
+1*2
G.K5
P
K
K
A5
K5
K5
Ma
(2.0
(0.6
0.004
0.006
0.5
12.3
0.10
0.25
0.005
0.019
I
11
I
11
I
I
1
II
319
320 ASTRONOMY TO-DAY
These stars are distant less than five parsecs
(about 16 light-years) from the sun, so they make
up the closest fringe of the stellar universe im-
mediately surrounding our system. The large num-
ber of binary systems is quite remarkable. Why
some stars are single and others double is not yet
known. By the spectroscopic method the propor-
tion is not so large; Campbell finding that about
one quarter of 1,600 stars examined are spectro-
scopic binaries, and Frost two-fifths to a half. The
exceptional number of large velocities is very re-
markable; the average transverse motion of the
nineteen stars is fifty kilometers per second, where-
as thirty is about what would have been expected.
As to star streams to which these nearest stars
belong, eleven are in Stream I and eight in Stream
II, in close accord with the ratio 3:2 given by the
6,000 stars of Boss's catalogue. "We are not able,"
says Eddington, "to detect any significant difference
between the luminosities, spectra, or speeds of the
stars constituting the two streams. The thorough
interpenetration of the two star streams is well
illustrated, since we find even in this small volume
of space that members of both streams are mingled
together in just about the average proportion."
THE RING NEBULA IN Lyra. This is the best example of the annular and
elliptic nebulae, which are not very abundant. (Photo, Mt. Wilson Solar
Observatory.)
CHAPTER L
ACTUAL DIMENSIONS OF THE STARS
WE have seen that the distances of the stars
from the solar system are immense beyond
conception, and millions upon millions of them are
probably forever beyond our power of ascertaining
by direct measurement what their distance really
is. After we had found the sun's distance and
measured the angle filled by his disk, it was easy
to calculate his actual size. This direct method,
however, fails when we try to apply it to the stars,
because their distances are so vast that no star's
disk fills an angle of any appreciable size ; and even
if we try to get a disk with the highest magnifying
powers of a great telescope our efforts end only in
failure. There is, indeed, no instrumentally appre-
ciable angle to measure.
How then shall we ascertain the actual dimen-
sions of the vast spheres which we know the stars
actually are, as they exist in the remotest regions
of space? Clearly by indirect methods only, and it
must be said that astronomers have as yet no gen-
eral method that yields very satisfactory results for
stellar dimensions. The actual magnitude of the
variable system of Algol, Beta Persei, is among the
best known of all the stars, because the spectro-
scope measures the rate of approach and recession
of Algol when its invisible satellite is in opposite
parts of the orbit ; the law of gravitation gives the
321 Sci. Vol. 211
322 ASTRONOMY TO-DAY
mass of the star and the size of its orbit, and so
the length of the eclipse gives the actual size of
the dark, eclipsing body. This figures out to be
practically the same size as that of our sun, while
Algol's own diameter is rather larger, exceeding a
million miles.
If we try to estimate sizes of stars by their bright-
ness merely, we are soon astray. Differences of
brightness are due to difference of dimensions, of
course, or of light-giving area; but differences of
distance also affect the brightness, inversely as the
squares of the distances, while differences of tem-
perature and constitution affect, in very marked
degree, the intrinsic brilliance of the light-emitting
surface of the star. There are big stars and little
stars, stars relatively near to us and stars exceed-
ingly remote, and stars highly incandescent as well
as others feebly glowing.
We have already shown how the angular diam-
eters subtended by many of the stars have been
estimated, through the relation of surface bright-
ness and spectral type. Antares and Betelgeuse
appear to be the most inviting for investigation, be-
cause their estimated angular diameters are about
one-twentieth of a second of arc. This is the
way in which their direct measurement is being
attempted.
As early as 1890, Michelson of Chicago suggested
the application of interference methods to the ac-
curate measurement of very small angles, such as
the diameters of the minor planets, and the satel-
lites of Jupiter and Saturn, as well as the arc dis-
tance between the components of double stars. Two
portions of the object glass are used, as far apart
as possible on the same diameter, and the inter-
ACTUAL DIMENSIONS OF STARS 323
ference fringes produced at the focus of the objec-
tive are then the subject of observation. These
fringes form a series of equidistant interference
bands, and are most distinct when the light comes
from a source subtending an infinitesimal angle.
If the object presents an appreciable angle, the vis-
ibility is less and may even become zero.
Michelson tested this method on the satellites of
Jupiter at the Lick Observatory in 1891, and
showed its accuracy and practicability. Neverthe-
less, the method has not been taken up by astron-
omers, until very recently at the Mount Wilson
Observatory, where Anderson has applied it to the
measurement of close double stars. It is found that,
contrary to general expectation, the method gives
excellent results, even if the "seeing" is not the best
2 on a scale of 10, for instance.
To simplify the manipulation of the interfero-
meter, a small plate with two apertures in it is
placed in the converging beam of light coming from
the telescope objective or mirror. The interference
fringes formed in the focal plane are then viewed
with an eyepiece of very high power, many thou-
sand diameters. The resolving power of the inter-
ferometer is found to be somewhat more than
double that of a telescope of the same aperture. By
applying the interferometer method to Capella, arc
distances of much less than one-twentieth of a
second of arc were measured. More recently the
method has been applied to the great star Betelgeuse
in Orion, whose angular diameter was found to be
0".46, corresponding to an actual diameter of 260,-
000,000 miles, if the star's parallax is as small as it
appears to be.
CHAPTER LI
THE VARIABLE STARS
OPECTACULAR as they are to the layman, novse,
O or temporary stars, are to the astronomers
simply a class among many thousands of stars
which they call variables, or variable stars. There
are a few objects classified as irregular variables,
one of which is very remarkable. We refer to Eta
Argus, an erratic variable in the southern constel-
lation Argo and surrounded by a well-known
nebula. There is a pretty complete record of this
star. Halley in 1677 when observing at Saint
Helena recorded Eta Argus as of the fourth magni-
tude. During the 18th century, it fluctuated be-
tween the fourth magnitude and the second. Early
in the 19th it rapidly waxed in brightness, fluctu-
ating between the first and second magnitudes
from 1822 to 1836. But two years later its light
tripled, rivaling all the fixed stars except Canopus
and Sirius. In 1843 it was even brighter for a few
months, but since then it has declined fairly stead-
ily, reaching a minimum at magnitude seven and a
half in 1886, with a slight increase in brightness
more recently. A period of half a century has been
suggested, but it is very doubtful if Eta Argus has
any regular period of va 
