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THE SIDEREAL MESSENGER

R I V I N G T O N S

3£onllon IV ater loo Place

©iforU Magdalen Street

([TambriaQf Trinity Street

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GALILEO'S BROKEN LENS.

EXHIBITED IN THE LOAN COLLECTION OF SCIENTIFIC APPARATUS
AT THE SOUTH KENSINGTON MUSEUM, 1676.

Franv a- pTwtcgraph,
oftht Science and. Art Depccrtjnenz, South Kensington.

5^.

THE SIDEREAL MESSENGER

GALILEO GALILEI

AND A PART OF THE F RE FACE TO KEPLER'S DIOPTRICS

CONTAINING THE ORIGINAL ACCOUNT OF GALILEO'S
ASTRONOMICAL DISCOVERIES.

a €rangiIation ioitl^ 3lntrotmction ann Bote^

EDWARD STAFFORD CARLOS, M.A.

HEAD MATHEMATICAL MASTER IN CHRIST's HOSPITAL.

RIVINGTONS

WATERLOO PLACE, LONDON

(©iforti anti ^ambtitise

MDCCCLXXX

PREFATORY NOTE.

About five years ago I was engaged in preparing
a catalogue of the ancient books which belong to
Christ's Hospital. One portion of these books
consisted of a collection of ancient mathematical
works presented at various times for the use of that
part of the school which is known as the Royal
Mathematical Foundation of King Charles ii.
Amongst them were some well known by name to
every mathematical student, but which few have
ever seen. Perhaps the most interesting of them
all was a little volume, printed in London in 1653,
containing Gassendi's Explanation of the Ptolemaic
and Gopernica7i Systems of Astronomy, as well as
that of Tycho Brahe, Galileo's Sidereiis Nicncius,
and Kepler's Dioptrics. I found Galileo's account

vi PRE FA TOR Y NO TE.

of liis astronomical discoveries so interesting, both
in matter and in style, that I translated it as a
recreation from school- work. I venture to think
that others also will be interested in following
Galileo through the apprehension of his famous
discoveries, and in reading the language in which
he announced them.

INTRODUCTION.

In 1609, Galileo, then Professor of Mathematics
at Padua, in the service of the Venetian Republic,
heard from a correspondent at Paris of the inven-
tion of a telescope, and set to work to consider how
such an instrument could be made. The result was
his invention of the telescope known by his name,
and identical in principle with the modern opera-
glass. In a maritime and warhke State, the advan-
tages to be expected from such an invention were
immediately recognised, and Galileo was rewarded
with a confirmation of his Professorship for life, and
a handsome stipend, in recognition of his invention
and construction of the first telescope seen at
Venice. In his pamphlet, The Sidereal Messenger,
here translated, Galileo relates how he came to
learn the value of the telescope for astronomical
research ; and how his observations were rewarded
by numerous discoveries in rapid succession, and at

INTRODUCTION

len^h by that of Jupiter's satellites. Galileo at
once saw the value of this discovery as bearing
upon the establishment of the Copernican system
of astronomy, which had met with shght acceptance,
and indeed as yet had hardly any recommendation
except that of greater simphcity. Kepler had just
published at Prague his work on the planet Mars
{Commentaria de motihus Stellce Martis), on which
he had been engaged apparently for eight years ;
there he heard of Galileo's discoveries, and at length
was invited by Galileo himself, through a common
friend, Giuliano de' Medici, ambassador of the Grand-
Duke of Tuscany, Cosmo de' Medici ii., to the
Emperor Eudolph ii., to correspond with Galileo
on the subject of these discoveries. The Emperor
also requested his opinion, and Kepler accordingly
examined Gahleo's Sidereal Messenger in a pam-
phlet, entitled A Discussion vjith the Sidereal
Messenger (Florence, 1610).

In this Discussion Kepler gives reasons for
accepting Galileo's observations — although he was
not able to verify them from want of a telescope —
and entirely supports Galileo's views and conclu-
sions, adducing his own previous speculations, or
pointing out, as in the case of Gahleo's idea of
earth-light on the moon, the previous conception of

INTRODUCTION. ix

the same explanation of the phenomenon. He
rejects, however, GaHleo's explanation of the copper
colour of the moon in echpses. Kepler ends by
expressing unbounded enthusiasm at the discovery
of Jupiter's satellites, and the argument it furnishes
in support of the Copernican theory.

Soon after, in 1611, Kepler pubHshed another
pamphlet, his Narrative, giving an account of actual
observations made in verification of Galileo's dis-
coveries by himself and several friends, whose
names he gives, with a telescope made by Gahleo,
and belonging to Ernest, Elector and Archbishop
of Cologne. Kepler and his friends saw the limar
mountains and three of the satellites of Jupiter, but
failed to make out any signs of the ring of Saturn
corresponding to the imperfect description of
Galileo.

Kepler had previously published a treatise on
Optics (Frankfort, 1604). He now extended it to
the consideration of the theory of the telescope, and
explained the principle of Galileo's telescope ; he
also showed another combination of lenses which
would produce a similar effect. This was the prin-
ciple of the common astronomical telescope, often
called, from this circumstance, Kepler's telescope,
though he did not construct it. The account of

X INTRODUCTION.

Galileo's later astronomical discoveries of Saturn's
ring and the phases of Venus is taken from the
preface of this work. — (Kepler's Dioptrics; Augs-
burg, 1611.)

In 1612 Galileo pubhshed a series of observations
of solar sjDots, and in 1618 some observations of
three comets. There exist also long series of
minute observations of Jupiter and his satellites,
continued to November 1619. — (Galileo's Worhs ;
Florence, 1845.)

Further astronomical researches may have been
hindered by failing sight. One more astronomical
discovery, however, that of the moon's librations,
was made as late as 1637, and the announcement
of it is dated " dalla mia carcere di Arcetri."
Galileo died January 8, 1642.

The following editions have been used for the
translation : —
Galileo's Worhs.

1. Florence, 1718.

2. Padua, 1744.

3. Florence, 1842-56.
Sidereus Nuncius,

1. Venice, 1610.

2. London, 1653.

INTRODUCTION. xi

Kepler's Works, ed. C. Friscli. Frankfurt a. M.,
1858-71.
Prodromus dissertationum mathematicarum conti-
nens Mysterium Cosmographicum de admira-
bili proportione orbium coelestium. Tubingen,
1596.
Astronomia nova alrLoXoyT^ro^ (Comment aria de
motibus stellse Martis). [Prague,] 1609.

THE

SIDEREAL MESSENGER

GALILEO GALILEI

THE

SIDEREAL MESSENGER

UNFOLDING GREAT AND MARVELLOUS SIGHTS,

AND PROPOSING THEM TO THE ATTENTION OF EVERY ONE,

BUT ESPECIALLY PHILOSOPHERS AND ASTRONOMERS,

BEING SUCH AS HAVE BEEN OBSERVED BY

GALILEO GALILEI

A GENTLEMAN OF FLORENCE,
PROFESSOR OF MATHEMATICS IN THE UNIVERSITY OF PADUA,

WITH THE AID OF A

TELESCOPE

lately invented by him.

Respecting the Moon^s Surface, an innumerable Jiumber of Fixed Stars,

the Milky IVay, and Nebulous Stars, but especially respecting

. Four Planets which revolve roiuid the Planet fitpiter at

different distances and in different periodic times, with

amazing velocity, and which, after refnaini?ig

unknown to every one up to this day, the

Author recently discovered, and

determined to iiame the

MEDICEAN STARS.

Venice i6io.

TO THE MOST SERENE

COSMO DE' MEDICI, THE SECOND,

FOURTH GRAND-DUKE OF TUSCANY.

THERE is certainly something very noble and large-
minded in the intention of those who have endea-
voured to protect from envy the noble achievements of
distinguished men, and to rescue their names, worthy
of immortality, from oblivion and decay. This desire
has given us the lineaments of famous men, sculptured
in marble, or fashioned in bronze, as a memorial of
them to future ages ; to the same feeling we owx the
erection of statues, both ordinary and equestrian; hence,
as the poet^ says, has originated expenditure, mount-
ing to the stars, upon columns and pyramids; with this
desire, lastly, cities have been built, and distinguished
by the names of those men, whom the gratitude of
posterity thought worthy of being handed down to all
ao^es. For the state of the human mind is such, that

1 Propertius, iii, 2. 17-22.
A

2 THE SIDEREAL MESSENGER.

unless it be continually stirred by the counterparts^ of
matters, obtruding themselves upon it from without,
all recollection of the matters easily passes away
from it.

But others, having regard for more stable and more
lasting monuments, secured the eternity of the fame
of great men by placing it under the protection, not of
marble or bronze, but of the Muses' guardianship and
the imperishable monuments of literature. But why
do I mention these things, as if human wit, content
with these regions, did not dare to advance further ;
whereas, since she well understood that all human
monuments do perish at last by violence, by weather,
or by age, she took a wider view, and invented more
imperishable signs, over which destroying Time and
envious Age could claim no rights ; so, betaking her-
self to the sky, she inscribed on the well-known orbs
of the brightest stars — those everlasting orbs — the
names of those who, for eminent and god-like deeds,
were accounted worthy to enjoy an eternity in com-
pany with the stars. Wherefore the fame of Jupiter,
Mars, Mercury, Hercules, and the rest of the heroes
by whose names the stars are called, will not fade

^ Compare Lucretius iv. 881 :

Dico auimo nostro primum simulacra meandi
Accidere, atque animum pulsare.

THE SIDEREAL MESSENGER. 3

until the extinction of the splendour of the constella-
tions themselves.

But this invention of human shrewdness, so par-
ticularly noble and admirable, has gone out of date
ages ago, inasmuch as primeval heroes are in posses-
sion of those bright abodes, and keep them by a sort
of right ; into whose company the affection of Augus-
tus in vain attempted to introduce Julius Caesar ; for
when he T\dshed that the name of the Julian constella-
tion should be given to a star, which appeared in his
time, one of those which the Greeks and the Latins
alike name, from their hair -like tails, comets, it
vanished in a short time and mocked his too eager
hope. But we are able to read the heavens for
your highness, most Serene Prince, far more truly
and more happily, for scarcely have the immortal
graces of your mind begun to shine on earth, w^hen
bright stars present themselves in the heavens, like
tongues to tell and celebrate your most surpassing
virtues to all time. Behold therefore, four stars
reserved for your famous name, and those not be-
longing to the common and less conspicuous multi-
tude of fixed stars, but in the bright ranks of the
planets — four stars which, moving differently from
each other, round the planet Jupiter, the most glori-
ous of all the planets, as if they were his own children

4 THE SIDEREAL MESSENGER.

accomplish the courses of their orbits with marvellous
velocity, while all the while with one accord they
complete all together mighty revolutions every ten
years round the centre of the universe, that is, round
the Sun.

But the Maker of the Stars himself seemed to direct
me by clear reasons to assign these new planets to the
famous name of your highness in preference to all
others. For just as these stars, like children worthy
of their sire, never leave the side of Jupiter by any
appreciable distance, so who does not know that
clemency, kindness of heart, gentleness of manners,
splendour of royal blood, nobleness in public functions,
wide extent of influence and power over others, all
of which have fixed their common abode and seat in
your highness, — who, I say, does not know that all
these qualities, according to the providence of God,
from whom all good things do come, emanate from
the benign star of Jupiter ? Jupiter, Jupiter, I
maintain, at the instant of the birth of your highness
having at length emerged from the turbid mists of
the horizon, and being in possession of the middle
quarter of the heavens, and illuminating the eastern
angle, from his own royal house, from that exalted
throne, looked out upon your most happy birth, and
poured forth into a most pure atmosphere all the

THE SIDEREAL MESSENGER. 5

brightness of his majesty, in order that your tender
body and your mind — though that was abeady adorned
by God with still more splendid graces — might imbibe
with your first breath the whole of that influence and
power. But why should I use only plausible argu-
ments when I can almost absolutely demonstrate my
conclusion ? It was the will of Almighty God that I
should be judged by your most serene parents not
unworthy to be employed in teaching your highness
mathematics, which duty I discharged, during the four
years just passed, at that time of the year when it is
customary to take a relaxation from severer studies.
Wherefore, since it evidently fell to my lot by God's
will, to serve your highness, and so to receive the rays
of your surpassing clemency and beneficence in a
position near your person, what wonder is it if you
have so warmed my heart that it thinks about scarcely
anything else day and night, but how I, who am
indeed your subject not only by inclination, but also
by my very birth and lineage, may be known to be
most anxious for your glory, and most grateful to you ?
And so, inasmuch as under your patronage, most
serene Cosmo, I have discovered these stars, which
were unknown to all astronomers before me, I have,
with very good right, determined to designate them
with the most august name of your family. And as

6 THE SIDEREAL MESSENGER.

I was the first to investigate them, who can rightly
blame me if I give them a name, and call them the
Medicean Stars, hoping that as much consideration
may accrue to these stars from this title, as other stars
have brought to other heroes? For not to speak of
your most serene ancestors, to whose everlasting glory
the monuments of all history bear witness, your
virtue alone, most mighty sire, can confer on those
stars an immortal name ; for who can doubt that you
will not only maintain and preserve the expectations,
high though they be, about yourself, which you have
aroused by the very happy beginning of your govern-
ment, but that you will also far surpass them, so that
when you have conquered others like yourself, you
may still vie with yourself, and become day by day
greater than yourself and your greatness ?

Accept, then, most clement Prince, this addition to
the glory of your family, reserved by the stars for
you ; and may you enjoy for many years those good
blessings, which are sent to . you not so much from
the stars as from God, the Maker and Governor of
the stars.

Your Highness's most devoted servant,

Galileo Galilel

Padua, Ilarrh 12, 1610.

THE ASTRONOMICAL MESSENGER

Containing and setting forth Observations lately made with the

aid of a' newly invented TELESCOPE respecting the Moo7i s

Surface, the Milky Way, Nebulous Stars, an

innumerable multitude of Fixed Stars, and

also respecting Four Planets never before

seen, which have been nained

THE COSMIAN STARS.^

IN the present small treatise I set forth some matters introduction.
of great interest for all observers of natural pheno-
mena to look at and consider. They are of great
interest, I think, first, from their intrinsic excellence ;
secondly, from their absolute novelty ; and lastly, also
on account of the instrument by the aid of which
they have been presented to my apprehension.

The number of the Fixed Stars which observers
have been able to see without artificial powers of
sight up to this day can be counted. It is therefore

1 The satellites of Jupiter are here called " the Cosmian Stars " in honour
of Cosmo de' Medici, but elsewhere Galileo calls them " the Medicean
Stars.'^ Kepler sometimes calls them ''the Medicean Sfars,'^ but more
often " satellites.''^

8 THE SIDEREAL MESSENGER.

decidedly a great feat to add to their number, and to
set distinctly before the eyes other stars in myriads,
which have never been seen before, and which sur
pass the old, previously known, stars in number more
than ten times.

Again, it is a most beautiful and delightful sight to
behold the body of the Moon, which is distant from
us nearly sixty seini- diameters^ of the Earth, as near
as if it was at a distance of only two of the same
measures ; so that the diameter of this same Moon
appears about thirty times larger, its surface about
nine hundred times, and its solid mass nearly 27,000
times larger than w^hen it is viewed only mth the
naked eye ; and consequently any one may know
with the certainty that is due to the use of our
senses, that the Moon certainly does not possess a
smooth and polished surface, but one rough and
uneven, and, just like the face of the Earth itself, is
everywhere full of vast protuberances, deep chasms,
and sinuosities.

Then to have got rid of disputes about the Galaxy
or Milky Way, and to have made its nature clear to
the very senses, not to say to the understanding,

^ Galileo says, " per sex denas fere terrestres diametros a nobis remotum'
by mistake for semi-diametros, and the same mistake occurs in p. 11.

THE SIDEREAL MESSENGER. 9

seems by no means a matter which ought to be con-
sidered of sKght importance. In addition to this, to
point out, as with one's finger, the nature of those stars
which every one of the astronomers up to this time
has called nebulous, and to demonstrate that it is very-
different from what has hitherto been believed, will be
pleasant, and very fine. But that which will excite
the greatest astonishment by far, and which indeed
especially moved me to call the attention of all astro-
nomers and philosophers, is this, namely, that I have
discovered four planets, neither known nor observed
by any one of the astronomers before my time, which
have their orbits round a certain bright star, one of
those previously known, like Venus and Mercury round
the Sun, and are sometimes in front of it, sometimes
behind it, though they never depart from it beyond
certain limits. All which facts were discovered and
observed a few days ago by the help of a telescope ^
devised by me, through God's grace first enlightening
my mind.

Perchance other discoveries still more excellent
will be made from time to time by me or by other

1 The words used by Galileo for " telescope" are perspicillum, specillum
instrumentum, organum, and occhiale (Ital.). Kepler uses also oculare
tuhtjLS, arundo dioptrica. The word " telescopium " is used by Gassendi,
1647.

lo THE SIDEREAL MESSENGER,

observers, with the assistance of a sirailar instrument,
so I will first briefly record its shape and preparation,
as well as the occasion of its being devised, and then
I will give an account of the observations made by
me.
Galileo's ac- About tcu months ago a report reached my ears

count of the O X J

invention of that a Dutchman had constructed a telescope, by the

his telescope. .... J. •/

aid of which visible objects, although at a great
distance from the eye of the observer, were seen
distinctly as if near ; and some proofs of its most
wonderful performances were reported, which some
gave credence to, but others contradicted. A few
days after, I received confirmation of the report in a
letter written from Paris by a noble Frenchman,
Jaques Badovere, which finally determined me to give
myself up first to inquire into the principle of the
telescope, and then to consider the means by which I
might compass the invention of a similar instrument,
which a little while after I succeeded in doing,
through deep study of the theory of Eefraction ;
and I prepared a tube, at first of lead, in the ends of
which 1 fitted two glass lenses, both plane on one
side, but on the other side one spherically convex,
and the other concave. Then bringing my eye to the
concave lens I saw objects satisfactorily large and
near, for they appeared one-third of the distance ofl"

THE SIDEREAL MESSENGER. ii

and nine times larger than when they are seen with the
natural eye alone. I shortly afterwards constructed
another telescope with more nicety, which magnified
objects more than sixty times. At length, by sparing
neither labour nor expense, I succeeded in construct-
ing for myself an instrument so superior that ob-
jects seen through it appear magnified nearly a
thousand times, and more than thirty times nearer
than if viewed by the natural powers of sight
alone.

It would be altogether a waste of time to enumer- oaiiieos
ate the number and importance of the benefits which tions with
this instrument may be expected to confer, when
used by land or sea. But without paying attention
to its use for terrestrial objects, I betook myself to
observations of the heavenly bodies ; and first of all,
I viewed the Moon as near as if it was scarcely two
semi- diameters^ of the Earth distant. After the
Moon, I frequently observed other heavenly bodies,
both fixed stars and planets, with incredible delight ;
and, when I saw their very great number, I began to
consider about a method by which I might be able to
measure their distances apart, and at length I found
one. And here it is fitting that all who intend to

^ " Vix per duas Telluris d'lamttros^'' by mistake for "semi-diametros."

12 THE SIDEREAL MESSENGER.

turn their attention to observations of this kind
should receive certain cautions. For, in the first
place, it is absolutely necessary for them to prepare
a most perfect telescope, one which will show very
bright objects distinct and free from any mistiness,
and will magnify them at least 400 times, for then
it will show them as if only one-twentieth of their
distance off. For unless the instrument be of such
power, it will be in vain to attempt to view all the
things which have been seen by me in the heavens,
or which will be enumerated hereafter.
Method of But in order that any one may be a little more
themTgn'ify- ccrtalu about the magnifying power of his instrument,
the telescope, hc shall fashlou two circles, or two square pieces of
paper, one of which is 400 times greater than the
other, but that will be when the diameter of the
greater is twenty times the length of the diameter of
the other. Then he shall view from a distance
simultaneously both surfaces, fixed on the same wall,
the smaller with one eye applied to the telescope, and
the larger with the other eye unassisted ; for that
may be done without inconvenience at one and the
same instant with both eyes open. Then both figures
will appear of the same size, if the instrument magni-
fies objects in the desired proportion.

After such an instrument has been prepared, the

THE SIDEREAL MESSENGER.

method of measuring distances remains for inquiry, Method of
and this we shall accomplish by the following con- L'S an^i
trivance : —

distances

between

heavenlj'

bodies by the

size of the
n .

^ aperture of

Q the telescope.

For the sake of being more easily understood, I
will suppose a tube A B c d/ Let E be the eye of the
observer ; then, when there are no lenses in the
tube rays from the eye to the object f g would be
drawn in the straight lines E c F, E d g, but when
the lenses have been inserted, let the rays go
in the bent lines E c H, E D i, — for they are con-
tracted, and those which originally, when unaffected
by the lenses, were directed to the object f g, will

The line c h in Galileo's figure represents the small pencil of rays
from H which, after refraction through the telescope, reach the eye e.
The enlarged figure shows that if op be the radius of the aperture
employed, the point H of the object would be just outside the field of
view. The method, however, is at best only a very rough one, as the
boundary of the field of view in this telescope is unavoidably indistinct.

14 THE SIDEREAL MESSENGER.

include only the part h i. Hence the ratio of the
distance e h to the line h i being known, we shall be
able to find, by means of a table of sines, the magni-
tude of the angle subtended at the eye by the object
H I, which we shall find to contain only some minutes.
But if we fit on the lens CD thin plates of metal,
pierced, some with larger, others with smaller aper-
tures, by putting on over the lens sometimes one
plate, sometimes another, as may be necessary, we
shall construct at our pleasure difierent subtending
angles of more or fewer minutes, by the help of
which w^e shall be able to measure conveniently the
intervals between stars separated by an angular
distance of some minutes, within an error of one or
two minutes. But let it suffice for the present to
have thus slightly touched, and as it were just put
our lips to these matters, for on some other oppor-
tunity I will publish the theory of this instrument in
completeness.

Now let m.e review the observations made by me
during the two months just past, again inviting the
attention of all who are eager for true philosophy to
the beginnings which led to the sight of most im-
portant phenomena.
The Moon. j^g^ j^g spcak first of the surface of the Moon,

Ruggedness "-

of its surface, whlch Is tumcd towards us. For the sake of being

THE SIDEREAL MESSENGER. 15

understood more easily, I distinguish two parts in it, Existence of

lunar moun-

which I call respectively the briohter and the darker, tains and

■^ "^ ^ valleys.

The brighter part seems to surround and pervade the
whole hemisphere ; but the darker part, like a sort of
cloud, discolours the Moon's surface and makes it
appear covered with spots. Now these spots, as they
are somewhat dark and of considerable size, are plain
to every one, and every age has seen them, wherefore
I shall call them great or ancient spots, to distinguish
them from other spots, smaller in size, but so thickly
scattered that they sprinkle the whole surface of the
Moon, but especially the brighter portion of it. These
spots have never been observed by any one before
me ; and from my observations of them, often repeated,
I have been led to that opinion which I have
expressed, namely, that I feel sure that the surface of
the Moon is not perfectly smooth, free from in-
equalities and exactly spherical, as a large school of
philosophers considers with regard to the Moon and
the other heavenly bodies, but that, on the contrary,
it is full of inequalities, uneven, full of hollows and
protuberances, just like the surface of the Earth itself,
which is varied everywhere by lofty mountains and
deep valleys.

The appearances from which we may gather these
conclusions are of the following nature : — On the

1 6 THE SIDEREAL MESSENGER.

fourth or fifth clay after new-moon, when the Moon
presents itself to us with bright horns, the boundary
which divides the part in shadow from the enlightened
part does not extend continuously in an ellipse, as
would happen in the case of a perfectly spherical body,
but it is marked out by an irregular, uneven, and very
wavy line, as represented in the figure given, for
several bright excrescences, as they may be called,
extend beyond the boundary of light and shadow
into the dark part, and on the other hand pieces
of shadow encroach upon the light : — nay, even a
great quantity of small blackish spots, altogether
separated from the dark part, sprinkle everywhere
almost the whole space which is at the time flooded
with the Sun's light, with the exception of that part
alone which is occupied by the great and ancient spots.
I have noticed that the small spots just mentioned
have this common characteristic always and in every
case, that they have the dark part towards the Sun's
position, and on the side away from the Sun they
have brighter boundaries, as if they were crowned
with shining summits. Now we have an appearance
quite similar on the Earth about sunrise, when we
behold the valleys, not yet flooded with light, but the
mountains surrounding them on the side opposite to
the Sun already ablaze mth the splendour of his

Sketches by Galileo to shew

the shape of a lunar mountain and of a walled plain. Galileo: Sidereua Nundus', Venice 1610.

THE SIDEREAL MESSENGER. 17

beams ; and just as the shadows in the hollows of the
Earth diminish in size as the Sun rises higher, so
also these spots on the Moon lose their blackness as the
illuminated part grows larger and larger. Again, not
only are the boundaries of light and shadow in the
Moon seen to be uneven and sinuous, but — and this
produces still greater astonishment — there appear very
many bright points within the darkened portion of the
Moon, altogether divided and broken off from the
illuminated tract, and separated from it by no incon-
siderable interval, which, after a little while, gradually
increase in size and brightness, and after an hour or
two become joined on to the rest of the bright portion,
now become somewhat larger ; but in the meantime
others, one here and another there, shooting up as if
growing, are lighted up within the shaded portion,
increase in size, and at last are linked on to the same
luminous surface, now still more extended. An
example of this is given in the same figure. Now,
is it not the case on the Earth before sunrise, that
while the level plain is still in shadow, the peaks of
the most lofty mountains are illuminated by the Sun's
raysl After a little while does not the light spread
further, while the middle and larger parts of those
mountains are becoming illuminated ; and at length,
when the Sun has risen, do not the illuminated parts

1 8 THE SIDEREAL MESSENGER.

of the plains and hills join together ? The grandeur,
however, of such prominences and depressions in the
Moon seems to surpass both in magnitude and extent
the ruggedness of the Earth's surface, as I shall here-
after show. And here I cannot refrain from mention-
ing what a remarkable spectacle I observed while the
Moon was rapidly approaching her first quarter, a
representation of which is given in the same illustra-
tion, placed opposite page 16. A protuberance of the
shadow, of great size, indented the illuminated part in
the neighbourhood of the lower cusp ; and when I had
observed this indentation longer, and had seen that it
was dark throughout, at length, after about two hours,
a bright peak began to arise a little below the middle
of the depression ; this by degrees increased, and
presented a triangular shape, but was as yet quite
detached and separated from the illuminated surface.
Soon around it three other small points began to
shine, until, when the Moon was just about to set,
that triangular figure, having now extended and
widened, began to be connected with the rest of the
illuminated part, and, still girt with the three bright
peaks already mentioned, suddenly burst into the
indentation of shadow like a vast promontory of
light.

At the ends of the upper and lower cusps also

THE SIDEREAL MESSENGER. 19

certain bright points, quite away from the rest of the
bright part, began to rise out of the shadow, as is seen
depicted in the same illustration.

In both horns also, but especially in the lower one,
there was a great quantity of dark spots, of which
those which are nearer the boundary of light and
shadow appear larger and darker, but those which are
more remote less dark and more indistinct. In all
cases, however, just as I have mentioned before, the
dark portion of the spot faces the position of the
Sun's illumination, and a brighter edge surrounds the
darkened spot on the side away from the Sun, and
towards the region of the Moon in shadow. This
part of the surface of the Moon, where it is marked
with spots like a peacock's tail with its azure eyes, is
rendered like those glass vases which, through being
plunged while still hot from the kiln into cold water,
acquire a crackled and wavy surface, from which cir-
cumstance they are commonly 0.2^^^ frosted glasses}
Now the great spots of the Moon observed at the same The lunar

spots are

time are not seen to be at all similarly broken, or full suggested to

be possibly

of depressions and prominences, but rather to be even seas bordered

by ranges of

and uniform ; for only here and there some spaces, mountains.
rather brighter than the rest, crop up ; so that if any

1 Specimens of frosted or crackled Venetian glass are to be seen in the
Slade Collection, British Museum, and fully justify Galileo's comparison.

20 THE SIDEREAL MESSENGER.

one wishes to revive the old opinion of the Pytha-
goreans, that the Moon is another Earth, so to say,
the brighter portion may very fitly represent the
surface of the land, and the darker the expanse of
water. Indeed, I have never doubted that if the
sphere of the Earth were seen from a distance, when
flooded with the Sun s rays, that part of the surface
which is land would present itself to view as brighter,
and that which is water as darker in comparison.
Moreover, the great spots in the Moon are seen to be
more depressed than the brighter tracts ; for in the
Moon, both when crescent and when waning, on the
boundary between the light and shadow, which pro-
jects in some places round the great spots, the adjacent
regions are always brighter, as I have noticed in
drawing my illustrations, and the edges of the spots
referred to are not only more depressed than the
brighter parts, but are more even, and are not broken
by ridges or ruggednesses. But the brighter part
stands out most near the spots, so that both before
the first quarter and about the third quarter also,
around a certain spot in the upper part of the figure,
that is, occupjdng the northern region of the Moon,
some vast prominences on the upper and lower sides
of it rise to an enormous elevation, as the illustrations
show. This same spot before the third quarter is seen

THE SIDEREAL MESSENGER. 21

to be walled round with boundaries of a deeper shade,
which just like very lofty mountain summits appear
darker on the side away from the Sun, and brighter
on the side where they face the Sun ; but in the case
of the cavities the opposite happens, for the part of
them away from the Sun appears brilliant, and that
part which lies nearer to the Sun dark and in shadow.
After a time, when the enlightened portion of the
Moon's surface has diminished in size, as soon as
the whole or nearly so of the spot already mentioned
is covered with shadow, the brighter ridges of the
mountains mount high above the shade. These two
appearances are shown in the illustrations which are
given.

There is one other point which I must on no Description

of a lunar

account forget, which I have noticed and rather "ater, per-

hapsTycho.i

wondered at. It is this : — The middle of the Moon, as
it seems, is occupied by a certain cavity larger than
all the rest, and in shape perfectly round. I have
looked at this depression near both the first and third
quarters, and I have represented it as well as I can
in the second illustration already given. It produces
the same appearance as to efi'ects of light and shade
as a tract like Bohemia would produce on the Earth,

1 Webb, Celestial Objects for Common Telescopes, p. 104, suggests this
identification.

22 777^ SIDEREAL MESSENGER.

if it were shut in on all sides by very lofty moun-
tains arranged on the circumference of a perfect
circle ; for the tract in the Moon is walled in with
peaks of such enormous height that the furthest side
adjacent to the dark portion of the Moon is seen
bathed in sunlight before the boundary between
light and shade reaches half-way across the circular
space. But according to the characteristic property
of the rest of the spots, the shaded portion of this too
faces, the Sun, and the bright part is towards the dark
side of the Moon, which for the third time I advise to
be carefully noticed as a most solid proof of the
ruggednesses and unevennesses spread over the whole
of the bright region of the Moon. Of these spots,
moreover, the darkest are always those which are
near to the boundary-line between the light and the
shadow, but those further off appear both smaller in
size and less decidedly dark ; so that at length, when
the Moon at opposition becomes full, the darkness of
the cavities differs from the brightness of the promi-
nences with a subdued and very slight difference.
Reasons for Thcsc phcnomeua which we have reviewed are
thatThere is a observed in the bright tracts of the Moon. In the
constitution grcat spots we do not see such differences of depres-

in various . ^ . n 1 j

parts of the sious auQ prommcuccs as we are compelled to recog-
fac°e°" ' '"' nise in the brighter parts, owing to the change of their

THE SIDEREAL MESSENGER. 23

shapes under different degrees of illumination by the
Sun's rays according to the manifold variety of the
Sun's position with regard to the Moon. Still, in the
great spots there do exist some spaces rather less
dark than the rest, as I have noted in \h^ illustrations,
but these spaces always have the same appearance,
and the depth of their shadow is neither intensified
nor diminished ; they do appear indeed sometimes a
little more shaded, sometimes a little less, but the
change of colour is very slight, according as the Sun's
rays fall upon them more or less obliquely ; and
besides, they are joined to the adjacent parts of the
spots with a very gradual connection, so that their
boundaries mingle and melt into the surrounding
region. But it is quite different with the spots which
occupy the brighter parts of the Moon's surface, for,
just as if they were precipitous crags with numerous
rugged and jagged peaks, they have well-defined
boundaries through the sharp contrast of light and
shade. Moreover, inside those great spots certain
other tracts are seen brighter than the surrounding
region, and some of them very bright indeed, but
the appearance of these, as well as of the darker parts,
is always the same ; there is no change of shape or
brightness or depth of shadow^, so that it becomes a
matter of certainty and beyond doubt that their

of the even-
ness of the
illuminated
part of the

24 THE SIDEREAL MESSENGER.

appearance is owing to real dissimilarity of parts, and
not to unevennesses only in their configuration, chang-
ing in different ways the shadows of the same parts
according to the variations of their illumination by the
Sun, which really happens in the case of the other
smaller spots occupying the brighter portion of the
Moon, for day by day they change, increase, decrease,
or disappear, inasmuch as they derive their origin
only from the shadows of prominences.
Explanation ^^^ \qyq I fccl that somc pcoplc may be troubled
with grave doubt, and perhaps seized with a difficulty
so serious as to compel them to feel uncertain about
ence^'ofThe tlic couclusion just cxplaincd and supported by so
byThYant. Hiauy phcnomcna. For if that part of the Moon's
Irfalphe^n"!^' surface which reflects the Sun's rays most brightly is
Tpoiibie^^ full of sinuosities, protuberances, and cavities innumer-
sphere^^"'° ^^Ic, why, whcn the Moon is increasing, does the outer
edge which looks toward the west, when the Moon is
waning, the other half-circumference towards the east,
and at full-moon the whole circle, appear not uneven,
rugged, and irregular, but perfectly round and circular,
as sharply defined as if marked out with a pair of
compasses, and without the indentations of any pro-
tuberances or cavities \ And most remarkably so,
because the whole unbroken edge belongs to that part
of the Moon's surface which possesses the property of

THE SIDEREAL MESSENGER. 25

appearing brighter than the rest, wliich I have said to
be throughout full of protuberances and cavities. For
not one of the Great Spots extends quite to the cir-
cumference, but all of them are seen to be together
away from the edge. Of this phenomenon, which
affords a handle for such serious doubt, I produce two
causes, and so two solutions of the difficulty.

The first solution which I offer is this : — If the pro-
tuberances and cavities in the body of the Moon
existed only on the edge of the circle that bounds the
hemisphere which we see, then the Moon might, or
rather must, show itself to us with the appearance
of a toothed wheel, being bounded with an irregular
and uneven circumference ; but if, instead of a single
set of prominences arranged along the actual circum-
ference only, very many ranges of mountains with
their cavities and ruggednesses are set one behind the
other along the extreme edge of the Moon, and that
too not only in the hemisphere w^hich we see, but also
in that which is turned away from us, but still near
the boundary of the hemisphere, then the eye, viewing
them afar off, will not at all be able to detect the
differences of prominences and cavities, for the inter-
vals between the mountains situated in the same
circle, or in the same chain, are hidden by the jutting
forward of other prominences situated in other ranges,

26 THE SIDEREAL MESSENGER.

and especially if the eye of the observer is placed in
the same line with the tops of the prominences men-
tioned. So on the Earth, the summits of a number of
mountains close together appear situated in one plane,
if the spectator is a long way off and standing at the
same elevation. So when the sea is rough, the tops of
the waves seem to form one plane, although between
the billows there is many a gulf and chasm, so deep
that not only the hulls, but even the bulwarks, masts,
and sails of stately ships are hidden amongst them.
Therefore, as mthin the Moon, as well as round her
circumference, there is a manifold arrangement of
prominences and cavities, and the eye, regarding them
from a great distance, is placed in nearly the same
plane with their summits, no one need think it strange
that they present themselves to the visual ray which
just grazes them as an unbroken line quite free from
unevennesses. To this explanation may be added
another, namely, that there is round the body of the
Moon, just as round the Earth, an envelope of some
substance denser than the rest of the ether, which is
sufficient to receive and reflect the Sun s rays, although
it does not possess so much opaqueness as to be able
to prevent our seeing through it — especially when it
is not illuminated. That envelope, when illuminated
by the Sun's rays, renders the body of the Moon

THE SIDEREAL MESSENGER.

apparently larger than it really is, and would be able
to stop our sight from penetrating to the solid body
of the Moon, if its thickness were greater ; now, it is
of greater thickness about the circumference of the
Moon, greater, I mean, not in actual thickness, but
with reference to our sight-rays, which cut it obliquely ;
and so it may stop our vision, especially when it is
in a state of brightness, and may conceal the true
circumference of the Moon on the side towards the
Sun.

This may be understood more clearly from the
adjoining figure, in which the body of the Moon, A B c.

is surrounded by an enveloping atmosphere, D E G.
An eye at f penetrates to the middle parts of the
Moon, as at a, through a thickness, da, of the at-
mosphere ; but towards the extreme parts a mass of
atmosphere of greater depth, E B, shuts out its boun-
dary from our sight. An argument in favour of this

28 THE SIDEREAL MESSENGER.

is, that the illuminated portion of the Moon appears
of larger circumference than the rest of the orb which
is in shadow.

Perhaps also some will think that this same cause
affords a very reasonable explanation why the greater
spots on the Moon are not seen to reach to the edge
of the circumference on any side, although it might
be expected that some would be found about the edge
as well as elsewhere ; and it seems credible that there
are spots there, but that they cannot be seen because
they are hidden by a mass of atmosphere too thick
and too bright for the sight to penetrate.
Calculation I thluk that It has been sufficiently made clear,
the height of from the explanation of phenomena which have been
given, that the brighter part of the Moon's surface is
dotted everywhere with protuberances and cavities ;

some lunar
mountains
exceeds four
Italian miles*

British feet). It ouly rcmalus for me to speak about their size, and
to show that the ruggednesses of the Earth's surface
are far smaller than those of the Moon's ; smaller, I
mean, absolutely, so to say, and not only smaller in
proportion to the size of the orbs on which they are.
And this is plainly shown thus : — As I often observed
in various positions of the Moon with reference to the

* In the list of the heights of lunar mountains determined by Beer and
Maedler, given in their work Dtr Mond (Berlin, 1837), there are six
which exceed 3000 toises, or 19,000 British feet.

THE SIDEREAL MESSENGER.

Sun, that some summits within the portion of the
Moon in shadow appeared illumined, although at
some distance from the boundary of the light (the
terminator), by comparing their distance with the
complete diameter of the Moon, I learnt that it some-
times exceeded the one-twentieth (^oth) part of the
diameter. Suppose the dis-
tance to be exactly -i^\h
part of the diameter, and let
the diagram represent the
Moon's orb, of which c A F is
a great circle, E its centre,
and c F a diameter, which
consequently bears to the
diameter of the Earth the
ratio 2:7; and since the diameter of the Earth, ac-
cording to the most exact observations, contains 7000
Italian miles, CF will be 2000, and CE 1000, and the
-2Vth part of the whole, CF, 100 miles. Also let CF
be a diameter of the great circle which divides the
bright part of the Moon from the dark part (for,
owing to the very great distance of the Sun from the
Moon this circle does not diflPer sensibly from a great
one), and let the distance of a from the point c be
-^Vth part of that diameter ; let the radius E A be
di-awn, and let it be produced to cut the tangent line

30 THE SIDEREAL MESSENGER.

G c D, which represents the ray that illumines the
summit, in the point D. Then the arc CA or the
straight line c d will be 100 of such units, as c e con-
tains 1000. The sum of the squares of DC, CE is
therefore 1,010,000, and the square of de is equal to
this; therefore the whole ED will be more than 1004;
and A D will be more than 4 of such units, as c E con-
tained 1000. Therefore the height of A D in the Moon,
which represents a summit reaching up to the Sun's
ray, G c d, and separated from the extremity c by
the distance c D, is more than 4 Italian miles ; but
in the Earth there are no mountains which reach to
the perpendicular height even of one mile. We are
therefore left to conclude that it is clear that the
- prominences of the Moon are loftier than those of
the Earth.
The faint \ wlsli iu tMs pkcc to asslgu the cause of another

illumination

of the Moon's luuar plicnomcnon well worthy of notice, and although

disc about 111

new-moon tlils pheuomcnon was observed by me not lately, but

explained to i i i • T

be due to many years ago, and has been pomted out to some
of my intimate friends and pupils, explained, and
assigned to its true cause, yet as the observation of it
is rendered easier and more vivid by the help of a
telescope, I have considered that it would not be
unsuitably introduced in this place, but I wish to
introduce it chiefly in order that the connection and

THE SIDEREAL MESSENGER.

resemblance between the Moon and the Earth may
appear more plainly.

When the Moon, both before and after conjunction,
is found not far from the Sun, not only does its orb
show itself to our sight on the side where it is fur-
nished with shining horns, but a slight and faint
circumference is also seen to mark out the circle of
the dark part, that part, namely, which is turned away
from the Sun, and to separate it from the darker back-
ground of the sky. But if we examine the matter
more closely, we shall see that not only is the extreme
edge of the part in shadow shining with a faint
brightness, but that the entire face of the Moon, that
side, that is, which does not feel the Suns glare, is
illuminated with a pale light of considerable bright-
ness. At the first glance only a fine circumference
appears shining, on account of the darker part of
the sky adjacent to it ; whereas, on the contrary,
the rest of the surface appears dark, on account of the
contiguity of the shining horns, which destroys the
clearness of our sight. But if any one chooses such
a position for himself, that by the interposition of a
roof, or a chimney, or some other object between
his sight and the Moon, but at a considerable distance
from his eye, the shining horns are hidden, and the
rest of the Moon's orb is left exposed to his view,

32 THE SIDEREAL MESSENGER.

then he will find that this tract of the Moon also,
although deprived of sunlight, gleams with consider-
able light, and particularly so if the gloom of the
night has already deepened through the absence of
the Sun ; for with a darker background the same
light appears brighter. Moreover, it is found that
this secondary brightness of the Moon, as I may call
it, is greater in proportion as the Moon is less distant
from the Sun ; for it abates more and more in pro-
portion to the Moon s distance from that body, so
much so that after the first quarter, and before the
end of the second, it is found to be weak and very
faint, although it be observed in a darker sky ;
whereas, at an angular distance of 60° or less, even
during twilight, it is wonderfully bright, so bright
indeed that, with the help of a good telescope, the
great spots may be distinguished in it.

This strange brightness has afforded no small per-
plexity to philosophical minds ; and some have
published one thing, some another, as the cause to
be alleged for it. Some have said that it is the
inherent and natural brightness of the Moon ; some
that it is imparted to that body by the planet Venus ;
or, as others maintain, by all the stars ; while some
have said that it comes from the Sun, whose rays,
they say, find a way through the solid mass of the

THE SIDEREAL MESSENGER. 33

Moon. But statements of tins kind are disproved
without much difficulty, and convincingly demons-
trated to be false. For if this kind of light were
the Moon's own, or were contributed by the stars,
the Moon would retain it, particularly in eclipses,
and would show it then, when left in an unusually
dark sky, but this is contrary to experience. For
the brightness which is seen on the Moon in eclipses
is far less intense, being somewhat reddish, and
almost copper-coloured, whereas this is brighter and
whiter ; besides, the brightness seen during an eclipse
is changeable and shifting, for it wanders over the
face of the Moon, so that that part which is near the
circumference of the circle of shadow thrown by the
Earth is bright, but the rest of the Moon is always
seen to be dark. From Avhich circumstance we
understand without hesitation that this brightness is
due to the proximity of the Sun's rays coming into
contact with some denser region which surrounds
the Moon as an envelope ; owing to which contact a
sort of dawn-lio;ht is diffused over the neio-hbourino^
regions of the Moon, just as the twilight spreads in
the morning and evening on the Earth ;^ but I will

1 The illumination of the Moon in eclipses, noticed by Galileo, is now
referred to the refraction of the sunlight by the earth's atmosphere, and
the reddish colour of the light is explained by Herschel {Outlines of

34 THE SIDEREAL MESSENGER.

treat more fully of this matter in my book upon the
System of the Universe}

Again, to assert that this sort of light is imparted
to the Moon by the planet Venus is so childish as to
be undeserving of an answer ; for who is so ignorant
as not to understand that at conjunction and within
an angular distance of 60° it is quite impossible for
the part of the Moon turned away from the Sun to be
seen by the planet Venus ?

But that this light is derived from the Sun pene-
trating with its light the solid mass of the Moon, and
rendering it luminous, is equally untenable. For
then this light would never lessen, since the hemi-
sphere of the Moon is always illumined by the Sun,
except at the moment of a lunar eclipse, yet really it
quickly decreases while the Moon is drawing near to
the end of her first quarter, and when she has passed

Astronomy, ch. vii.) to be clue to tlie absorption of the violet and blue
rays by the aqueous vapour of the Earth's atmosphere. The idea of
a sensible lunar atmosphere is not in accordance with the observed
phenomena of the occultations of stars.

1 Galileo's Systema Mundi. Owing to the violent opposition provoked
by the discussion of the discoveries of Galileo, and their bearing on the
Copernican system of astronomy, Galileo seems to have found it neces-
sary to delay the publication of this work until 1632, when, belie \^ng
himself safe under the friendship and patronage of Pope Urban viii. and
others in power at Rome, he at length published it. Urban, however,
now turned against him, denounced the book and its author, and sum-
moned him to Rome, where the well-known incidents of his trial and
condemnation took place.

THE SIDEREAL MESSENGER. 35

her first quarter it becomes quite dull. Since, there-
fore, this kind of secondary brightness is not inherent
and the Moon's own, nor borrowed from any of the
stars, nor from the Sun, and since there now remains
in the whole universe no other body whatever except
the Earth, w^hat, pray, must we conclude ? What must
we assert ? Shall we assert that the body of the
Moon, or some other dark and sunless orb, receives
light from the Earth? Why should it not be the
Moon ? And most certainly it is. The Earth, ^dth
fair and grateful exchange, pays back to the Moon an
illumination like that which it receives from the
Moon nearly the whole time during the darkest
gloom of night. Let me explain the matter more
clearly. At conjunction, when the Moon occupies
a position between the Sun and the Earth, the
Moon is illuminated by the Sun's rays on her half
towards the Sun which is turned away from the Earth,
and the other half, ^dth which she regards the Earth,
is covered mth darkness, and so in no degree illumines
the Earth's surface. When the Moon has slightly
separated from the Sun, straightway she is partly
illumined on the half directed towards us ; she turns
towards us a slender silvery crescent, and slightly
illumines the Earth ; the Sun's illumination increases
upon the Moon as she approaches her first quarter,

36 THE SIDEREAL MESSENGER.

and the reflexion of that light increases on the Earth ;
the brightness in the Moon next extends beyond the
semicircle, and our nights grow brighter; at length
the entire face of the Moon looking towards the Earth
is irradiated with the most intense brightness by the
Sun, which happens when the Sun and Moon are on
opposite sides of the Earth ; then far and wide the
surface of the Earth shines with the flood of moon-
light ; after this the Moon, now waning, sends out
less powerful beams, and the Earth is illumined less
powerfully ; at length the Moon draws near her first
position of conjunction with the Sun, and forthwith
black night invades the Earth. In such a cycle the
moonlight gives us each month alternations of brighter
and fainter illumination. But the benefit of her light
to the Earth is balanced and repaid by the benefit of
the light of the Earth to her ; for while the Moon is
found near the Sun about the time of conjunction, she
has in front of her the entire surface of that hemi-
sphere of the Earth which is exposed to the Sun, and
vividly illumined with his beams, and so receives
light reflected from the Earth. Owing to such re-
flexion, the hemisphere of the ]\Ioon nearer to us,
though deprived of sunlight, appears of considerable
briohtness. Ao'ain, when removed from the Sun
through a cjuadrant, the Moon sees only one-half of

THE SIDEREAL MESSENGER. yj

tlie Earth's liemispliere illuminated, namely the
western half, for the other, the eastern, is covered
with the shades of night ; the Moon is, therefore, less
brightly enlightened by the Earth, and accordingly
that secondary light appears fainter to us. But if
you imagine the Moon to be set on the opposite side
of the Earth to the Sun, she will see the hemis23here
of the Earth, now^ between the Moon and the Sun,
quite dark, and steeped in the gloom of night ; if,
therefore, an ecli^^se should accompany such a posi-
tion of the Moon, she will receive no light at all, being
deprived of the illumination of the Sun and Earth
together. In any other position, with regard to the
Earth and the Sun, the Moon receives more or less
light by reflexion from the Earth, according as she
sees a greater or smaller portion of the hemisphere of
the Earth illuminated by the Sun ; for such a law is
observed between these two orbs, that at whatever
times the Earth is most brightly enlightened by the
Moon, at those times, on the contrary, the Moon is
least enlightened by the Earth ; and contrariwise.
Let these few words on this subject suffice in this
place ; for I will consider it more fully in my System
of the Universe, where, by very many arguments and
experimental proofs, there is shown to be a very
strong reflexion of the Sun's light from the Earth, for

38 THE SIDEREAL MESSENGER.

the benefit of those Avho urge that the Earth must
be separated from the starry host, chiefly for the
reason that it has neither motion nor light, for I will
prove that the Earth has motion, and surpasses the
Moon in brightness, and is not the place where the
dull refuse of the universe has settled down; and I
will support my demonstration by a thousand argu-
ments taken from natural phenomena.
Stars. Their Hlthcrto I havc spoken of the observations which I

appearance ^ i-nr jit t*ii

in the have made concerning the Moon s body ; now i will
briefly announce the phenomena which have been,
as yet, seen by me with reference to the Fixed Stars.
And first of all the following fact is worthy of con-
sideration : — The stars, fixed as well as erratic, when
seen with a telescojDe, by no means appear to be
increased in magnitude in the same proportion as
other objects, and the Moon herself, gain increase of
size ; but in the case of the stars such increase appears
much less, so that you may consider that a telescope,
which (for the sake of illustration) is powerful enough
to magnify other objects a hundred times, will scarcely
render the stars magnified four or five times. But
the reason of this is as follows : — When stars are
viewed with our natural eyesight they do not present
themselves to us of their bare, real size, but beaming
with a certain vividness, and fringed with sparkling

THE SIDEREAL MESSENGER. 39

rays, especially when the night is far advanced ; and
from this circumstance they appear mnch larger than
they would if they were stripped of those adventitious
fringes, for the angle which they subtend at the eye
is determined not by the primary disc of the star, but
by the brightness which so widely surrounds it.
Perhaps you T\ill understand this most clearly from
the well-known circumstance that when stars rise
just at sunset, in the beginning of twilight, they
appear very small, although they may be stars of the
first magnitude ; and even the planet Venus itself, on
any occasion when it may present itself to view in
broad daylight, is so small to see that it scarcely
seems to equal a star of the last magnitude. It is
different in the case of other objects, and even of the
Moon, which, whether viewed in the light of midday
or in the depth of night, always appears of the same
size. We conclude therefore that the stars are seen
at midnight in un curtailed glory, but their fringes are
of such a nature that the daylight can cut them off,
and not only daylight, but any slight cloud which
may be interposed between a star and the eye of the
observer. A dark veil or coloured glass has the same
effect, for, upon placing them before the eye between
it and the stars, all the blaze that surrounds them
leaves them at once. A telescope also accomplishes

40 THE SIDEREAL MESSENGER.

the same result, for it removes from the stars their
adventitious and accidental splendours before it
enlarges their true discs (if indeed they are of that
shape), and so they seem less magnified than other
objects, for a star of the fifth or sixth magnitude seen
through a telescope is shown as of the first magnitude
only.

The difference between the appearance of the
j)lanets and the fixed stars seems also deserving of
notice. The planets present their discs perfectly
round, just as if described with a pair of compasses,
and appear as so many little moons, completely illu-
minated and of a globular shape ; but the fixed stars
do not look to the naked eye bounded by a circular
circumference, but rather like blazes of light, shooting
out beams on all sides and very sparkling, and with a
telescope they appear of the same shape as when they
are viewed by simply looking at them, but so much
laro^er that a star of the fifth or sixth mao^nitude
seems to equal Sirius, the largest of all the fixed stars. -^
Telescopic But bevoud the stars of the sixth mao-nitude you

Stare • tKpIr J ' O J

will behold through the telescope a host of other stars,

1 The immense distance of stars makes it impossible for them to be
magnified by any telescope, however powerful ; the apparent or spurious
disc is an optical effect, which depends on the telescope used, and is
smallest with the largest aperture.

Stars : their
infinite

Or(ons 6e/f and Sword; 83 Stars

Pleiades, 3b Stars
Galileo^Sidereus Nunc/us."

THE SIDEREAL MESSENGER. 41

wliicli escape tlie unassisted sio;lit, so numerous as to multitude.

-L '-' As examples,

be almost beyond belief, for you may see more than 0"°^'^ Beit

•J J J J and Sword

six otlier differences of mao-nitude, and tlie laro-est of ^"^ the

^ ^ O _ Pleiades are

these, whicb I may call stars of the seventh maoiii- described

•^ *-^ as seen by

tude, or of the first magnitude of invisible stars, ap- Gaiiieo.
pear with the aid of the telescope larger and brighter
than stars of the second masfnitude seen with the
unassisted sight. But in order that you may see one
or two proofs of the inconceivable manner in which
they are crowded together, I have determined to
make out a case against two star-clusters, that from
them as a specimen you may decide about the rest.

As my first example I had determined to depict
the entire constellation of Orion, but I was over-
whelmed by the vast quantity of stars and by want
of time, and so I have deferred attempting this to
another occasion, for there are adjacent to, or scattered
among, the old stars more than five hundred new
stars within the limits of one or two degrees. For
this reason I have selected the three stars in Orion's
Belt and the six in his Sword, which have been long
well-known groups, and I have added eighty other
stars recently discovered in their vicinity, and I have
preserved as exactly as possible the intervals between
them. The well-knoT\Ti or old stars, for the sake of
distinction, I have depicted of larger size, and I have

42 THE SIDEREAL MESSENGER.

outlined tliem witli a double line ; the others, invisible
to the naked eye, I have marked smaller and with
one line only. I have also preserved the differences
of magnitude as much as I could.

As a second example I have depicted the six stars

/ of the constellation Taurus, called the Pleiades (I say

/ six intentionally, since the seventh is scarcely ever

visible), a group of stars which is enclosed in the heavens

within very narrow precincts. Near these there lie

more than forty others invisible to the naked eye, no

one of which is much more than half a degree off any

of the aforesaid six ; of these I have noticed only

thirty-six in my diagram. I have preserved their

intervals, magnitudes, and the distinction between the

old and the new stars, just as in the case of the

constellation Orion.

The Milky Thc ucxt objcct which I have observed is the

Way consists

entirely of esscucc or substaucc of the Milky Way. By the aid

stars in

countless of a tclcscopc auy one may behold this in a manner

numbers and

of various wdilch SO dlstluctly appeals to the senses that all the

magnitudes.

disputes which have tormented philosophers through
so many ages are exploded at once by the irrefragable
evidence of our eyes, and we are freed from wordy
disputes upon this subject, for the Galaxy is nothing
else but a mass of innumerable stars planted together
in clusters. Upon whatever part of it you direct the

Star-duster in. Orion's Head'

-T-

+ + ^
^ +

:K-

+ +

+ 4-

Star-cluster of Praesepo ,n Cane
Galileo"Sidereus Nuncius, Yen ice, 1610.

THE SIDEREAL MESSENGER. 43

telescope straightway a vast crowd of stars presents
itself to view ; many of them are tolerably large and
extremely bright, but the number of small ones is
quite beyond determination.

And whereas that milky brightness, like the bright- Nebuise re-

_ , . , , . -, , . . 1 solved into

ness 01 a white cloud, is not only to be seen m the dusters of
Milky Way, but several spots of a similar colour shine examples.
faintly here and there in the heavens, if you turn the orioasHead
telescope upon any of them you will find a cluster of
stars packed close together. Further — and you will
be more surprised at this, — the stars which have been
called by every one of the astronomers up to this day
nebulous, are groups of small stars set thick together
in a wonderful way, and although each one of them
on account of its smallness, or its immense distance
from us, escapes our sight, from the commingling of
their rays there arises that brightness which has
hitherto been believed to be the denser part of the
heavens, able to reflect the rays of the stars or the
Sun.

I have observed some of these, and I wish to subjoin
the star-clusters of two of these nebulae. First, you
have a diagram of the nebula called that of Orion's
Head, in which I have counted twenty-one stars.

The second cluster contains the nebula called Pr^e-
sepe, which is not one star only, but a mass of more

Jan. 7, 1610
record of
Galileo's
observations
during two
months.

44 THE SIDEREAL MESSENGER.

than forty small stars. I have noticed thirty-six
stars, besides the Aselli, arranged in the order of the
accompanpng diagram.
Discovery of I have now finished my brief account of the
sSkeJ, observations which I have thus far made with re-
gard to the Moon,- the Fixed Stars, and the Galaxy.
There remains the matter, which seems to me to
deserve to be considered the most important in this
work, namely, that I should disclose and publish to
the world the occasion of discovering and observing
four PLAXETS, never seen from the very beginning of
the world up to our own times, their positions, and
the observations made during the last two months
about their movements and their chansres of mas^ni-
tude ; and I summon all astronomers to apply them-
selves to examine and determine their periodic times,
which it has not been permitted me to achieve up
to this day, owing to the restriction of my time.
I give them warning however again, so that they may
not approach such an inquiry to no purpose, that they
will want a very accurate telescope, and such as I
have described in the beginning of this account.

On the 7th day of January in the present year,
1610, in the first ^ hour of the follomng night, when I

1 The times of Galileo's observations are to be understood as reckoned
from sunset.

THE SIDEREAL MESSENGER. 45

was vie win o^ the constellations of the heavens throuoh
a telescope, the planet Jupiter presented itself to my
view, and as I had prepared for myself a very excel-
lent instrument, I noticed a circumstance which I had
never been able to notice before, owing to want of
power in my other telescope, namely, that three little
stars, small but very bright, were near the planet ;
and although 1 believed them to belong to the num-
ber of the fixed stars, yet they made me somewhat
wonder, because they seemed to be arranged exactly
in a straight line, parallel to the ecliptic,^ and to be
brighter than the rest of the stars, equal to them
in magnitude. The position of them with reference
to one another and to Jupiter was as follows (Fig. l).

On the east side there were two stars, and a single
one towards the west. The star which was furthest
toAvards the east, and the western star, appeared
rather larger than the third.

I scarcely troubled at all about the distance between
them and Jupiter, for, as I have already said, at first
I believed them to be fixed stars ; but when on Janu-
ary 8th, led by some fatality, I turned again to look

^ The satellites of Jupiter revolve in planes very nearly, although not
exactly, coincident with that of the equator of the planet, which is in-
clined 3° 5' 30^'' to the orbit of the planet, and the plane of the orbit is
inclined 1° 18' 51" to the ecliptic.

46 THE SIDEREAL MESSENGER.

at the same part of the heavens, I found a very differ-
ent state of things, for there were three little stars all
west of Jupiter, and nearer together than on the
previous night, and they were separated from one
another by equal intervals, as the accompanying illus-
tration (Fig. 2) shows.

At this point, although I had not turned my
thoughts at all upon the approximation of the stars to
one another, yet my surprise began to be excited, how
Jupiter could one day be found to the east of all the
aforesaid fixed stars when the day before it had been
west of two of them ; and forthwith I became afraid
lest the planet might have moved differently from the
calculation of astronomers, and so had passed those
stars by its own proper motion. I therefore waited
for the next night with the most intense longing, but
I was disappointed of my hope, for the sky was
covered with clouds in every direction.

But on January 10th the stars appeared in the
following position with regard to Jupiter ; there were
two only, and both on the east side of Jupiter, the
third, as I thought, being hidden by the planet
(Fig. 3). They were situated just as before, exactly
in the same straight line with Jupiter, and along
the Zodiac.

When I had seen these phenomena, as I knew that

THE SIDEREAL MESSENGER. 47

corresponding changes of position could not by any
means belong to Jupiter, and as, moreover, I perceived
that the stars which I saw had been always the same,
for there were no others either in front or behind,
within a great distance, along the Zodiac, — at length,
changing from doubt into surprise, I discovered that
the interchange of position which I saw^ belonged not
to Jupiter, but to the stars to which my attention had
been drawn, and I thought therefore that they ought
to be observed henceforward mth more attention
and precision.

Accordingly, on January 11th I saw an arrange-
ment of the following kind (Fig. 4), namely, only
two stars to the east of Jupiter, the nearer of which
was distant from Jupiter three times as far as from
the star further to the east ; and the star furthest to
the east was nearly t^ice as large as the other one ;
whereas on the previous night they had ajDpeared
nearly of equal magnitude. I therefore concluded,
and decided unhesitatingly, that there are three stars
in the heavens moving about Jupiter, as Venus and
Mercury round the Sun ; which at length was estab-
lished as clear as daylight by numerous other subse-
quent observations. These observations also estab-
lished that there are not only three, but four, erratic
sidereal bodies performing their revolutions round

48 THE SIDEREAL MESSENGER.

Jupiter, observatioDs of whose changes of position
made with more exactness on succeeclinor nights the
following account will supply. I have measured also
the intervals between them with the telescope in the
manner already explained. Besides this, I have given
the times of observation, especially when several were
made in the same night, for the revolutions of these
planets are so swift that an observer may generally
get differences of position every hour.

Jan. 12. — At the first hour of the next nig;ht I saw
these heavenly bodies arranged in this manner (Fig. 5).
The satellite^ furthest to the east was greater than the
satellite furthest to the west ; but both were very
conspicuous and bright ; the distance of each one from
Jupiter was two minutes. A third satellite, certainly
not in view before, began to appear at the third hour ;
it nearly touched Jupiter on the east side, and was
exceedingly small. They were all arranged in the
same straight line, along the ecliptic.

Jan. 13. — For the first time four satellites were in
view in the following position with regard to Jupiter
(Fig. 6).

1 Galileo continues to call these bodies sto.rs, perhaps meaning "Mecli-
cean stars," throughout the description of their configurations, but as he
had now detected their nature, it is more convenient to call them satellites,
the term introduced by Kepler,

THE SIDEREAL MESSENGER. 49

There were three to the west, and one to the east ;
they made a straight line nearly, but the middle
satellite of those to the west deviated a little from the
straight line towards the north. The satellite furthest
to the east was at a distance of 2' from Jupiter ; there
were intervals of 1' only between Jupiter and the
nearest satellite, and between the satellites themselves,
west of Jupiter. All the satellites appeared of the same
size, and though small they were very brilliant, and
far outshone the fixed stars of the same masfnitude.

Jan. 14. — The weather was cloudy.

Jan. 15. — At the third hour of the night the four
satellites were in the state depicted in the next diagram
(Fig. 7) with reference to Jupiter.

All were to the west, and arranged nearly in the
same straight line ; but the satellite which counted
third from Jupiter was raised a little to the north.
The nearest to Jupiter was the smallest of all; the
rest appeared larger and in order of magnitude ;
the intervals between Jupiter and the three nearest
satellites were all equal, and were of the magni-
tude of 1' each; but the satellite furthest to the
west was distant ^ from the satelHte nearest to
it. They were very brilliant, and not at all twink-
ling, as they have always appeared both before and
since. But at the seventh hour there were only

50 THE SIDEREAL MESSENGER.

three satellites, presenting with Jupiter an appear-
ance of the following kind (Fig. 8). They were, that
is to say, in the same straight line to a hair ; the
nearest to Jupiter was very small, and distant from
the planet 3^; the distance of the second from this
one was l' \ and of the third from the second 4' 30".
But after another hour the two middle satellites were
still nearer, for they were only 3 O", or less, apart.

Jan. 16. — At the first hour of the night I saw three
satellites arranged in this order (Fig. 9). Jupiter was
between two of them, which were at a distance of
0' 40" from the planet on either side, and the third
was west of Jupiter at a distance of 8'. The satel-
lites near to Jupiter appeared brighter than the satel-
lite further off, but not larger.

Jan. 17, after sunset 0 hours 30 minutes, the
configuration was of this kind (Fig. 10). There was
one satellite only to the east, at a distance of o from
Jupiter ; to the west likewise there was only one
satellite, distant 11^ from Jupiter. The satellite on
the east appeared twice as large as the satellite to the
west ; and there were no more than these two. But
four hours after, that is, nearly at the fifth hour, a
third satellite began to emerge on the east side, which,
before its appearance, as I think, had been joined with
the former of the two other satellites, and the position

THE SIDEREAL MESSENGER. 51

was of this kind (Fig. 11). The middle satelHte was
very near indeed to the satellite on the east, and was
only 20" from it; and was a little towards the south
of the straight line drawn through the two extreme
satellites and Jupiter.

Jan. 18, at 0 h. 20 m. after sunset, the appearance
was such as this (Fig. 12), The satellite to the east
was larger than the western one, and was at a dis-
tance from Jupiter of 8', the western one being at a
distance of 10'.

Jan. 19. — At the second hour of the night the
relative position of the satellites was such as this
(Fig. 13) ,* that is, there were three satellites exactly
in a straight line with Jupiter, one to the east, at a
distance of 6^ from Jupiter ; between Jupiter and the
first satellite to the west in order, there was an interval
of 5'; this satellite was 4' off the other one more to
the west. At that time I was doubtful whether or no
there was a satellite between the satellite to the east and
Jupiter, but so very close to Jupiter as almost to touch
the planet ; but at the fifth hour I saw this satelHte dis-
tinctly, by that time occupying exactly the middle posi-
tion between Jupiter and the eastern satellite, so that
the configuration was thus (Fig. 14). Moreover, the
satellite which had just come into view Vv^as very small ;
yet at the sixth hour it was nearly as large as the rest.

52 THE SIDEREAL MESSENGER.

Jan. 20 : 1 h. 15 m. — A similar arrangement was
seen (Fig. 15). There were three satellites, so small
as scarcely to be distinguishable ; their distances
from Jupiter, and from one another, were not more
than l\ I was doubtful whether on the western side
there were two satellites or three. About the sixth
hour they were grouped in this way (Fig. 16). The
eastern satellite was twice as far away from Jupiter as
before, that is 2^; on the western side, the satellite in
the middle was distant from Jupiter O' 40", and from
the satellite still further to the west O' 20"; at length,
at the seventh hour, three satellites were seen on the
western side (Fig. 17). The satellite nearest to Jupiter
was distant from the planet 0^ 20"; between this one
and the satellite furthest to the west there was an
interval of 40", but between these another satellite was
in view shghtly southward of them, and not more
than 1 0" off the most westerly satellite.

Jan. 21 : Oh. 30 m. — There were three satellites
on the east side ; the satelhtes and Jupiter were at
equal distances apart (Fig. 18). The intervals were by
estimation 50" each. There was also one satellite on
the west, distant 4/ from Jupiter. The satellite on
the east side nearest to Jupiter was the least of all.

Jan. 22 : 2 h. — The grouping of the satellites was
similar (Fig. 19). There was an interval of 5^ from

THE SIDEREAL MESSENGER. 53

the satellite on the east to Jupiter ; from Jupiter to
the satellite furthest to the west 1' . The two interior
satellites on the western side were O' 40" apart, and
the satellite nearer to Jupiter was 1^ from the planet.
The inner satellites were smaller than the outer ones,
but they were situated all in the same straight line,
along the ecliptic, except that the middle of the three
western satellites was slightly to the south of it,
but at the sixth hour of the night they appeared in
this position (Fig. 20). The satellite on the east was
very small, at a distance from Jupiter of 0' as before ;
but the three satellites on the west were separated by
equal distances from Jupiter and from each other ;
and the intervals were nearly \' 20" each. The
satellite nearest Jupiter appeared smaller than the
other two on the same side, but they all appeared
arranged exactly in the same straight line.

Jan. 23, at 0 h. 40 m. after sunset, the grouping of
the satellites was nearly after this fashion (Fig. 21).
There were three satellites with Jupiter in a straight
line along the ecliptic, as they always have been ; two
were on the east of the planet, one on the west ; the
satellite furthest to the east was 1' from the next one,
and this satellite 2' 40" from Jupiter; Jupiter was
3'' 2 0" from the satellite on the west ; and they were
all of nearly the same size. But at the fifth hour the

54 THE SIDEREAL MESSENGER.

two satellites which had been previously near Jupiter
were no longer visible, being, as I suppose, hidden
behind Jupiter, and the appearance presented was
such as this (Fig. 22).

Jan. 24. — Three satellites, all on the east side, were
visible, and nearly, but not quite, in the same straight
line with Jupiter, for the satellite in the middle was
slightly to the south of it (Fig. 23). The satellite
nearest to Jupiter w^as 2^ distant from the planet ;
the next in order O' 30" from this satellite, and
the third was 9^ further off still ; they were all very
bright. But at the sixth hour two satellites only
presented themselves to view in this position, namely
in the same straight line with Jupiter exactly, and
the distance of the nearest to the planet was length-
ened to o'\ the next was 2' further off, and unless
I am mistaken, the two satellites previously observed
in the middle had come together, and appeared as
one.

Jan. 25, at 1 h. 40 m., the satellites were grouped
thus (Fig. 24). There w^ere only two satellites on the
east side, and these were rather large. The satellite
furthest to the east was 5' from th<e satellite in the
middle, and it was 6' from Jupiter.

Jan. 26, at 0 h. 40 m., the relative positions of
the satellites were thus (Fig. 25). Three satellites

THE SIDEREAL MESSENGER. 55

were in view, of which two were east and the
third west of Jupiter ; this one was distant o' from
the planet. On the east side the satellite in the
middle was at a distance of 5' 20"; the fmther satel-
lite was 6' beyond ; they w^ere arranged in a straight
line, and were of the same size. At the fifth hour the
arrangement was nearly the same, with this difference
only, that the fourth satellite was emerging on the east
side near Jupiter. It was smaller than the rest, and
was then at a distance of O' 30" from Jupiter; but
was raised a little above the straight line towards the
north, as the accompanying figure shows (Fig. 26).

Jan. 27, 1 h. after sunset, a single satellite only
was in view, and that on the east side of Jupiter in
this position (Fig. 27). It was very small, and at a
distance of 7^ from Jupiter.

Jan. 28 and 29. — Owing to the intervention of
clouds, I could make no observation.

Jan. 30. — At the first hour of the nio^ht the satel-
lites were in view, arranged in the following way
(Fig. 28). There was one satellite on the east side, at
a distance of 2^ 30" from Jupiter; and there were two
satellites on the west, of which the one nearer to
Jupiter was Z' off the planet, and the other satellite \'
further. The places of the outer satellites and Jupiter
were in the same straight line ; but the satellite in

56 THE SIDEREAL MESSENGER.

the middle was a little al30ve it to the north. The
satellite furthest to the west was smaller than the
rest.

On the last day of the month, at the second hour,
two satellites on the east side were visible, and one
on the west (Fig. 29), Of the satellites east of the
planet, the one in the middle was 2' 20" distant from
Jupiter ; and the satellite further to the east was 0^ 30"
from the middle satellite ; the satellite on the west
was at a distance of 10' from Jupiter. They were in
the same straight line nearly, and would have been
exactly so, only the satellite on the east nearest to
Jupiter was raised a little towards the north. At the
fourth hour, the two satellites on the east w^ere still
nearer together, for they were only 20" apart (Fig. 30).
The western satellite appeared rather small in these
two observations.

Feb. 1. — At the second hour of the night the
arrangement was similar (Fig. 31). The satellite
furthest to the east was at a distance of 6' from
Jupiter, and the western satellite 8^ On the east
side there was a very small satellite, at a distance of
20" from Jupiter. They made a perfectly straight
line.

Feb. 2. — The satellites were seen arranged thus
(Fig. 32). There was one only on the east, at a dis-

THE SIDEREAL MESSENGER. 57

tance of 6' from Jupiter. Jupiter was 4' from the
nearest satellite on the west ; between this satellite
and the satellite further to the west there was an
interval of 8' ; they were in the same straight line
exactly, and were nearly of the same magnitude.
But at the seventh hour four satellites were there
— two on each side of Jupiter (Fig. 33). Of
these satellites, the most easterly was at a distance
of a! from the next ; this satellite was \' 40" from
Jupiter ; Jupiter was 6' from the nearest satellite on
the west, and this one from the satellite further to
the west, 8^ ; and they were all alike in the same
straight line, drawn in the direction of the Zodiac.

Feb. 3 : 7 h. — The satellites were arranged in the
following way (Fig. 34) : — The satellite on the east
was at a distance of \' 30" from Jupiter ; the nearest
satellite on the west, 2', and there was a long dis-
tance, 10^, from this satellite to the satellite further
to the west. They were exactly in the same straight
line, and of equal magnitude.

Feb. 4 : 2 h. — Four satellites attended Jupiter, two
on the east and two on the west, arranged in one per-
fectly straight line, as in the adjoining figure (Fig. 35).
The satellite furthest to the east was at a distance of
o' from the next satellite. This one was O' 40" from
Jupiter ; Jupiter i' from the nearest satellite on the

58 THE SIDEREAL MESSENGER.

west ; and tliis one from the satellite further to the
west 6^ In magnitude they were nearly equal ; the
satellite nearest to Jupiter was rather smaller in ap-
pearance than the rest. But at the seventh hour (Fig.
36) the eastern satellites w^ere at a distance of only
0' 30" apart. Jupiter was 2^ from the nearest satellite
on the east ; and from the satellite on the west, next
in order, A! \ this one was distant Z' from the satellite
further to the w^est. They were all equal in magni-
tude, and in a straight line, drawn in the direction of
the ecliptic.

Feb. 5. — The sky -was cloudy.

Feb. 6. — Two satellites only appeared, with Jupiter
between them, as is seen in the accompanying figure
(Fig. 3 7). The satellite on the east was 2' from
Jupiter, and that on the west 3'. They were in the
same straight line with Jupiter, and were equal in
magnitude.

Feb. 7. — There were two satellites by the side of
Jupiter, and both of them on the east of the planet,
arranged in this manner (Fig. 38). The intervals
between the satellites and Jupiter were equal, and of
\' each ; and a straight line would go through them
and the centre of Jupiter.

Feb. 8 : 1 h. — Three satellites were there, all on
the east side of Jupiter, as in the diagram (Fig. 39).

THE SIDEREAL MESSENGER. 59

The nearest to Jupiter, a rather small one, was dis-
tant from the planet \' 20" ; the middle one was 4^
from this satellite, and was rather large ; the satellite
furthest to the east, a very small one, was at a dis-
tance of 0^ 20" from this satellite. It was doubtful
whether there was one satellite near to Jupiter or two,
for sometimes it seemed that there was another satel-
lite by its side towards the east, wonderfully small, and
only 1 0" from it. They were all situated at points in
a straight line drawn in the direction of the Zodiac.
At the third hour the satellite nearest to Jupiter was
almost touching the planet, for it was only distant lO"
from it ; but the others had become further off, for the
middle one was 6' from Jupiter. At length, at the fourth
hour, the satellite which was previously the nearest to
Jupiter joined wdth the planet and disappeared.

Feb. 9: Oh. 30m. — There were two satellites on
the east side of Jupiter, and one on the west, in an
arrangement such as this (Fig. 40). The satellite
furthest to the east, which was a rather small one, was
distant 4' from the next satellite ; the satellite in
the middle was laro;er, and at a distance of 7' from
Jupiter. Jupiter was distant 4' from the w^estern
satellite, which was a small one.

Feb. 10: 1 h. 30 m. — A pair of satellites, very
small, and both on the east of the planet, were

6o THE SIDEREAL MESSENGER.

visible, in the following position (Fig. 41). The
further satellite was distant from Jupiter lO', the
nearer 0^ 20", and they were in the same straight
line ; but at the fourth hour the satellite nearest to
Jupiter no longer appeared, and the other seemed so
diminished that it could scarcely be kept in sight,
although the atmosphere was quite clear, and the
satellite was further from Jupiter than before, since
its distance was now 12^

Feb. 11:1 h. — There were two satellites on the
east, and one on the west (Fig. 42). The western
satellite was at a distance of 4' from Jupiter. The
satellite on the east, nearest to the planet, was
likewise A! from Jupiter ; but the satellite further
to the east was at a distance from this one of
8' ; they were fairly clear to view, and in the same
straight line ; but at the third hour the fourth satel-
lite was visible near to Jupiter on the east, less in
magnitude than the rest, separated from Jupiter by a
distance of 0' 30^ and slightly to the north out of
the straight line drawn through the rest (Fig. 43).
They were all very bright and extremely distinct,
but at 5 h. 30 m. the satellite on the east nearest
to Jupiter had moved further aw^ay from the planet,
and was occupying a position midway between the
planet and the neighbouring satellite further to the

THE SIDEREAL MESSENGER. 6i

east. They were all in the same straight line exactly,
and of the same magnitude, as may be seen in the
accompanying diagram (Fig. 44).

Feb. 12 : Oh. 40 m. — A pair of satellites on the
east, a pair likewise on the west, were near the planet
(Fig. 45). The satellite on the east furthest removed
from Jupiter was at a distance of 10^, and the further
of the satellites on the west was 8' ofi'. They were
both fairly distinct. The other two were very near to
Jupiter, and very small, especially the satellite to the
east, which was at a distance of O' 40" from Jupiter.
The distance of the western satellite was V , But at
the fourth hour the satellite which was nearest to
Jupiter on the east was visible no longer.

Feb. 13 : Oh. 30 m. — Two satellites were visible
in the east, two also in the west (Fig. 46). The
satellite on the east near Jupiter was fairly distinct ;
its distance from the planet was 2^ The satelHte
further to the east was less noticeable ; it was distant
a! from the other. Of the satellites on the west, the
one furthest from Jupiter, which was very distinct,
was parted from the planet 4^ Between this satellite
and Jupiter intervened a small satellite close to the
most westerly satellite, being not more than 0^ 3" off.
They were all in the same straight line, corresponding
exactly to the direction of the ecli|)tic.

62 THE SIDEREAL MESSENGER.

Feb. 15 (for on the 14tli the sky was covered with
clouds), at the first hour, the position of the satellites
was thus (Fig. 4*7) ; that is, there were three satellites
on the east, but none were visible on the west. The
satellite on the east nearest to Jupiter was at a dis-
tance of O' 50" from the planet ; the next in order was
O' 20" from this satellite, and the furthest to the east
was 2' from the second satellite, and it was larger than
the others, for those nearer to Jupiter were very small.
But about the fifth hour only one of the satellites
which had been near to Jupiter was to be seen ; its
distance from Jupiter w^as 0^ 30". The distance of
the satellite furthest to the east from Jupiter had
increased, for it was then 4' (Fig. 48). But at the
sixth hour, besides the two situated as just described
on the east, one satellite was visible towards the west,
very small, at a distance of 2' from Jupiter (Fig. 49).

Feb. 16 : 6 h. — Their places were arranged as
follows (Fig. 50) ; that is, the satellite on the east was
l' from Jupiter, Jupiter 5^^ from the next satellite on
the west, and this %' from the remaining satellite still
further to the west. They were all of the same mag-
nitude nearly, rather bright, and in the same straight
line, corresponding accurately to the direction of the
Zodiac.

Feb. 17 : 1 h. — Two satellites were in view, one on

THE SIDEREAL MESSENGER. 63

the east, distant Z' from Jupiter; tlie. other on the
west, distant 10^ (Fig. 51). The latter was somewhat
less than the satellite on the east ; but at the sixth
hour the eastern satellite was nearer to Jupiter, being
at a distance of O' 50", and the western satellite was
further off, namely 12^ At both observations they
were in the same straight line with Jupiter, and were
both rather small, especially the eastern satellite in the
second observation.

Feb. 18 : 1 h. — Three satellites were in view, of
which two were on the west and one on the east ; the
distance of the eastern satellite from Jupiter was 3^,
and of the nearest satellite on the west 2' ; the remain-
ing satellite, still further to the west, was 8' from the
middle satellite (Fig. 52). They were all in the same
straight line exactly, and of about the same magnitude.
But at the second hour the satellites nearest to the
planet were at equal distances from Jupiter, for the
western satellite was now also 3' from the planet. But
at the sixth hour the fourth satellite was visible be-
tween the satellite on the east and Jupiter, in the fol-
lowing configuration (Fig. 53). The satellite furthest
to the east was at a distance of o' from the next in
order; this one was at a distance of \' 50" from
Jupiter ; Jupiter was at a distance of o' from the
next satellite on the west, and this 7^ from the satellite

64 THE SIDEREAL MESSENGER.

still further to the west. These were nearly equal in
magnitude, only the satellite on the east nearest to
Jupiter was a little smaller than the rest, and they were
all in the same straight line parallel to the ecliptic.

Feb. 19 : 0 h. 40 m. — Two satellites only were in
view, west of Jupiter, rather large, and arranged
exactly in the same straight line with Jupiter, in the
direction of the ecliptic (Fig. 54). The nearer satellite
was at a distance of 7' from Jupiter and of 6' from the
satellite further to the west.

Feb. 20. — The sky was cloudy.

Feb. 21: 1 h. 3 0 m. — Three satellites, rather small,
were in view, placed thus (Fig. 55). The satel-
lite to the east was 2' from Jupiter ; Jupiter was Z'
from the next, which was on the west ; and this one was
7' from the satellite further to the west. They were
exactly in the same straight line parallel to the ecliptic.

Feb. 25 : 1 h. 30 m. (for on the three previous
nights the sky was overcast). — Three satellites ap-
peared, two on the east, which were at a distance of
4' apart, the same as the distance of the nearer satelhte
from Jupiter ; on the west there was one satellite at a
distance of 2' from Jupiter. They were exactly in
the same straight line in the direction of the ecliptic
(Fig. 56).

Feb. 26 : 0 li. 30 m. — A pair of satellites only

THE SIDEREAL MESSENGER. 65

were present, one on the east, distant 10' from
Jupiter ; the other was on the west, at a distance of
6' (Fig. 57). The eastern satellite was slightly smaller
than the western. At the fifth hour three satellites
were visible ; for, besides the two already noticed, a
third satellite was in view, on the west, near Jupiter,
very small, which previously had been hidden behind
Jupiter, and it was at a distance of l' from the planet
(Fig. 58).

But the satellite on the east was seen to be further
off than before, being at a distance of 11' from
Jupiter. On this night, for the first time, I deter-
mined to observe the motion of Jupiter and the
adjacent planets (his satellites) along the zodiac, by
reference to some fixed star; for there was a fixed
star in view, eastwards of Jupiter, at a distance of
11' from the satellite on the east, and a little to the
south, in the following manner (Fig. 59).

Feb. 27: 1 h. 4 m. — The satellites appeared in the
following configuration. The satellite furthest to the
east was at a distance of 1 0' from Jupiter ; the next
in order was near Jupiter, being at a distance of
0' 30" from the planet. The next satellite was on the
western side, at a distance of 1' 30" from Jupiter;
and the satellite further to the west was at a distance
of \' from this. The two satellites near to Jupiter

66 THE SIDEREAL MESSENGER.

appeared small, especially the satellite on the east ;
but the satellites furthest oflf were very bright, par-
ticularly that on the west, and they made a straight
line in the direction of the ecli23tic exactly. The
motion of the planets towards the east was plainly
seen by reference to the aforesaid j&xed star, for Jupiter
and his attendant satellites were nearer to it, as may
be seen in the accompanying figure (Fig. 60). At the
fifth hour the satellite on the east, near to Jupiter,
was 1' from the planet.

Feb. 28:1 h. — Only two satellites were visible, one
on the east, at a distance of 9' from Jupiter, and
another on the west, at a distance of 2' ; they were
both rather bright, and in the same straight line with
Jupiter, and a straight line drawn from the fixed
star perpendicular to this straight line fell upon the
satellite on the east, as in the figure (Fig. 61). At
the fifth hour a third satellite was seen at a distance
of 2' from Jupiter, on the east, in the position shown
in the figure (Fig. 62).

March 1 : 0 h. 40 m. — Four satellites, all on the
east of the planet, were seen ; the satellite nearest to
Jupiter was 2' from the planet ; the next 1' from
this; the third was 0' 20" from the second, and was
brighter than the others ; and the satellite still further
to the east was at a distance of 4' from it, and was

THE SIDEREAL MESSENGER. 67

smaller than the others (Fig. 63). They made a
straight line very nearly, only the satellite third from
Jupiter was slightly above the line. The fixed star
formed with Jupiter and the most easterly satellite
an equilateral triangle, as in the figure.

March 2 : Oh. 40 m. — Three satellites were in
attendance, two on the east and one on the west, in
the configuration shown in the diagram (Fig. 64).
The satellite furthest to the east was 7' from
Jupiter ; from this satellite the next was distant 0' 30",
and the satellite on the west was separated from
Jupiter by an interval of 2'. The satellites furthest
ofi* were brighter and larger than the remaining
satellite, which appeared very small. The satellite
furthest to the east seemed to be raised a little
towards the north, out of the straight line drawn
through the other satellites and Jupiter.

The fixed star already noticed was at a distance of
8' from the western satellite, that is, from the per-
pendicular drawn from that satellite to the straight
line drawn through all the system, as shown in the
figure given.

These determinations of the motion of Jupiter and
the adjacent planets (his satellites) by reference to a
fixed star, I have thought well to present to the
notice of astronomers, in order that any one may be

68 THE SIDEREAL MESSENGER.

able to understand from them that the movements
of these planets (Jupiter s satellites) both in longitude
and in latitude agree exactly with the motions [of
Jupiter] which are extracted from tables.

These are my observations upon the four Medicean
planets, recently discovered for the first time by me ;
and although it is not yet permitted me to deduce by
calculation from these observations the orbits of these
bodies, yet I may be allowed to make some state-
ments, based upon them, well worthy of attention.

And, in the first place, since they are sometimes
behind, sometimes before Jupiter, at like distances,
and withdraw from this planet towards the east and
lnd°periods towards the west only within very narrow limits
of divergence, and since they accompany this planet
alike when its motion is retrograde and direct, it can
be a matter of doubt to no one that they perform
their revolutions about this planet, while at the same
time they all accomplish together orbits of twelve
years' length about the centre of the world. More-
over, they revolve in unequal circles, which is evi-
dently the conclusion to be drawn from the fact that
I have never been permitted to see two satellites in
conjunction when their distance from Jupiter was
great, whereas near Jupiter two, three, and sometimes
all (four), have been found closely packed together.
Moreover, it may be detected that the revolutions of

Deductions
from the
previous
observations
concerning

of Jupiter
satellites.

THE SIDEREAL MESSENGER. 69

the satellites which describe the smallest circles round
Jupiter are the most rapid, for the satellites nearest
to Jupiter are often to be seen in the east, when the
day before they have appeared in the west, and con-
trariwise. Also the satellite moving in the greatest
orbit seems to me, after carefully weighing the
occasions of its returning to positions previously
noticed, to have a periodic time of half a month. -^
Besides,'' we have a notable and splendid argument to
remove the scruples of those who can tolerate the
revolution of the planets round the Sun in the Coper-
nican system, yet are so disturbed by the motion of
one Moon about the Earth, while both accomplish an
orbit of a year's length about the Sun, that they
consider that this theory of the constitution of the
universe must be upset as impossible ; for now we
have not one planet only revolving about another,
while both traverse a vast orbit about the Sun, but
our sense of sight presents to us four satellites circling

^ In the edition of Galileo's works published at Florence, 1854, there are
given the tables of the hourly movements of the satellites of Jupiter, from
which Galileo determined their periods of revolution. In the beginning
of his treatise on floating bodies, Discorso intorno i Galleggianti, 1611-12,
Galileo gives the times of rotation as approximately, (i.) Id. 18^ h. ;
(ii.) 3 d. l.Si h. ; (iii.) 7 d. 4 h. ; (iv.) 16 d. 18 h. ; he also published
configurations of the satellites calculated for March, April, and a part of
May 1613. The periodic times of the satellites, as corrected by later
observers, are, (i.) 1 d. 18 h. 28 m. ; (ii.) 3 d. 13 h. 15 m. ; (iii.) 7 d.
3 h. 43 m. ; (iv.) 16 d. 16 h. 32 m.

Explanation
of the varia-
tions in
brightness
jf Jupite
satellites

70 THE SIDEREAL MESSENGER.

about Jupiter, like the Moon about the Earth, while
the whole system travels over a mighty orbit about
the Sun in the space of twelve years. ^

Lastly, I must not pass over the consideration of
the reason why it happens that the Medicean stars, in
ufTupiter^ performing very small revolutions about Jupiter,
seem sometimes more than twice as large as at other
times. We can by no means look for the explanation
in the mists of the Earth's atmosphere, for they appear
increased or diminished, while the discs of Jupiter
and neighbouring fixed stars are seen quite unaltered.
That they approach and recede from the Earth at the
points of their revolutions nearest to and furthest
from the Earth to such an extent as to account for so
great changes seems altogether untenable, for a strict
circular motion can by no means show those pheno-
mena ; and an elliptical motion (which in this case
would be nearly rectilinear) seems to be both unten-
able and by no means in harmony with the pheno-
mena observed. But I gladly publish the explanation
which has occurred to me upon this subject, and
submit it to the judgment and criticism of all true
philosophers. It is certain that when atmospheric
mists intervene the Sun and Moon appear larger, but
the fixed stars and planets less than they really are ;
hence the former luminaries, when near the horizon,
are larger than at other times, but stars appear

THE SIDEREAL MESSENGER. *j\

smaller, and are frequently scarcely visible ; also they
are still more diministied if those mists are bathed in
light ; so stars appear very small by day and in the
twilight, but the Moon does not appear so, as I have
previously remarked. Moreover, it is certain that
not only the Earth, but also the Moon, has its own
vaporous sphere enveloping it, for the reasons which I
have previously mentioned, and especially for those
which shall be stated more fully in my System ; and
we may consistently decide that the same is true with
regard to the rest of the planets ; so that it seems to
be by no means an untenable opinion to place round
Jupiter also an atmosphere denser than the rest of
the ether,^ about which, like the Moon about the
sphere of the elements, the Medicean planets (Jupiter's
satellites) revolve; and that by the intervention of
this atmosphere they appear smaller when they are in
apogee ; but when in perigee, through the absence or

1 Modern astronomers agree in assigning an atmosphere to Jupiter, but
consider it not extensive enough to afifect the brightness of the satellites.
— (Webb, Celestial Objects/or Common Telescopes.) Their absolute magni-
tudes are different, and their surfaces have been observed to be obscured
by spots, which may account for the variations of their brightness. These
spots, like the lunar spots, are probably due to variations of reflective power
at different parts of their surfaces, for as they always turn the same face to
Jupiter, they present diflferent portions of their surfaces to us periodically,
and it has been ascertained by observation that "these fluctuations in
their brightness are periodical, depending on their position with respect to
the Sun." — (Herschel, Outlines of Astronomy ; Arago, Astronomie Popu-
laire, 1854.)

THE SIDEREAL MESSENGER.

attenuation of that atmosphere, they appear larger.
Want of time prevents my going further into these
matters ; my readers may expect further remarks
upon these subjects in a short time.

Original ConfigMrations of Jupiter s Satellites ob-
served by Galileo in the months of January^
February, and March 1610, and published with
the 1st edition of his book Sidereus Nuncius,
Venice, 16 10.

Fig.

Date.

East.

West.

Jan. 7

• 0

0 • • •

• • 0

• -o

0 • • •

0 • • • •

0 . . .

• o*

• 0

THE SIDEREAL MESSENGER.

Fig.

Date.

East.

West.

Jan. 17

•.O

. . o • •

. o • •

• o ••

• o • .•

• ••o •

o*.

• o • • •

. . o •

.•o

• o

. 'o

• o • •

• • o

THE SIDEREAL MESSENGER.

Fig.

Date.

East.

(

West.

Jan. 31

• • O

Feb. 1

• O

O •

. • o •

• o •

. .Q •

. . o • •

• o

• • o

. o

. . o •

. .o

. o

. 'o

. . . o •

• o*

• o

• • 'O

• o

THE SIDEREAL MESSENGER.

Fig. Date.

East.

West.

Feb. 15

• o •

o • •

• o

• o •

• • o •

• o •

o •

0 star.

• o • •

star ©

O •

star ©

• o •

Mar. 1

. • . . o

star ©

O •

Star®

A PART OF THE PREFACE TO
KEPLER'S DIOPTRICS

FORMING

A CONTINUATION OF GALILEO'S
SIDEREAL MESSENGER.

In the preface to Kepler's Dioptrics there are introduced
letters of Galileo about the new and astonishing discoveries
which he had made in the heavens by the aid of the telescope
since the publication of his work, The Sidereal Messenger. The
portion of the preface which refers to Galileo, containing these
letters and Kepler's remarks upon them, is added here, as
continuing the original account of Galileo's astronomical dis-
coveries.

Extract f 7^0111 the Preface of Kepler s Dioptrics.
Augsburg, 1611.

" The Sidereal Messenger^' of Galileo has been for a Kepi

er re-
marks on the

long time in everybody's hands, also my *' Discussion, importanc

, , . . . of the appli-

such as it is, ivith this Messenger'' and my ^We/* cation of the
Narrative in confirmation of Galileo's Sidereal Mes- astronomical

investiga-

senger, so any reader may briefly weigh the chief t^ons as in-

n T t r T 1 dicated by

points of that Messenger and see the nature and the oaiueo's

. . discoveries,

value of the discoveries made by the aid of the tele- published in

his Sidereal

scope, the theory of which I am intending to demon- Messenger.
strate in this treatise. Actual sight testified that
there is a certain bright heavenly body which we call
the Moon. It was demonstrated from the laws of
optics that this body is round ; also Astronomy, by

8o THE SIDEREAL MESSENGER.

some arguments founded upon optics, had built up
the conclusion that its distance from the earth is
about sixty semi-diameters of the earth. Various spots
showed themselves in that body ; and the result was a
dubious opinion among a few philosophers, derived
from Hecataeus' account of the stories about the island
of the Hyperboreans,^ that the reflected images of
mountains and valleys, sea and land, were seen there ;
but now the telescope places all these matters before
our eyes in such a way that he must be an intellectual
coward who, while enjoying such a view, still thinks
that the matter is open to doubt. Nothing is more
certain than that the southern parts of the moon teem
with mountains, very many in number, and vast in
size ; and that the northern parts, inasmuch as they
are lower, receive in most extensive lakes the water
flowing down from the south. The conclusions which
previously Pena published as disclosed by the aid of
optics, started indeed from certain slight supports,
rather than foundations, afibrded by actual sight, but
were proved by long arguments depending one upon
another, so that they might be assigned to human
reason rather than to sight ; but now our very eyes, as
if a new door of heaven had been opened, are led to the
view of matters once hidden from them. But if it
should please any one to exhaust the force of reason-

^ Diodorus Siculus, ii. 47.

KEPLER'S CONTINUATION. 8i

ing upon these new observations, who does not see
how far the contemplation of nature will extend her
boundaries, when we ask, '' What is the use of the
tracts of mountains and valleys, and the very wide
expanse of seas in the moon V and ''May not some
creature less noble than man be imagined such as
might inhabit those tracts V

With no less certainty also do we decide by the
use of this instrument even that famous question,
which, coeval with philosophy itself, is disputed to
this day by the noblest intellects — I mean, " Whether
the earth can move (as the theory of the Planets
greatly requires) without the overthrow of all bodies
that have weight, or the confusion of the motion of the
elements ? For if the earth were banished from the
centre of the universe, some fear lest the water should
leave the orb of the earth and flow to the centre of the
universe ; and yet we see that in the moon, as well as
in the earth, there is a quantity of moisture occupying
the sunken hollows of that globe ; and although this
orb revolves actually in the ether, and outside the
centres not merely of the universe, but even of our
earth, yet the mass of water in the moon is not at all
hindered from cleaving invariably to the orb of the
moon, and tending to the centre of the body to
which it belongs. So, by this instance of the pheno-

82 THE SIDEREAL MESSENGER.

mena of the moon, the science of optics amends the
received theory of mechanics, and confirms on this
point my introduction to my Commentaries upon the
Motions of the Planet Mars}

The followers of the Samian philosophy (for I may
use this epithet to designate the philosophy originated
by the Samians, Pythagoras and Aristarchus) have a
strong argument against the apparent immobility of
the earth provided in the phenomena of the moon.
For we are taught by optics that if any one of us was
in the moon, to him the moon, his abode, would seem

^ Kepler says in his introduction to his Commentaries upon the Motions
of the Planet Mars, that the theory of gravitation depends on certain
axioms, one of which is that " heavy bodies do not tend to the centre of
the universe, supposing the earth to be placed there, because that point is
the centre of the universe, but because it is the centre of the earth. So,
wherever the earth be set, or whithersoever it be transported, heavy bodies
have a continual tendency to it." Kepler's object in this work was to
correct the methods for determining the apparent places of the planets
according to the three theories then current — the Ptolemaic, the Coperni-
can, and that of Tycho Brahe.

In 1593 the observed place of the planet Mars differed by nearly 5°
from the place calculated for it. Kepler accordingly studied the motions
of this planet, and " by most laborious demonstrations and discussions of
many observations," arrived at the conclusions known as Kepler's first and
second laws ; according to which the Copernican system of eccentric and
epicycles was replaced by an ellipse whose centre and eccentricity were
the same as the centre and eccentricity of the eccentric in the older
method, and the Sun therefore was in one of the foci. Also the motion
of the planet in its orbit was such that equal areas were described about
the Sun by the radius vector of the planet in equal times. — Kepler,
Astronomia Nova alToXoyrjros (Prague), 1609.

KEPLER'S CONTINUATION. 83

quite immovable, but our earth and sun ard all the
rest of the heavenly bodies movable ; for the conclu-
sions of sight are thus related.

Pena has noticed how astronomers, using the prin-
ciples of optics, have by most laborious reasoning
removed the Milky Way from the elementary uni-
verse, where Aristotle had placed it, into the highest
region of the ether ; but now, by the aid of the tele-
scope lately invented, the very eyes of astronomers
are conducted straight to a thorough survey of the
substance of the Milky Way ; and whoever enjoys
this sight is compelled to confess that the Milky
Way is nothing else but a mass of extremely small
stars.

Again, up to this time the nature of nebulous stars
had been entirely unknown ; but if the telescope be
directed to one of such nebulous balls, as Ptolemy calls
them, it again shows, as in the case of the Milky Way,
three or four very bright stars clustered very close
together.

Again, who without this instrument would have
believed that the number of the fixed stars was ten
times, or perhaps twenty times, more than that which
is given in Ptolemy's description of the fixed stars ?
And whence, pray, should we seek for conclusive
evidence about the end or boundary of this visible

84 THE SIDEREAL MESSENGER.

universe, proving that it is actually the sphere of the
fixed stars, and that there is nothing beyond, except
from this very discovery by the telescope of this
multitude of fixed stars, which is, as it were, the
vaulting of the mobile universe % Again, how greatly
an astronomer would go wrong in determining the
magnitude of the fixed stars, except he should survey
the stars all over again with a telescope, also may be
seen in Galileo's treatise, and we will also hereafter
produce in proof a letter from a German astronomer.

But no words can express my admiration of that
chapter of the Sidereal Messenger where the story is
told of the discovery, by the aid of a very highly
finished telescope, of another world, as it were, in the
planet Jupiter. The mind of the philosopher almost
reels as he considers that there is a vast orb, which is
equal in mass to fourteen orbs like the earth (unless
on this point the telescope of Galileo shall shortly
reveal something more exact than the measurements
of Tycho Brahe) round which circle four moons, not
unlike this moon of ours; the slowest revolving in
the space of fourteen of our days, as Galileo has
published ; the next to this, by far the brightest of
the four, in the space of eight days, as I detected in
last April and May ; the other two in still shorter
periods. And here the reasoning of my Commen-

KEPLER'S CONTINUATION. 85

taries about the Planet Mars, applied to a similar
case, induces me to conclude also that the actual
orb of Jupiter rotates with very great rapidity, most
certainly faster than once in the space of one of
our days ; so that this rotation of the mighty orb
upon its own axis is accompanied wherever it goes
by the perpetual circuits of those four moons. More-
over, this sun of ours, the common source of heat
and light for this terrestrial world as well as for that
world of Jupiter, which we consider to be of the
augular magnitude of 30' at most, there scarcely sub-
tends more than 6' or 7^ and is found again in the
same position among the fixed stars, having com-
pleted the zodiac in the interval, after a period of
twelve of our years/ Accordingly, the creatures
which live on that orb of Jupiter, while they con-
template the very swift courses of those four moons
among the fixed stars, while they behold them and
the sun rising and setting day by day, would swear

^ The degree of accuracy attained by Kepler and Galileo witli their
imperfect instruments will be appreciated by comparing these statements
with the determinations of later astronomers. Jupiter is about 1300
times the size of the Earth. Its diameter is about 87,000 miles ; time of
rotation, 9 h. 55 m. 21 sec. ; time of revolution, 4333 days nearly. The
angular diameter of the sun, seen from Jupiter, is between 6' and 7'.
The times of revolution of the four satellites are, as already given : (i.)
1 d. 18 h. 28 m., (ii.) 3 d. 13 h. 15 m., (iii.) 7 d. 3 h. 43 m., (iv.) 16 d.
6 h. 32 m.

86 THE SIDEREAL MESSENGER.

by Jupiter-in- stone, like the Eomans (for I have lately
returned from those parts), that their orb of Jupiter
remains immovable in one spot, and that the fixed
stars and the sun, which are the bodies really at rest,
no less than those four moons of theirs, revolve round
that abode of theirs with manifold variety of motions.
And from this instance now, much more than before
from the instance of the moon, any follower of the
Samian philosophy will learn what reply may be made
to any one objecting to the theory of the motion of
the earth as absurd, and alleging the evidence of our
sight. 0 telescope, instrument of much knowledge,
more precious • than any sceptre ! Is not he who
holds thee in his hand made king and lord of the
works of God % Truly

" All that is overhead, the miojhty orbs
With all their motions, thou dost subjugate
To man's intelligence."

If there is any one in some degree friendly to Coper-
nicus and the lights of the Samian philosophy, who
finds this difficulty only, that he doubts how it can
happen, supposing the earth to perform again and
again her course among the planets through the
ethereal plains, that the moon should keep so con-
stantly by her side, like an inseparable companion,
and at the same time fly round and round the actual

KEPLER S CONTINUA TION. 87

orb of the earth, just like a faithful dog which goes
round and round his master on some journey, now
running in front, now deviating to this side or that,
in ever- varying mazes, let him look at the planet
Jupiter, which, as this telescope shows, certainly
carries in its train not one such companion only, like
the earth, as Copernicus showed, but actually four,
that never leave it, though all the time hastening
each in its own orbit.

But enough has been said about these matters in
my Discussion with the Sidereal Messenger, It is
time that I should turn to those discoveries which
have been made since the publication of Galileo's
Sidereal Messenger, and since my Discussion wHth it,
by means of this telescope.

It is now just a year since Galileo wrote to Prague, Gaineos dis-
and gave full notice that he had detected something new satums ring
in the heavens beyond his former discoveries ; and that ""'''^
there might not be any one who, with the intention of
detracting from his credit, should try to pass himself
off as an earlier observer of the phenomenon, Galileo
gave a certain space of time for the publication of the
new phenomena which any one had seen ; he himself
meanwhile described his discovery in letters transposed
in this manner : smaismrmilmepoetale
u m i b u n e n u g t t a u i r a s. Out of these

88 THE SIDEREAL MESSENGER.

letters I made an uncouth verse which I inserted in
my Short Account in the month of September of last
year : —

Salve umbistineum^ geminatum Martia proles.
Hail, twin companionship, cMdren of Mars.

But I was a very long way from the meaning of
the letters ; it contained nothing to do with Mars ;
and, not to detain you, reader, here is the solution of
the riddle in the words of Galileo himself, the author
ofit:^—

'' Di Firenze li 13 di Novembre 1610. — Ma pas-
sando ad altro, giacche il Sig. Keplero ha in questa sua
ultima narrazione stampate le lettere che io mandai
trasposte a Yostra Signoria Illustrissima e Reveren-
dissima venendomi anco significato come Sua Maesta
ne desidera il senso, ecco che io lo mando a Yostra
Signoria Illustrissima per participarlo con Sua Maesta
col Sig. Keplero e con chi piacerk a Yostra Signoria
Illustrissima bramando io che lo sappia ognuno. Le

^ Unibistlneum. Apparently this is some German word with a Latin
ending, such as um-hei-stehn ; Kepler fancied that Galileo had discovered
two satellites of Mars.

■^ The text of the four letters of Galileo followed here is that given in
the edition of Galileo's works published at Florence, 1842-56 ; that in
the edition of Kepler's Dioptrics, published at Augsburg, 1611, is very
inaccurate. These letters were written to Giuliano de' Medici, ambas-
sador of the Grand-Duke of Tuscany to the Emperor Eudolf ii. at Prague.

KEPLER S CONTINUATION. 89

lettere dunque combinate nel lor vero senso dicono
cos\,

Altissimum planetam tergeminum observavi.

Questo e, clie Saturno con mia grandissima ammira-
zione ho osservato essere non una stella sola, ma tre
insieme, le quali quasi si toccano ; e sono tra di loro
totalmente immobili, e constituite in questa guisa
oQo- Quella di mezzo e assai piii grande delle
laterali ; sono situate una da oriente, I'altra da occi-
dente, nella medesima linea retta a capello ; non sono
giustamente secondo la dirittura del Zodiaco, ma
Toccidentale si eleva alquanto verso Borea ; forse sono
parallele all' Equinoziale. Se si guarderanno con un
occhiale che non sia di grandissima moltiplicazione,
non appariranno tre stelle ben distinte, ma parrk, che
Saturno sia una stella lunghetta in forma di un' oliva,
cosi, O^. Ma servendosi di un occhiale che moltiplichi
pill di mille volte in superficie, si vedranno tre globi
distintissimi, che quasi si toccano, non apparendo tra
essi maggior divisione di un sottil filo oscuro. Or
ecco trovata la corte a Giove, e due Servi a questo
Vecchio che I'aiutano a camminare ne mai se gli
staccano dal fianco. Intorno agli altri Pianeti non ci
e novita alcuna, ec."

Although these words differ little from Latin, yet I
will translate them that no difficulty may hinder my

90 THE SIDEREAL MESSENGER.

reader from following me. Thus then Galileo writes :
— " But to come now to my second topic. Since
Kepler has published in that recent 'Narrative' of
his the letters which I sent as an anagram to your
illustrious Lordship, and since an intimation has been
given me that his Majesty desires to be taught the
meaning of those letters, I send it to your illustrious
Lordship, that your Lordship may communicate it to
his Majesty, to Kepler, and to any one your Lordship
may wish.

''The letters when joined together as they ought to
be, say this,

* Altissimum planetam tergeminum observavi,'
' I have observed the most distant of the planets to have a triple
form.'

" For in truth I have found out with the most in-
tense surprise that the planet Saturn is not merely
one single star, but three stars very close together,
so much so that they are all but in contact one
mth another. They are quite immovable with
regard to each other, and are arranged in this man-
ner, oOo. The middle star of the three is by far
greater than the two on either side. They are situ-
ated one towards the east, the other towards the
west, in one straight line to a hairs-breadth ; not, how-
ever, exactly in the direction of the Zodiac, for the

KEPLER'S CONTINUATION. 91

star furthest to the west rises somewhat towards the
north ; perhaps they are parallel to the equator. If
you look at them through a glass that does not
multiply much, the stars will not appear clearly
separate from one another, but Saturn's orb will
appear somewhat elongated, of the shape of an olive,
thus, (^. But if you should use a glass which multi-
plies a surface more than a thousand times, there will
appear very distinctly three orbs, almost touching
one * another ; and they will be thought to be not
further apart than the breadth of a very fine and
scarcely visible thread. So you see a guard of satel-
lites has been found for Jupiter, and for the decrepit
little old man two servants to help his steps and never
leave his side. Concerning the rest of the planets I
have found nothing new."

So says Galileo ; but if I may do so, I will not
make an old dotard out of Saturn, and two ser-
vants for him out of his companion orbs, but rather out
of those three united bodies I will make a triple Geryon,
out of Galileo Hercules, and out of the telescope
his club, armed with which, Galileo has conquered
that most distant of the planets, drawn it out of
the furthest recesses of nature, dragged it down 'to
earth, and exposed it to the gaze of us all. It pleases
me too, now that the nest has been found, to consider

92 THE SIDEREAL MESSENGER.

with curiosity what kind of brood must be in it, what
kind of life, if there be any life there, between orbs
which all but touch each other two and two, where
not even

" a space
Of sky extends not more than three elLs wide,"^

but where there is scarcely a chink of a nail's-breadth
all round.

Do indeed the astrologers rightly ascribe to Saturn
the guardianship of miners, who, accustomed to spend
their lives, like moles, underground, seldom breathe
the free air under the open sky 1 Although the
darkness here is rather more supportable than in
Saturn, because the sun, which appears there only as
large as Venus appears to us on the earth, continually
casts its rays through the spaces between the different
orbs in such a way that those inhabitants who are
situated on one orb are covered by the other as by a
ceiling ; while those on the latter orb, on the top of
this roof of theirs, exposed as it is to the full light of
the sun, receive a glare as if from very firebrands.
But I must draw in the reins and check my mind in
its enjoyment of the free fields of ether ; for fear,
perchance, later observations should report something

1 Virgil, Edog. iii. 105.

KEPLER'S CONTINUATION. 93

different from the first account, something changed in *
course of time.^

At the end of his letter Galileo seemed to think
that he had come to the end of his reports about the
planets, and observations of new phenomena respect-
ing them, but ever on the watch, that eye of his, that
one not of Nature's making — I mean his telescope — in
a short time made more discoveries, concerning which
read the following letter of Galileo : —

" Di Firenze li 11 di Decemhre 1610. — Sto con desi- ^^^^^""^ of

Galileo's dis-

derio, attendendo la risposta a due mie scritte ultima- foveryofthe

phases of

mente per sentire quello, che avera detto il Sig. venus.

1 The completion of Galileo's observations on Saturn depended on the
improvement of astronomical instruments, as will be evident from the
following sketch. Galileo made out the first indications of Saturn's ring
in 1610, as narrated in his letter, with a power of thirty ; but in December
1612 he wrote to one of his friends, Marco Velseri, that he could no
longer see these indications, and began to imagine that his telescope had
deceived him, and apparently abandoned further researches. Hevelius in
1642 saw the ring more clearly, but figured it as two crescents attached
to Saturn by their cusps. At length, in 1653, Huyghens provided him-
self with a power of one hundred, having made the lenses with his own
hands, and immediately discovered the explanation of the phenomena
which had baffled jJrevious observers. He published his explanation of
Saturn's ring, and his discovery of the first satellite, in his Systema Satur-
nium, 1659. Cassini, with still more powerful instruments, discovered
four more satellites in 1671, 1672, 1684. Sir William Herschel in 1789
detected two more, " which can only be seen with telescopes of extraordi-
nary power and perfection, and under the most favourable atmospheric
circumstances." — (Herschel, Outlines of Astronomy, § 548.) And the last
of the eight satellites was discovered in 1848 by Lassell of Liverpool, and
Bond of Cambridge, U.S., simultaneously.

94 THE SIDEREAL MESSENGER.

Keplero della stravaganza di Saturno. Intanto mando
[a Vostra Signoria Illustrissima e Eeverendissima] la
cifra di un altro particolare osservato da me nuova-
mente, il quale si tira dietro la decisione di grandis-
sime controversie in Astronomia, ed in particolare
contiene in se un gagliardo argomento per la con-
stitutione [Pitagorica e Copernicana] dell' Universe ;
e a suo tempo pubblicherb la deciferazione ed altri
particolari. Spero che avero trovato il metodo per
definire i periodi dei quattro Pianeti Medicei, stimati
con gran ragione quasi inesplicabili dal Sig. Keplero,
al quale piacera, ec.

" Le lettere trasposte sono queste :

" Haec immatura a me jam frustra leguntur, o.y."

Which, may be translated thus : —

'' I am anxiously looking for an answer to my last
two letters, that I may learn what Kepler says about
the marvels of Saturn's orb. In the meantime I send
him a riddle concerning a certain new and splendid
observation which tends to decide great controversies
in astronomy ; and especially contains a fine argument
in favour of the Pythagorean and Copernican system
of the universe. At the proper time I will publish the
solution of the riddle, and some other particulars. I
hope that I have found a method of determining the
periodic times of the four Medicean planets, which

KEPLER'S CONTINUATION. 95

Kepler, not without very good reason, thought inex-
plicable, etc.

" The letters turned into an anagram, are these :

" Haec immatura a me jam frustra leguntur, o.y."

So far Galileo. But if, reader, this letter has filled
YOU with a desire to know the meaning contained in
those letters, then you must read another letter of
Galileo which follows.

But before you do so, I should like you to notice, by
the way, what Galileo says about the Pythagorean and
Copernican system of the universe. For he points to
my Mystery of the Universe^ published fourteen years
ago, in which I took the dimensions of the Planetary
orbits according to the astronomy of Copernicus, who
makes the sun immovable in the centre, and the earth

1 Kepler, in his Mystery of the Universe, endeavoured to connect the
orbits of the planets with the five regular solids, thus : If in a sphere (i.)
a cube be inscribed, and in the cube a sphere (ii.); and in that sphere a
tetrahedron, and in the tetrahedron a sphere (iii.); and in that sphere a
dodecahedron, and in the dodecahedron a sphere (iv.); and in that sphere
an icosahedron, and in the icosahedron a sphere (v.); and in that sphere
an octahedron, and in the octahedron a sphere (vi.), the diameters of these
six spheres will be proportional to the diameters of the orbits of Saturn,
Jupiter, Mars, the Earth, Venus, and Mercury respectively ; or, as Kepler
expresses it, the common centre of these spheres represents the position
of the Sun, and the six spheres represent the spheres of the planets.

By these considerations, however, Kepler was led to enunciate his third
law, that the squares of the periodic times of planets are proportional to
the cubes of their mean distances from the sun. — Kepler, Prodromti^
D'lssertationum Mathematicarum continens Mysterium Cosmographicum, etc.
(Tubingen, 1596.)

96 THE SIDEREAL MESSENGER.

movable both round the sun and upon its own axis ;
and I showed that the differences of their orbits cor-
responded to the five regular Pythagorean figures, which
had been already distributed by their author among
the elements of the world, though the attempt was
admirable rather than happy or legitimate, and for
which fio;ures' sake Euclid wrote the whole of his
Geometry. Now, in that Mystery you may find a sort
of combination of Astronomy and Euclid's Geometry,
and through this combination a most thorough com-
pletion and finishing of them both ; and this was the
reason why I waited with intense longing to see what
sort of an argument Galileo would produce in favour
of the Pythagorean system of the universe. After
this explanation, Galileo's letter about this argument
was as follows : —

" Illustrissimo e Keverendissimo Signore mio colen-
dissimo.

"E tempo che io deciferi a Vostra Signoria lUustris-
sima e Eeverendissima e per lei al Sig. Keplero le
lettere trasposte le quali alcune settimane sono le
inviai ; e tempo dico, giacche sono interamente chiaro
della verity del fatto, sicche non ci resta un minimo
scrupolo, o dubbio. Sapranno dunque come circa a
tre mesi fa vedendosi Venere vespertina la comin-
ciai ad osservar diligentemente coll' occhiale, per

JS the Earth (centre of universe). S the Sun. 0 centre of eccentric. D centre erf
Planet's epicycle. YY' stationary points. S v v' projections of SYY'on the ecliptic of which
^ IS the centre.

S the Sun, centre of solar system, v e positions of planet and Earth at conjunction.
VV^stationary points of planet, ^^'corresponding positions of the Earth.

KEPLER'S CONTINUATION. 97

veder col sen so stesso quello di die non dubitava
punto rintelletto. La vidi dunque sul principio di
figura rotonda, pulita e terminata, ma molto pic-
ciola; di tal figura si mantenne sino che comincib
ad avvicinarsi alia sua massima digressione, ma tra
tanto and5 crescendo in mole. Comincio poi a man-
care dalla rotondita nella sua parte orientale ed
avversa al Sole, e in poclii giorni si ridusse ad esser un
mezzo cerchio perfettissimo, e tale si mantenne, senza
punto alterarsi, finche incomincib a ritirarsi verso il
Sole, allontanandosi dalla tangente. Ora va calando
dal mezzo cerchio, e si mostra cornicolata, e andera
assottigliandosi sino all' occultazione, riducendosi
allora con coma sottilissime. Quindi passando air
apparizione mattutina, la vedremo pur falcata, e sot-
tilissima e colle corna avverse al Sole ; andera poi
crescendo fino alia massima digressione, dove apparira
semicircolare, e tale senza alterarsi si manterra molti
giorni, e poi dal mezzo cercliio passer^ presto al tutto
tondo, e cosi rotonda si conservera poi per molti mesi.
II suo diametro adesso e circa cinque volte maggiore
di quello, clie si mostrava nella sua prima apparizione
vespertina ; dalla quale mirabile esperienza abbiamo
sensata, e certa dimostrazione di due gran questioni
state fin qui dubbie tra i maggiori ingegni del Mondo.
L'una e, die i Pianeti tutti son di lor natura tenebrosi

98 777^ SIDEREAL MESSENGER.

(accadendo anco a Mercurio Tistesso, che a Venere).
L'altra, che Venere necessarissimamente si volge
intorDO al Sole, come anco Mercurio, cosa, che degli
altri Pianeti, fu creduta da' Pitagorici, dal Coper-
nico, dal Keplero e da' loro seguaci, ma non sensata-
mente provata, come ora in Venere, ed in Mercurio.

" Averanno dunqne il Sig. Keplero, e gli altri Coper-
nicani da gloriarsi di aver creduto e filosofato bene,
sebbene ci e toccato, e ci e per toccare ancora ad esser
reputati dall' nniversita dei Filosofi in libris, per poco
intendenti, e poco meno che stolti.

'*Le parole dunque, che mandai trasposte, e che
dicevano,

Haec immatura a me jam frustra leguntur, o.y.

dicono ordinate

Cyntliiae figuras aemulatur mater amorum.

Cioe, che Venere imita le figure della Luna. Osser-
vai tre notti sono I'ecclisse, nella quale non vi e
cosa notabile, solo si vede il taglio dell' ombra indis-
tinto, confuso e come annebbiato, e questo per derivare
essa ombra dalla Terra lontanissima da essa Luna.
Voleva scrivere altri particolari, ma essendo stato trat-
tenuto molto da alcuni gentiluomini, ed essendo I'ora
tardissima, son forzato a finire. Favoriscami salutare in
mio nome i SS. Keplero, Asdale e Segheti, ed a Vostra
Signoria Illustrissima con ogni reverenza bacio le

KEPLER'S CONTINUATION. 99

mani, e dal Signore Dio gli prego felicita. Di Firenze
il primo di Gennaio 1610. Ab Incarnatione.

"Di Yostra Signoria Illustrissima e Eeverendissima
Servidore obbligatissimo. Galileo Galilei."

Such is Galileo's letter ; but let me give you the
substance of it : —

" It is time for me to disclose the method of reading
the letters which some weeks since I sent you as an
anagram. It is time now, I mean, after I have
become quite certain about the matter, so much so
that I have no lono;er even a shadow of doubt. You
must know then that about three months ago, when
the star of Venus could be seen, I began to look at it
through a telescope with great attention, so that I
might grasp with my physical senses an idea which I
was entertaining as certain. At first then you must
know the planet Venus appeared of a perfectly
circular form, accurately so, and bounded by a
distinct edge, but very small ; this figure Venus kept
until it began to approach its greatest distance from
the sun, and meanwhile the apparent size of its orb
kept on increasing. From that time it began to lose
its roundness on the eastern side, which was turned
away from the sun, and in a few days it contracted
its visible portion into an exact semicircle ; that

loo THE SIDEREAL MESSENGER.

figure lasted without the smallest alteration until it
began to return towards the sun where it leaves the
tangent drawn to its epicycle.-^ At this time it loses
the semicircular form more and more, and keeps on
diminishing that figure until its conjunction, when
it will wane to a very thin crescent. After completing
its passage past the sun, it will appear to us, at its

1 In the Ptolemaic system the earth's centre was regarded as the
centre of the universe, and the movements of the heavenly bodies were
explained by eccentrics and epicycles. The sun was conceived to describe
a circle about a point not exactly coinciding with the centre of the earth,
called the sun's eccentric. The planets described epicycles (circles) whose
centres described eccentrics (circles), and the centres of these eccentrics
coincided with the centre of the sun's eccentric. In the case of Mercury
and Venus the centre of the epicycle was always on the line drawn from
the centre of the eccentric to the sun's centre. In the case of the other
planets the construction was more complicated. The stationary points
were determined by drawing tangents from the earth's centre (or the
observer) to the epicycle, as in the figure (1). — (Gassendi, Instltutlo Astro-
nomica, 1647.) This will explain Kepler's description of the stationary
points as the points where the planet leaves the tangent to its epicycle,
supposing that he uses the terms of the current {i.e. Ptolemaic) astronomy.
Copernicus placed the sun instead of the earth at the centre of the
universe, but to determine the positions of the planets at any given time
with as much accuracy as was attainable with the Ptolemaic system, he
was obliged to use a similar method of eccentrics and ejncycles, so that
Kepler's expression may be understood to describe the stationary points
according to the Copernican theory, though it is still strange that he
should not recognise the elliptical form of the planetary orbits, which he
had lately demonstrated after most laborious reasoning in his Commen-
taries on the Motion of the Planet Mars, 1 609, Galileo's own expression
seems to describe the stationary points according to the Copernican
system, as would be expected, as the points where the planet leaves the
tangent drawn to its orbit from the earth (Fig. 2).

KEPLER S CONTINUATION. loi

appearance as a morning star, as only sickle-shaped,
turning a very thin crescent away from the sun ;
afterwards the crescent will fill up more and more
until the planet reaches its greatest distance from the
sun, in which position it will appear semicircular, and
that figure will last for many days without appreci-
able variation. Then by degrees, from being semi-
circular it will change to a full orb, and will keep
that perfectly circular figure for several months ; but
at this instant the diameter of the orb of Venus is
about five times as laro;e as that which it showed at
its first appearance as an evening star.

"From the observation of these wonderful phenomena
we are supplied with a determination most conclusive,
and appealing to the evidence of our senses, of two
very important problems, which up to this day were
discussed by the greatest intellects with different con-
clusions. One is that the planets are bodies not self-
luminous (if we may entertain the same views about
Mercury as we do about Venus). The second is that
we are absolutely compelled to say that Venus (and
Mercury also) revolves round the sun, as do also all
the rest of the planets. A truth believed indeed by
the Pythagorean school, by Copernicus, and by
Kepler, but never proved by the evidence of our
senses, as it is now proved in the case of Venus and

I02 THE SIDEREAL MESSENGER.

Mercury. Kepler therefore and the rest of the school
of Copernicus have good reason for boasting that
they have shown themselves good philosophers, and
that their belief was not devoid of foundation ; how-
ever much it has been their lot, and may even here-
after be their lot, to be regarded by the philosophers
of our times, who philosophise on paper, with an
universal agreement, as men of no intellect, and little
better than absolute fools.

" The words which I sent with their letters trans-
posed, and which said,

Haec immatura a me jam frustra leguntur, o.y.

when reduced to their proper order, read thus,

Cynthiae figuras aemulatur mater amorum :

The mother of the Loves rivals the phases of Cynthia :

that is,

Venus imitates the phases of the Moon.

Three days ago I observed an eclipse of the moon,
but not anything worthy of special notice occurred in
it. Only the edge of the shadow appeared indistinct,
blurred, and hazy ; the cause of the phenomenon no
doubt is that the shadow has its origin at the earth,
at a great distance from the body of the moon.

" I have some other particulars, but I am prevented
by time from writing about them, etc."

So writes Galileo.

KEPLER'S CONTINUATION. 103

What now, dear reader, shall we make out of our
telescope \ Shall we make a Mercury's magic-wand
to cross the liquid ether with, and, like Lucian,^ lead
a colony to the uninhabited evening star, allured by
the sweetness of the place ? or shall we make it a
Cupid's arrow, which, entering by our eyes, has
pierced our inmost mind, and fired us with a love of
Venus \ For what lano;uao;e is too strong: for the
marvellous beauty of this orb, if, having no light of
its o^vTi, it can attain simply by the borrowed light of
the sun to such splendour, as Jupiter has not, nor the
moon, though enjoying a proximity to the sun as
close as Venus ; for if the moon's light be compared
with the light of Venus, it will be seen to be certainly
greater on account of the apparent magnitude of the
moon, but, in comparison with the light of Venus,
dull, dead, and leaden. 0 truly golden Venus !
Will any one doubt any more that the whole orb of
Venus is wrought most smoothly out of pure unal-
loyed gold, since its surface, when only placed in the
sunlight, reflects a splendour so intense ! And here
let me add my experiments about the alteration of
the light of Venus on blinking the eye, w^hich I
have examined in the part of my Astronomy which

1 LuciaD, Ver. Hist. i. 12.

I04 THE SIDEREAL MESSENGER.

treats of Optics. Eeasoning will be able to conclude
nothing else but this, that the orb of Venus turns on
its own axis with an exceedingly swift rotation, dis-
playing one after another different parts of its surface
which are more or less capable of retaining the sun's
light.'

But enough of my own conclusions. Let us now
hear as an epilogue Galileo's conclusions built up out
of all the observations which he has made with his
telescope, and announced from time to time. Thus
he writes once more : —
Galileo's con- '' Illustrlssimo e Keverendissimo Si^nore mio colen-

clusions with ^

regard to the dlssimO.

inherent

nature of the "JJo ricevuto ^usto, 6 coutcuto particolarissimo nella

brightness of o ' j.

the stars. Icttura dcll' ultima di Vostra Signoria Illustrissima
e Keverendissima delli 7 stante, ed in particolare in
quella parte dove ella m'accenna la favorevole in-
clinazione dell' lUustriss. Sig. Cons. "Wackher, verso

^ The first scientific determination of the period of the rotation of
Venus was made by Dominique Cassini in 1666, from observations of
spots on the planet, and concluded to be about 24 hours ; but in 1726
Bianchini deduced a period of 24 d. 8 h. from similar observations. The
true period is considered to be 23 h. 21 m., determined by Schroeter
by a series of observations lasting from 1788 to 1793 on the periodicity
of the deformation of the horns of Venus. — (Arago, Astronomie Populaire,
1854.)

Kepler's statements can only be regarded as anticipations of phenomena
not yet actually observed.

KEPLER'S CONTINUATION. 105

di me, la quale io infinitamente stimo, eel apprezzo ;
e poiche quella ha principalmente origine dall' aver
io incontrate osservazioni necessariamente dimostranti
conclusioni per avanti tenute vere da sua Signoria
Illustrissima per confermarmi maggiormente il pos-
sesso di grazia tanto pregiata da me, prego Vostra
Signoria Illustrissima e Eeverendissima a fargli in-
tendere per mia parte come conforme alia credenza di
Sua Signoria Illustrissima ho dimostrazione certa, che
siccome tutti i Pianeti ricevono il lume dal Sole,
essendo per se stessi tenebrosi, e opachi ; cosi le
Stelle fisse risplendono per lor natura, non bisognose
dell' illustrazione de' raggi solari, li quali, Dio sa, se
arrivino a tanta altezza, piu di quello, che arrivi a noi
il lume di una di esse fisse. II principal fondamento
del mio discorso e nelF osservare io molto evidente-
mente con gli occhiali che quel Pianeti di mano in
mano, che si trovano piu vicini a noi, o al Sole,
ricevono maggiore splendore, e piii illustremente ce
Io riverberano ; e percio Marte perigeo, e a noi vicin-
issimo si vede assai piii splendido, che Giove ; benche
a quello di mole assai inferiore ; e dijficilmente se gli
pub coir occhiale levare quella irradiazione, che im-
pedisce il vedere il suo disco terminato, e rotondo ; il
che in Giove non accade, vedendosi esquisitamente
circolare. Saturno poi per la sua gran lontananza si

io6 THE SIDEREAL MESSENGER.

vede esattamente terminato, si la Stella mao^more di
mezzo, come le due piccole lateral! ; ed appare il suo
lume languido, ed abbaeinato e senza niuna irradiazione,
die impedisca il distinguere i suoi tre piccoli globi
terminatissimi. Ora poiche apertameute veggiamo,
che il Sole molto splendidamente illustra Marte vicino,
e clie molto piii languido e il lume di Giove (sebbene
senza lo strumento appare assai cbiaro, il clie accade
per la grandezza, e candore della Stella) languidissimo,
e fosco quello di Saturno, come molto piii lontano,
quali doveriano apparirci le Stelle fisse lontane indici-
bilmente piii di Saturno, quando il lume loro derivasse
dal Sole % Certamente debolissime, torbide e smorte.
Ma tutto I'opposito si vede, peroccbe se rimireremo
per esempio il Cane, incontreremo un fulgore vivis-
simo, cbe quasi ci toglie la vista, con una vibrazione
di raggi tanto fiera, e possente, che in comparazione di
quello rimangono i Pianeti, e dico Giove e Yenere
stessa, come un impurissimo vetro appresso un lim-
pidissimo e finissimo diamante. E benche il disco di
esso Cane apparisca non maggiore della cinquantesima
parte di quello di Giove, tuttavia la sua irradiazione
e grande, e fiera in modo, che I'istesso globo tra i
proprii crini s'implica, e quasi si perde, e con qualche
difficolta si distingue ; dove che Giove (e molto piii
Saturno) si vedono e terminati, e di una luce languida.

KEPLER S CONTINUATION. 107

e per cosi dire quieta. E per tanto io stimo, clie bene
filosoferemo, referendo la causa della scintillazione
delle Stelle fisse, al vibrare, che elle fanno dello
splendore proprio e native dall' intima loro sustanza ;
dove che nella superficie de' Pianeti termina piii
presto, e si finisce Tilluminazione, che dal Sole deriva,
e si parte. Se io sentirb qualche particolare questione
ricevuta dal medesimo Sig. Wackher, non resterb d'
afFaticarmivi intorno, per dimostrarmi, quale io sono
desiderosissimo di servire un tanto Signore, e non gia
con isperanza di aggiungere al termine conseguito dal
suo discorso, perche benissimo comprendo, che a
quant 0 sia passato per Io finissimo cribro del giudizio
di esso, e del Sig. Keplero, non si puo aggiungere di
squisitezza ; ne io pretenderei altro, che col dubitare, e
mal filosofare, eccitar loro al ritrovamento di nuove
sottigliezze. Gl' ingegni singolari, che in gran numero
fioriscono nelF Alemagna, mi hanno lungo tempo
tenuto in desiderio di vederla, il qual desiderio ora si
raddoppia per la nuova grazia dell' lllustrissimo Sig.
AVackher, la quale mi farebbe divenir grande ogni
picciola occasione, che mi si presentasse. Ma ho di
soverchio occupata Vostra Signoria Illustrissima e
Keverendissima. Degnisi per fine di ofFerirmi e dedi-
carmi devotissimo servidore all' lllustrissimo Sig.
AVackher, salutando anco caramente il Sig. Keplero,

io8 THE SIDEREAL MESSENGER.

ed a lei con ogni riverenza bacio le mani, e dal Signore
Dio le prego somma felicita.

''[Di Firenze li 26] di Marzo 1611. Di Yostra
Signoria lUustrissima e Eeverendissima obbligatissimo
Servidore, Galileo Galilel"

Wben translated, the meaning is as follows : —
" Your last letter lias exceedingly pleased me,
especially tbat part which assures me of the friendly
feeling entertained towards me by the most illus-
trious Imperial Counsellor, AYagher, which I for my
part highly appreciate. And since the cause of this
friendliness is, that I have incontestably demon-
strated by some observations of mine certain con-
clusions which he had long held as true, I wish to
confirm my possession of favour, which I value so
much, and accordingly I ask you to give him this
piece of news from me ; that I have most conclusive
arguments ready, showing clearly that, just as he
holds, all the planets receive their light from the
sun, being by constitution bodies dark and devoid
of light ;^ but that the fixed stars shine by their own
proper light, not needing to be illuminated by the

1 Proctor [Other Worlds than Ours, 1875) has given some reasons for
believing that Jupiter and Saturn shine in part with their own light,
owing to their great internal heat.

KEPLER S CONTINUATION, 109

snn s rays, since God knows whether they reach the
very remote region of the fixed stars with intensity
even equal to the small intensity with which the
rays of the fixed stars come down to ns.

*'My demonstration depends chiefly on this fact, that
with the telescope I have distinctly observed that the
planets receive greater brightness, and reflect it more
intensely, in proportion as each one is nearer to us and
to the sun. So Mars in perigee, that is, when nearest
to the earth, greatly surpasses Jupiter in brightness,
although in actual size it is far inferior to Jupiter ;
and in consequence it is difficult to receive the efful-
gence of this planet in the telescope, for it is so great
as to prevent the eye from being able to distinguish
the circular termination of the planet's disc. This
does not happen in the case of Jupiter, for it appears
quite circular. The next planet, Saturn, on account
of its great distance likewise — for indeed it is the most
remote of the planets, — appears bounded by a well-
defined edge, both the greater orb in the middle and
the two small orbs at its sides. Indeed, it appears to
shine with a faint and delicate light, without any
efiulgence to prevent an observer recognising the
well-defined termination of its three orbs. Since,
then, we see that Mars, the nearest of the three, is
illumined by the sun with very great splendour, and

no THE SIDEREAL MESSENGER.

that the light of Jupiter, at a greater distance, is much
more faint (although without the use of an instrument
it appears tolerably bright, which is due to the size
and brilliancy of its body), and that the light of
Saturn, at the greatest distance, is most faint, and
almost watery, of what kind, do you think, would
appear the light of the fixed stars, which are at an
immeasurable distance further from the sun than
Saturn, if they only received light from the sun ?
Most certainly, extremely feeble, indistinct, and pallid.
And yet we find quite the contrary ; for, let us look
with our eyes at the Dog- Star, for example. We shall
encounter a most vivid brilliancy, which almost pricks
the eye with the rapid sparkling of its rays, of such
intensity that, in comparison with it, the planets, even
Jupiter, and Venus too, are as thoroughly outshone as
common and bad glass compared with a highly polished
and most sparkling diamond. And although the orb
of the Dog-Star appears no larger than the fiftieth
part of Jupiter's disc, nevertheless its brilliancy is
great and very strong ; so that the form of its disc,
which you expect to see, hides itself among the rays
of its own refulgence, envelops itself in them, and
almost disappears ; and it is not distinguished without
some difiiculty from the rays which surround it.
Whereas Jupiter, and still more Saturn, are seen well

KEPLER S CONTINUATION. in

defined ; and their light is without intensity, and, if
I may say so, quiescent. "Wherefore I think that we
shall rightly apply our philosophy if we refer the cause
of the twinkling of the fixed stars to vibrations of a
brilliancy, which is their own, belonging to their con-
stitution, and inherent in their substance, and say, on
the other hand, that the illumination of the planets,
which is derived from the sun, and distributed to the
world, is limited to their surface."

These are the scientific conclusions in Galileo's'
letter ; the rest I omit.

You see then, studious reader, how the subtle mind
of Galileo, in my opinion the first philosopher of the
day, uses this telescope of ours like a sort of ladder,
scales the furthest and loftiest walls of the visible
world, surveys all things with his own eyes, and, from
the position he has gained, darts the glances of his
most acute intellect upon these petty abodes of ours —
the planetary spheres I mean, — and compares with
keenest reasoning the distant with the near, the lofty ,
with the deep.

VALE ET DEUM IN OPERIBUS SUIS CELEBEAIIE NUNQUAM DESINE.

Kepler, Narratio.

THOMAS AND ARCHIBALD CONSTABLE, PRINTERS TO HER MAJESTY,

,G15 «

3 5002 00064 7516

Galilei, Galileo

The sidereal messenger of Galileo Galile

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Full text of "The sidereal messenger of Galileo Galilei : and a part of the preface to Kepler's Dioptrics containing the original account of Galileo's astronomical discoveries"

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