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TRANSIT OF VENUS, 1874.
PUBLISHED BY
JAMES MACLEHOSE, GLASGOW.
MACMILLAN AND CO., LONDON.
Bdifiburgk^ . • Edmonston &* Douglas.
Dublin^ . . ,1V.//. Smith &* Son.
bRCBMBER 9, 1874.
TRANSIT OF VENOS, 1874.
THE TRANSIT OF VENUS
In 1874.
BY
ROBERT GRANT, M.A. LL.D. F.R.S.,
PROFESSOR OF ASTRONOMY IN THE UNIVERSITY
OF GLASGOW.
/0>/
GLASGOW: JAMES MACLEHOSE,
PUBLISHER TO THE UNIVERSITY.
LONDON : MACMILLAN AND CO.
1874.
ISU. A. 60.
TRANSIT OF VENUS, 1874.
10 TRANSIT OF VENUS,
According to the science of modem
astronomy, the Sun occupies the centre
of the planetary system, and the Earth
is a planet, revolving round the Sun like
the other planets of the system; on the
other hand, the innumerable stars are
supposed to be situated in space at a
remote distance apart from the planetary
system. But the Earth is a round body
several thousand miles in diameter. We
may therefore reasonably infer that the
other planets are similarly bodies of vast
dimensions, and this conclusion appUes
with still greater force to the Sun, the
central body of the planetary system.
Furthermore, the theory of modem
TRANSIT OF VENUS,- 1 1
astronomy, which places the Sun in the
centre of the planetary system, assigns to
the stars of the celestial vault the rtle
of so many resplendent suns, each con-
stituting the centre of a retinue of re-
volving bodies. In like manner, then,
as we are led to suppose that the Sun
is a body of great magnitude, so we
infer, by a similar train of reasoning,
that the stars are also bodies of vast
dimensions.
But in order to ascertain the magni^
tudes of the celestial bodies, we must
know their distances from the Earth.
We are thus led to consider the supreme
importance of the astronomical problem
12 TRANSIT OF VENUS.
which is to form the groundwork of our
explanations. When we have once made
some progress in a knowledge of the
distances of the celestial bodies, we are
in a position to form a conception of
the amazing extent of the physical
universe. We thus come to learn that
the Sun is a stupendous globe more
than 800,000 miles in diameter, and
that its distance from the Earth is more
than ninety-one millions of miles. We
learn, furthermore, that the planets are
bodies of immense size revolving round
the Sun, that the extreme planet of the
system, the planet Neptune, revolves at a
distance of two thousand eight hundred
TRANSIT OF VENUS. ^l
millions of miles from the central body,
and that the orbits of many comets ex-
tend even much farther into space.
Finally, we arrive at the conclusion that
the stars are in reality suns, exceeding
in many instances the great central body
of the planetary system in magnitude,
and traversing space at an almost incon-
ceivable distance from the Earth.
Beaearches of the Ancient Astronomers on the
Distances of the Celestial Bodies,
The Greek astronomers made some in-
genious attempts to determine the distance
of the Sun from the Earth, but in no case
was either the method of solution or the
14 TRANSIT OF VENUS.
existing state of astronomy adequate to
meet the requirements of a problem of
such difficulty. In the case of the Moon
they were more successful. The method
employed by them was exactly the same
in principle as that used by modem astro-
nomers, and forms indeed the basis of all
researches having for their object the
determination of the distances of the
celestial bodies from the Earth. A brief
explanation of it will presently be
given.
i
TRANSIT OF VENUS, 15
Relation between the Mean Distances of the
Planets from the Sun, and their Times of
Revolution round that Body.
It is a remarkable fact that although the
ancient astronomers made no progress in the
determination of the absolute distances ot
the celestial bodies from the Earth, with the
single exception of the Moon, they arrived
at a very approximate estimate of the
relative distances of the planets from the
Sun. Ptolemy, who is, next to Hippar-
chus, the greatest astronomer of antiquity,
has given a statement of the relative dis-
tances of the planets in a very important
work which he has written on the science of
astronomy; and those results, after receiv-
i6
TRANSIT OF VENUS.
ing a correction derived from the observa-
tions of the Danish astronomer, Tycho
Brah^, were instrumental in conducting
Kg. 1.
the renowned astronomer Kepler to one
of the greatest discoveries recorded in
the annals of science. Fig. 1 shows how
TRANSIT OF VENUS. 17
the relative distances of Venus and the *
Earth from the Sun may be readily
derived from observation. S represents
the Sun, E the Earth, and V Venus. The
time chosen for the observation is that
at which the angular distance of the planet
from the Sun is the greatest possible. The
angle SVE being then a right angle, it
suffices to determine by observation the
magnitude of the angle SEV in order to
ascertain the proportion of SE to SV,
which gives the relative distances of the
Earth and planet from the Sun.
With the view of enabling the reader
to form an idea of the importance of the
discovery of Kepler above alluded to/ we
l8 TRANSIT OF VENUS.
shall now lay before him a statement of
the times of revolution of the eight prin-
cipal planets, and their relative distances
from the Sun, as derived from the most
recent researches : —
Time of Mean
Bevolution. Distance.
Mercury, 0-241 0-388
Yenus, 0-615 0-723
The Earth, .... 1-000 1-000
Mars, 1-884 1-524
Jupiter, ..... 11-868 5-203
Saturn, 29-456 9-539
Uranus, ..... 84-014 19-182
Neptune, 164-610 30-037
It will be readily seen from this table
that, as the time of revolution of a planet
increases, its mean distance from the Sun
TRANSIT OF VENUS. 19
increases also. It is manifest, however,
that the mean distance increases in a
much slower proportion than the time of
revolution. Thus, while the time of revo-
lution of the planet Neptune exceeds
the time of revolution of the Earth in
the proportion of 164 to 1, the mean
distance of Neptune exceeds the mean
distance of the Earth only in the pro-
portion of 30 to 1. It was reserved
for Kepler to discover a relation be- ^
tween the times of revolution and the
mean distances of the planets, by means
of which one of these elements can be
readily ascertained from a knowledge
of the other. This theorem is generally
^O TRANSIT OF VENUS.
called Kepler's third law of the planetary
movements. It may be thus enunciated :
— The squares of the times of revolution
of the playlets are proportional to the
cubes of their mean distcmces from the
Sun. The significance of this law will
be readily understood by a reference
to the foregoing table. Thus, to take
the case of the planet Mars — ^its time of
revolution is 1*884, the square of which
is 3*53 ; again, its mean distance is
1*524, the cube of which is 3'53, a result
exactly equal to the square of the time
of revolution. In the same way, if we
take any other planet, and if we square
the time of revolution and cube the
TRANSIT OF VENUS. 21
mean distance, we shall obtain two sets,
of numbers which will be identical, or
very nearly identical, with each other.
Furthermore, it is plain from this law
that, if we know the time of revolution
of a planet, we have only to square it,
and we obtain a result, the cube root of
which wiU give the mean distance of the
planet from the Sun. Conversely, if we
know the mean distance, we have only to
form the cube of it, and the square root
of the result will give the time of
revolution.
It is important to bear in mind that
in the table which we have given repre-
senting the times of revolution and mean
22 TRANSIT OF VENUS,
distances from the Sun of the principal
planets, it is only the relative distances
which are set down. Thus we learn from
the table that the mean distance of Jupiter
is 5*203 — the Earth's mean distance being
represented by unity ; but the table gives
us no information respecting the absolute
value of this unit. It is clear, however,
that if we assign a certain numerical
value to it, we are then in a position to
determine the absolute numerical value of
the mean distance of any planet, from
the sun. Thus, if the unit be expressed
by ninety-one and a half milHons of
miles, this number will then represent
the absolute mean distance of the Earth
TRANSIT OF VENUS. 23
from the Sun; and similarly the mean
distance of Jupiter, as given in the
table, upon being multiplied by ninety-
one and a half millions of miles, will
give, in round numbers, four hundred
and seventy-six millions of miles as
the absolute mean distance of the
planet from the Sun. We thus learn the
supreme importance of ascertaining the
absolute mean distance of any one planet
from the Sun ; for this object being once
achieved, the mean distances of all the
other planets from the Sun may be readily
computed by an arithmetical process, either
from the table of relative distances, or
by means of Kepler's third law.
24 TRANSIT OF VENUS,
Eayplanation of the Method for Fivding the
Distance of an Inaccessible Object
The determination of the distance of an
inaccessible object appears to many per-
sons to be an undertaking of insuperable
difficulty. Nothing can be more simple
than the principle which underlies the
solution of this problem. Let A represent
the position of an observer who wishes
to ascertain his distance from an inac-
cessible object O. He first carefully
measures the distance between the station
A and another station B. The line thus
measured is called a hose line. With a
theodolite he then measures the bearing
TRANSIT OF VENUS.
25
of O relatively to the station B. He next
w-
•K
proceeds to the station B, and similarly
measures the bearing of O with respect
c
26 TRANSIT OF VENUS.
to the station A. In this manner
he determines the two angles at the
base of the triangle AOB, and having
already ascertained by measurement the
length of the base AB, he is in a posi-
tion to compute the remaining sides and
angles of the triangle. He thus arrives
at a knowledge of the lengths of the
lines AO, BO, which represent the dis-
tance of the object from each of the
two stations.
In Fig. 2, the letters S, E, N, W,
denote the cardinal points of the horizon,
South, East, North, West. Now it is
clear that when the object O is viewed
from A, it appears in the direction north-
TRANSIT OF VENUS. 27
east ; on the other hand, when observed
from B, it appears in the direction north-
west. The angle AOB, formed by the
lines AO, BO, indicates therefore the
change of direction which the object under-
goes in consequence of the observer shift-
ing his position from A to B. It has
received an important designation. It is
called the parallax of the object.
Difficulty experienced in Applyi/ng the fore-
going Principle to the Celestial Bodies,
In endeavouring to ascertain the dis-
tances of the celestial bodies by the ap-
plication of the principle explained above,
we at once encounter a grave diflficulty.
2S
TRANSIT OF VENUS.
Fig. 3.
S
B A-
kB
TRANSIT OF VENUS, 29
In Fig. 3, let S, S, S, represent a
celestial body at diflferent distances from
a base line AB of invariahle length.
The parallax of the object is obviously
represented by the angle SBC, formed
by drawing BC parallel to AS. Now,
it is clear, by an inspection of the figure,
that the more distant the object is, the
smaller does the angle of parallax be-
come, insomuch that finally the distance
of S may be so great that the line drawn
parallel to AS coincides sensibly with
BS, and the angle of parallax vanishes
altogether. Now, if we fail to determine
the angle at S, which represents the par-
allax of the object, we have no means
30 TRANSIT OF VENUS,
of solving the triangle ASB, and con-
sequently we are unable to ascertain the
value of the distance AS or BS.
We have hitherto supposed the base
line to be invariable. Let us now con-
sider what would be the effect produced
by assigning to the base line a variable
length, and placing the celestial object S
at the same constant distance. It is
manifest, by referring to Fig. 4, that
as the ba^e Une diminishes the angle
of parallax diminishes also, insomuch
that finally we may imagine S to be so
remote that the line BC, drawn par-
allel to AS, coincides sensibly with BS, and
the angle of parallax vanishes altogether.
TRANSIT OF VENUS,
31
We now perceive clearly the main source
Fig. 4.
of the difficulty experienced in determin-
ing the distances of the celestial bodies
32 TRANSIT OF VENUS,
from the Earth. It arises in the first
place from the extreme remoteness of
those bodies, and secondly from the com-
parative smallness of the base from which
they are measured. The Earth is a body
of only eight thousand miles in diameter ;
consequently any base line drawn on its
surface cannot possibly exceed a few
thousand miles in length. This, however,
is an insignificant magnitude compared
with the immense distance of the celes-
tial bodies. In consequence of this cir-
cumstance the parallax of a celestial
object is so excessively small that, imtil
towards the close of the seventeenth
century, its determination even approx-
TRANSIT OF VENUS, 33
imately in any case, with the single
exception of the Moon, had steadily
continued to baffle the eflforts of
astronomers.
In observing the celestial bodies, it is
usual for astronomers to refer their ap-
parent positions to the centre of the Earth
which is the true physical centre of the
terrestrial globe. Indeed, since the Earth
is a body of considerable dimensions, it
is plain that observations made from
different places on its surface could not
be comparable unless they were referred
to some common centre. In Fig. 5 let
P, P', be two celestial bodies ; A the
position of an observer on the Earth's
34
TRANSIT OF VENUS,
surface, and let C denote the centre of
the Earth. P is supposed to be in the
zenith ; consequently P ' is supposed to be
Fig. 6.
removed from the zenith by the angle
PAP'. Now, the object P being in the
zenith, occupies the same apparent posi-
tion when viewed from A, as it would
TRANSIT OF VENUS, 35
do if seen from the centre of the
Earth. The object P', on the other
hand, when viewed from A, is dis-
placed relatively to the Earth's centre
by the angle AP^C. This angle is called
the diurnal parallax of the object. While
the parallax vanishes for an object in the
zenith, it attains its maximum value for
an object which is in the horizon, as
Fig. 6.
in Fig. 6, which exhibits the horizontal
parallax P, namely, the angle APC.
36 TRANSIT OF VENUS,
It appears then that, in order to deter-
mine the distance of a celestial body from
the Earth, two things are necessary. First,
we must know the exact length of the
base line from which the observations of
the object are made ; secondly, we must
detect an appreciable parallax of the
object depending upon the observations
made at the two extremities of the base.
As regards the base line, we are enabled
to compute its length exactly when we
once know the magnitude and figure of
the Earth, and the longitude and latitude
of each of the two extremities of the
base. The measurement of the angle of
parallax is manifestly an operation of
TRANSIT OF VENUS, 37
much delicacy, in consequence of its ex-
treme minuteness ; for, except under ex-
ceptionally favourable circumstances, it
eludes the eflforts of the most skilful
astronomer.
It has been already stated that, if
we once ascertained the absolute dis-
tance of any one of the planets from
the Sun, the table of relative distances
would enable us to compute the absolute
distances of all the others/ Now, the
Earth bemg a planet, it is clear that we
could eflfect this important object if we
succeeded in determining the exact value
of the solar parallax. We now begin to
perceive the immense magnitude of the
A
38 TRANSIT OF VENUS.
results derivable from a knowledge of
this element.
To ascertain tlie value of the solar
parallax by direct observations of the Sun
has been found impracticable for various
reasons, which it would be out of place
to attempt explaining here. Instead of
attacking the problem in this way, a;Stro-
nomers have skilfully evaded its more
formidable difficulties by deducing the
value of the solar parallax from obser-
vations of certain of the planets. It is
clear from what has been already stated
that the nearer a planet is to the Earth
the more favourable are the circumstances
for the determination of its parallax.
TRANSIT OF VENUS, 39
Now, there are two planets which occa-
sionally approach comparatively near to
the Earth. These are the planets Venus
and Mars — ^the one revolving immediately
within the Earth's orbit, the other re-
volving immediately beyond it. We
shall commence with a remark or two
on Mars, which was the planet first em-
ployed for determining the solar paral-
lax.
Fig. 7 represents the orbits of the
Earth and Mars, on the supposition that
they are both circles, having the Sun in
their common centre S. Let the Earth
be travelling in its orbit at E ; join SE,
and imagine it to be extended so as
^
40
TRANSIT OF VENUS,
to meet the orbit of Mars in M. Now
it is plain that when the planet is
Fig. 7.
at M, it is nearer to the Earth (sup-
posed for the sake of explanation to be
always at E) than when it is in any
other part of its orbit. In this position
TRANSIT OF VENUS, 41
the planet is technically said to be in
opposition, the reason being that when
viewed from the Earth it then appears
in the opposite region of the heavens
to that in which the Sun is situate. Thus,
if at the time when the planet is in opposi-
tion it is midnight, the Sun being then
due north and under the horizon, the
planet will appear due south and above
the horizon. It is obvious therefore that
when the planet is in opposition it is
much nearer to the Earth than when it
is in any other part of its orbit. But
in point of fact the circumstances are
much more favourable than we have
imagined. We have assumed that the
D
42 TRANSIT OF VENUS,
orbits of the Earth and the planet are
both circles. In reality, however, they
are ellipse^. The orbit of Mars is con-
siderably excentric; that of the Earth is
less so : but the two orbits are so placed
relatively to each that their excentricities
combine together in producing occasionally
a comparatively near approach of the
two planets. Thus, when the planet is
in the 'perihelion^ and consequently in
the position where it is nearest posdhle
to the Sun, and if it be at the same time
in opposition, the Earth will be very near
the aphelion of its orbit, and consequently
will be the farthest possible from the
Sun. In this position then the planet
TRANSIT OF VENUS, 43
will as it were retire (within its supposed
circular orbit) to meet the Earth, while
the Earth will advance outwards (beyond
its supposed circular orbit) to meet the
planet. The consequence of this favour-
able state of matters is that, while in
general the distance between the Earth
and planet at the time of opposition
amounts to fifty or sixty milUons of
miles, it occasionally diminishes so as
not to exceed thirty-five millions of miles.
This near approach of the two planets
happens at intervals of fifteen or seven-
teen years. It was first taken advantage
of for determining the solar parallax in the
seventeenth century by Cassini, an eminent
44 TRANSIT OF VENUS.
French astronomer, who obtained 9"*5
for the value of the solar parallax, whence
the distance of the Sun from the Earth
would be eighty-five millions of miles.
This method of determining the Sun's
distance from the Earth has been used
on several subsequent occasions.
But the planet Venus furnishes a
method ' of a peculiar kind for ascertain-
ing the value of the solar parallax^
which has been considered to be more
entitled to confidence than any other
method heretofore employed for the pur-
pose. This beautiful planet, as has been
already stated, revolves immediately with-
in the Earth's orbit. Fig. 8 gives a
TRANSIT OF VENUS.
45
graphic representation of the various
positions which it assumes as it revolves
round the Sun, the Earth being assumed,
for the sake of illustration, to be station-
4^ TRANSIT OF VENUS,
ary at E. When the planet is at '^y it
is then immediately beyond the Sun, and
the Earth, Sun, and planet are in the
same straight line. In this position the
planet is said to be in superior conjunc-
tion. The illuminated hemisphere being
turned wholly towards the Earth, the
planet, if it were possible to see it, would
present a round disc, like the full Moon.
It is, however, immersed in the efiulgence
of the Sun's light, and is consequently
invisible. In this position both Sun and
planet rise and set together. As the
planet revolves in its orbit, the illumi-
nated hemisphere is gradually turned
away from the Earth, and the planet as-
TRANSIT OF VENUS, 47
sumes a gibbous aspect, as at ^; it also
now begins to set after the Sun, and is
therefore an evening star. At "ifi it as-
sumes the appearance of the half moon ;
the time of its visibility above the hori-
zon after sunset is also now the longest
possible. In this position the planet
is said to be at its greatest eastern
elongation. After quitting this position
the planet assumes the form of a beau-
tiful crescent, as at v^; it also now
gradually approaches the Sun, continuing
a shorter time above the horizon on each
successive night. When the planet ar-
rives at 1;^ the Sun, Earth, and planet
are again in the same straight line. The
48 TRANSIT OF VENUS,
planet is now said to be in inferior con-
junction. In this position it comes di-
rectly between the Sun and the Earth.
The Sun and the planet now rise and set
together. When the planet has advanced
beyond this position it rises and sets
before the Sun, and is, consequently, now
a morniiig star; and the same succession
of phases is reproduced in a reverse
order, until the planet finally arrives in
superior conjunction at v^, when both Sun
and planet again rise and set together.
Now, it is clear from this explanation that
when the planet is in inferior conjunction, it
is nearer to the Earth than in any other
part of its orbit. The occasion is there-
TRANSIT OF VENUS, 49
fore especially favourable for the determi-
nation of its parallax. But, unfortunately,
the same cause which prevents the planet
from being generally visible, when it is
in superior conjunction, is equally efficacious
in this case also. There are, however,
certain rare occasions when the planet
may be seen in this position, not however
as a star, but as a ovund black spot passing
over the Sun's disc. Since Mercury and
Venus both revolve within the Earth's
orbit, they may occasionally be seen in
this manner between the Earth and the
Sun. A phenomenon of this kind is
technically termed a transit of the planet.
The importance of the transits of Venus
so TRANSIT OF VENUS,
over the Sun's disc for ascertaining the
value of the solar parallax was first pointed
out by James Gregory, and was after-
wards insisted upon more fully by Halley.
In 1761 and 1769 there occurred transits
of Venus over the Sun's disc, and in
both cases, the occasion was deemed to be
of so great importance that the principal
nations of Europe despatched observers
to various parts of the world for the pur-
pose of observing the phenomenon.
The phenomenon to be observed may
be readily understood by reference to
Figure 9. The large circle represents
the Sun. The smaller circle represents
the planet. The planet enters upon the
TRANSIT OF VENUS, 5 1
Sun^s disc, making exterior contact with
it at 1, and interior contact at 2. It
then travels along and leaves the solar
disc on the right, making interior contact
with the solar limb at 5, and exterior
contact at 6. The object of the observer
is especially to note the precise instant
i
52 TRANSIT OF VENUS,
when the planet makes interior contact
with the Sun's limb as at 2 and 5.
Two methods of determining the solar
parallax, on the basis of observations of
the transit of Venus over the Sun's disc,
have been devised by astronomers. The
Fig. 10.
earliest of these methods was that pro-
posed by Halley. It may be thus
explained: — ^Let E, E' be two places of
observation in opposite parts of the
Earth, the one being near the north pole,
TRANSIT OF VENUS, 53
and the other near the south pole. An
observer at the centre of the Earth would
see the planet travel along the dotted
chord included between the chords OP,
QR. The observer at E sees the
planet describe upon the Sun's disc the
chord QR. The observer at E' sees
the planet describe the chord OP.
Now, the difference between the times
of describing the two chords QR,
OP, constitutes an indication of the
displacement in the path of the planet
resulting from the difference in position
between the two stations of observation
E, E'. But this displacement depends
upon the absolute distance of Venus and
^
54 TRANSIT OF VENUS.
the Earth from the Sun. By means,
therefore, of the duration of the transit
of Venus as observed from two distant
stations on the Earth's surface, an indi-
cation is obtained of the value of the
solar parallax, and consequently of the
Sun's distance from the Earth.
The other method of determining the
solar parallax by observations of the
transit of Venus, is founded upon observ-
ing the internal contact of the planet with
the sun at two distant stations on the
Earth's surface. Thus, let E, E', be two
such stations ; the observer at E sees the
ingress of the planet upon the Sun's disc
when it arrives at a; on the other hand,
TRANSIT OF VENUS. 55
the observer at E' does not see the
ingress of the planet until it arrives at 6.
The interval of time which the planet
Fig. 11.
thus occupies in passing from a to 6
constitutes the groundwork of the solution
of the problem for finding by this method
the Sun's distance from the Earth.
The problem may be solved in a similar
w^y l>y observations made, at two distant
stations, of the egress of the • planet from
the Sun's disc. In this case the interval
56 TRANSIT OF VENUS,
of time, which the planet occupies in
describing the arc cc2, as observed at
EE', supplies an indication of the Sun's
distance from the Earth. This is tenned
Dehsle's method, because it was first
suggested by Delisle, a French astro-
nomer, in contradistinction to the method
based upon the observed duration of the
planet on the Sun's disc, which is due to
Halley.
The method of Delisle requires a
comparison of the exax^t times of ingress
or egress of the planet, as observed at
the two stations. To effect this object
we must determine the longitudes of
the stations relatively to Greenwich or
TRANSIT OF VENUS, 57
some known meridian. This is in all
cases an operation of much delicacy.
Careful preparations were made to
observe the transit of Venus, which
occurred in 1761 ; but the weather was
generally unfavourable for the purpose.
Stm more systematic and extensive
arrangements were made to observe
the transit of 1769. In this instance
the operations were successful, the
phenomenon having been satisfactorily
observed by a great number of persons
in different parts of the world. In
1824, the totality of the observations
of the transits of 1761 and 1769
were submitted to a profound discussion
E
i
53 TRANSIT OF VENUS.
by the celebrated German astronomer,
Encke, who deduced from them a
parallax mdicating the Sun's distance
from the Earth to be, in round num-
bers, ninety-five millions of milea This
result was speedily adopted by astro-
nomers, and continued to be inserted
in all text-books on astronomy until
quite a recent period. It is now ascer-
tained beyond all doubt that the value of
the solar parallax assigned by the German
astronomer is considerably erroneous.
If the planet Venus revolved in the
plane of the ecliptic, it would be seen as
a round spot passing over the Sun's disc
every time it arrived in inferior conjuno-
TRANSIT OF VENUS. $9
tion. But, in point of fact, the orbit of
the planet is inclined to the plane of
the Earth's orbit, and the result conse-
quently is that the planet can be seen
on the Sun only when, at the time of
inferior conjunction, it is passing through,
or very near, either of the nodes of its
orbit. Let us suppose two hoops, one
of which is a little larger than the
other. Let them both have a common
centre, and let them be inclined to each
other at a given angle. Further, while
the Sun is supposed to be in the common
centre of the hoops, let us assume that
Venus revolves in the inner, and the
Earth in the outer, hoop. Let a
1
6o TRANSIT OF VENUS,
common diameter of the two hoops be
drawn through the points where the
inner hoop intersects the plane of the
outer hoop. The points here referred
to represent the nodes of the planet's
orbit, and the diametral line pass-
ing through them is called the line of
nodes. A transit of the planet can
happen only in June or December,
because the Earth can only then be
situated in the line of nodes of the
planet's orbit. When the planet is pass-
ing through its ascending node, the
transit happens in December ; when it
is passing through the descending node,
the transit necessaxUy occurs in June.
TRANSIT OF VENUS, 6 1
Transits generally occur at intervals of
105^ years, 8 years, 121^ years, 8 years,
105^ years, 8 years, &c. Frequently,
however, instead of the transits occurring
in pairs, there may be only one transit
and then a long interval of more than a
hundred years. The recurrence of transits
at intervals of only eight years arises
from the fact that thirteen revolutions of
the planet are eflfected in almost exactly
the same time as eight revolutions of
the Earth; consequently, if the planet
should be in either of the nodes of its
orbit at the time of inferior conjunction,
it will be very nearly in the node after
the lapse of eight years, sufficiently near>
62 TRANSIT OF VENUS.
perhaps, to bring about another transii
of the planet. If the planet shoulc
pass OTer the Sun's disc very near th«
centre, its displacement after the lapsi
of eight years may be too great to alloT
of another transit.
Earrlier Transits of Venue.
The earhest prediction of a transit o
Venus over the Sun's disc is due t"
Kepler, In 1629 he announced tha
a transit of the planet would occu
in 1631. "When the time asaignei
for the occurrence of the phenomeno]
finally arrived, astronomers searched fo
the planet on the Sun's disc, but withou
TRANSIT OF VENUS, 63
success. It is now known that the
transit really did occur, but that the
planet passed over the Sun's disc in
the night time, and was consequently
invisible to the astronomers of Europe.
Another transit of Venus occurred in
1639. The phenomenon appears to
have wholly escaped the attention
of astronomers on this occasion, with
the exception of two young English-
men, Jeremiah Horrocks and William
Crabtree, who alone had the privi-
lege of witnessing a spectacle, the like
of which no mortal had hitherto
ever seen. Horrocks was curate of
Hoole, near Preston. He was endowed
\
64 TRANSIT OF VENUS.
with an original genius of a high order,
and an ardent enthusiasm in the pursuit
of science; and although he died in the
very flower of his age, lie has left
behind hun a name which wiU Uve
imperishablj in the annals of science.
By means of his own calculations he
discovered to his great delight that there
would be a transit of the planet in
November 24 (O.S.), 1639, and he hastened
to make suitable preparations for observ-
ing the phenomenon.
The plan of observation devised by
him consisted in admitting the Sun's
light into a dark room through an
aperture in the window, and receiving
TRANSIT OF VENUS, 65
the image of the Sun upon a white screen
attached to the opposite wall. The
transit of the planet over the Sun's disc
would then be indicated by the presence
of a round black spot traversing the
white circle.
Horrocks watched the solar image
carefully throughout the whole of the
23rd of November, but no trace of
the planet was seen. On the morning
of the 24th, which was Sunday, he simi-
larly scrutinized the image of the solar
disc, but failed to obtain any indication
of the presence of the planet. After an
absence of some time, caused by the
necessity of attending to his clerical
i
66 TKAXSIT OF rEXVS.
duties he repaired again with eager
anxiety to the darkened chamber, when,
'' Oh^ most grati^ring spectacle ! " said
he, '^ the object of so many esunest
wishes^ I perceived a new spot of unnsoal
magnitude, and of a perfectly round form,
that had just wholly entered upon the
left limb of the Sun, so that the margin
of the Sun and the spot coincided with
each other, forming the angle of contacf
Owing to the near approach of sunset,
Horrocks was unable to observe the planet
longer than half an hour. Crabtree, who
resided near Manchester, had, in accord-
ance with instructions from Horrocks,
made similar preparations for observing
TRANSIT OF VENUS, 67
the phenomenon, and he also enjoyed the
gratification of seeing the planet on the
Sun's disc; but a cloud came over the
Sun s face, and he was unable to make
any precise measures. The observations
of Horrocks have furnished valuable
materials in recent years for correcting
the elements of the orbit of the planet.
Horrocks, as already stated, died young,
but he has left behind him unmistake-
able proofs of a thoroughly original
genius, and a capacity for the cultiva-
tion of physical science which have
earned for him a lasting reputation. In
the present day, when a transit of the
planet is close at hand, steps have been
J
68 TRANSIT OF VENUS.
taken by the men of science of his
country to erect a fitting tribute to his
memory in Westminster Abbey.
The Trandts of 1761 and 1769.
It has been abready stated that the
weather was generally unfavourable for
the observation of the transit of 1761.
The preparations for observing the transit
of 1769 were upon a more extensive scale,
and led to a more successful issue. The
British Government despatched observers
to Otaheite, in the Pacific Ocean, for the
purpose of observing the phenomenon.
The ship in which they sailed, the
*' Endeavour," was commanded by the
TRANSIT OF VENUS, 69
celebrated navigator, Captain James Cook.
The other principal nations of Europe
made similar preparations, with a view to
the observation of the transit. The
weather on the whole was favourable for
the observation of the phenomenon, and
the operations were skilfully executed.
The method chiefly relied upon was that of
durations, as illustrated in Fig. 10. Two
of the most important stations were
Otaheite, in the southern hemisphere,
and Wardhus, in Lapland, in the northern.
The duration of the transit at Wardhus
was 5 hours 54 minutes ; the duration at
Otaheite was 5 hours 32 minutes. The
diflference of durations amounted therefore
A
7o TRANSIT OF VENUS,
to 22 minutes. This interval of 22
minutes constituted the result leading to
the determination of the solar parallax.
If the absolute distances of the Sun
and the planet from the Earth were
gradually increased (the relative distances
of the two bodies remaining unchanged),
the difference of duration of the transit
would gradually diminish, until ultimately
it would be so small as to be inappreciable.
Now 22 minutes constitute a considerable
interval of time for measurement, and it
happens that a small error committed in
its determination would entail a rela-
tively much smaller error in the compu-
tation of the solar parallax.
TRANSIT OF VENUS. 7 ^
Shortly after the occurrence of the
transits of 1761 and 1769 various evalua-
tions of the solar parallax were deduced
from the resulting observations, but the
discordances of the results thus obtained
were greater than was deemed satisfac-
tory. It has been already mentioned
that Encke submitted the totality of
the observations of both transits to a
comprehensive discussion. The German
astronomer obtained 8"*5776 for the
definitive value of the solar parallax.
This would indicate the Sun's distance
from the Earth to be, in round numbers,
ninety-five millions of miles.
A difficulty of an unexpected and
72 TRAXSIT OF rEXUS.
serious nature occurred to the observers
of the transit of Venus on these occasions.
It was found that the ingress of the planet
on the Sun's disc was characterized bj the
presence of a pear-shaped, dark ligament,
connecting the planet with the Sun's limb,
and rendering it a matter of exceeding
difficulty to pronounce upon the precise
instant when the two bodies formed
internal contact. It was to the un-
certainty of the observations arismg from
this cause (a purely optical one) that the
discordance between the results obtained
for the value of the solar parallax by
different astronomers was in a great
measure attributable.
TRANSIT OF VENUS. 73
Mcyre Recent Determiifiations of the Value
of the Solar Parallax.
The value of the Sun's distance from the
Earth resulting from the researches of
Encke in 1824 continued to be adopted
in all popular treatises on astronomy as
representing the most trustworthy value
of that element which had hitherto been
arrived at. In recent years, however,
astronomers have found reason to suspect
that the evaluation of Encke is considerably
in error. As early as 1854, Hansen, in
a letter to the Astronomer Royal, an-
nounced that his researches in the lunar
theory seemed to indicate that Encke's
F
74 TRANSIT OF VENUS.
value of the solar parallax was too small
(and consequently that the resulting dis-
tance of the Sun from the Earth was too
great). Subsequently, he determined in
this manner the value of the solar parallax^
and he found it to be S^'SIG. Stone, the
first Assistant at the Royal Observatory,
Greenwich (now Her Majesty*s Astro-
nomer at the Cape of Grood Hope), deduced
a similar result from observations of the
planet Mars, made in 1862, when it ap-
proached very near to the Earth. His
computations gave 8''*943 for the resulting
value of the solar parallax. About the
same time Le Verrier, the eminent French
Astronomer, obtained S^'SSQ from his re-
TRANSIT OF VENUS, 7 5
searches in the planetary theory. These
various determinations concurred in in-
dicating that the true distance of the Sun
from the Earth was not ninety -five
millions of miles, as had been hither-
to supposed, but rather • somewhere
about ninety-one and a half millions
of miles, or about ^Vth less than the
assumed value. This conclusion received
a striking confirmation from an un-
expected source. The velocity of light
may be determined astronomically in
two different ways. One of these
is founded upon observations of the
eclipses of Jupiter's satellites. When the
Earth is in the part of its orbit which is
76 TRANSIT OF VENUS,
nearest to the planet, the eclipses occur
earlier than the predicted times. On the
other hand, when it is in the more remote
part of its orbit, the eclipses occur later
than the times computed from theory.
These discordances may all be got rid of
by assuming that light is not propagated
instantaneously, but, on the contrary,
occupies some time in passing through
space. Now, it is found in this way that
light occupies sixteen minutes in travers-
ing a diameter of the Earth's orbit. As-
suming, then, that the radius of the
Earth's orbit is ninety-five milUons of
miles, we hence arrive at the conclusion
that light traverses space with the amaz-
TRANSIT OF VENUS, 77
ing velocity of one hundred and ninety-
two thousand miles in a second.
The other method is founded upon a
curious phenomenon of the stars. When
their apparent positions are carefully
scrutinized, it is observed that the stars
all describe a very small ellipse in the
heavens, a fact which may be satisfactorily
explained by assuming that light occupies
a certain interval of time in passing from
a star to the earth. Now, the magni-
tude of the major axis of the ellipse
described by a star in this case depends
upon the proportion which the orbital
velocity of the earth bears to the
velocity of light. Here, then, we have
78 mANSIT OF VENUS,
three quantities, from any two of which we
can derive the third — ^namely, the major
axis of the ellipse of aberration as it is
called, the velocity of the Earth in its
orbit, and the velocity of light. Now,
observations of the stars give us the
value of the first of these three quantities,
and if we assume the radius of the Earth's
orbit to be ninety-five millions of miles,
we can readily compute from it the orbital
velocity of the Earth. Knowing then
these two quantities, we are in a position
to determine the third, and thus we arrive
at the conclusion that light travels
through space at the rate of one hundred
and ninety-two thousand miles in a
TRANSIT OF VENUS. 79
second, as indicated by the eclipses of
Jupiter's satellites.
But, wonderful to relate, the velocity
of light has been determined by an
experimental process, conducted upon the
Earth's surface within the compass of a
few^ hundred yards. This has been ac-
complished by two distinct methods,
both due to two French physicists,
Fizeau and Foucault. The results ob-
tained in the two instances agree in
assigning to light a velocity of one
hundred and eighty-five thousand miles
in a second. Here, then, we have pre-
sented to us a striking discordance
between the value of the velocity of
3o TRANSIT OF VENUS.
light deduced astronomicallj^ as stated
above, and the value obtained by the
French physicists. But in computing
the velocity of light astronomically, we
assumed that the radius of the Earth's
orbit was ninety-five millions of miles.
Let us, however, suppose the radius' of
the orbit to be ninety-one and a half
millions of miles, as pointed out by the
recent researches of astronomers, and we
obtain by both astronomical methods a
velocity amounting to one hundred and
eighty-five thousand miles in a second^
precisely the same value as that indi-
cated by the experiments of the French
physicists.
TRANSIT OF VENUS, 8 1
Thus it appears that the terrestrial
experiments for ascertaining the velocity
of light concurred with recent astrono-
mical researches in indicating the ne-
cessity of adopting a larger value of
the solar parallax than the value hitherto
employed by astronomers.
Finally, Stone having investigated anew
the question, in so far as concerned the
transit of 1769, ascertained that, by a juster
interpretation of some of the observations
the resulting value of the solar parallax
agreed very nearly with the value derived
from other sources. In fact, he found
in this manner the value of the solar
parallax to be 8"*91, indicating the Sun's
{
%
32 TRAXSIT OF VENUS.
distance {rom the Earth to be ninet
million six hundred thousand mile&
Details respecting the Transit of Venus in
December 8, 1874.
It has been already stated that the
observation of the transit of Venus con-
sists in noting the precise instant when
the planet^ in its passage over the Sun s
disc^ forms internal contact with the
margm of the Sun, first at ite ingress
upon the solar disc, and secondly, at
its egress from the disc. If we suppose
an observer to be situated at the centre
of the Earth, he would see the inter-
nal contact at ingress on the morning
TRANSIT OF VENUS. 83
of the 9th of December at 2 h. 15 m.
Greenwich Mean Time, and he would
see the internal contact at egress at
5 h. 57 m. The included interval of
time is therefore 3 hours 42 minutes.
But the interval of time which elapses be-
tween the instant when the planet first
impinges on the solar disc, and the
instant when it finally leaves the disc is
necessarily somewhat greater, as may be
seen by referring to Fig. 9. It amounts,
in fact, to 4 hours 41 minutes. An
observer on the Earth's surface, having
the Sun in his zenith, would see the
different phases of the transit exactly
as an observer at the centre of the
34 TRANSIT OP VENUS.
Earth would see them. But the result
would be different if he were stationed
at any other place on the Earth's
surface, as may be readily understood by
referring to Figures 10 and 11. Let us
confine our attention to the instant of
internal contact at ingress and egress.
Of course, the transit can only be visible
from the illuminated hemisphere of the
Earth ; or, in other words, the hemis-
phere which is turned towards the Sun.
Now, there are certain places on the
Earth's surface from which the internal
contact of the planet with the Sun at
ingress may be seen earlier than it
would be seen to an observer stationed
TRANSIT OF VENUS, 85
at the centre of the Earth; and again,
there are other places where the internal
contact at ingress would be seen later
than it would be seen from the centre
of the Earth, Now, if the Sun had no
sensible parallax, the time of internal
contact, whether at ingress or egress,
would be the same everywhere on the
Earth's surface as at the centre. It is
clear, then, that the difference between
the times of internal contact at ingress
or egress, as observed from two distant
stations on the Earth's surface constitutes
the datum available to the astronomer
for the solution of the problem of the
Sun's distance from the Earth. The
\
S6 TRANSIT OF VENUS.
following plan of localization as r^ards
stations accordingly offers itself as most
suitable for observing the phenomenon.
1. Stations where the internal contact
at ingress is accelerated.
2. Stations where the internal contact
at ingress is retarded.
3. Stations where the internal contact
at egress is accelerated.
4. Stations where the internal contact
at egress is retarded.
It is upon this principle that astrono-
mers have selected a great number of
stations in various parts of the world, for
the purpose of observing the transit.
Let us take^ for example, Woahoo in the
i
TRANSIT OF VENUS. 87
Sandwich Isles, and Kerguelen Island.
The internal contact will be seen eleven
minutes earlier from Woahoo, and twelve
minutes later from Kerguelen Island than
if it were viewed from the Earth's centre.
Hence the interval between the times of
internal contact, as seen at Woahoo and
Kerguelen Island, amounts to twenty-
three minutes. Similarly, the astronomer
combines the observations made at two
stations, where the internal contact at
egress is seen earlier in the one case, and
later in the other, than if the phenomenon
was seen at the centre of the Earth.
1
88 TRANSIT OF VENUS,
ArrangeTnenta for observing the TramsU of
Venus in 1874.
As early as 1857, the Astronomer
Royal, in a paper communicated to the
Royal Astronomical Society, drew the
attention of astronomers to the approach-
ing transits of Venus over the Sun's
disc in 1874 and 1882; and on several
subsequent occasions he explained his
views on the subject to the Society.
The Government having been made to
understand the importance attached to
the proper observation of the transit of
1874, induced Parliament to vote a con-
siderable sum of money for defraying the
TRANSIT OF VENUS. 89
necessary expenses. The whole of the
arrangements connected with the different
expeditions for observing the pheno-
menon have been planned and executed
under the superintendence of Sir George
Airy, the Astronomer Royal. Five
stations were originally chosen for the
observation of the transit. These were,
Alexandria, Honolulu, Rodriguez, New
Zealand, and Kerguelen Island. It
was subsequently considered desirable to
supplement the station at Honolulu
by two additional stations at some
distance apart. These are Hawaii and
Kauai. An additional station has
also been attached to Kerguelen Island,
G
9© TRAXSIT OF VEXUS.
and one at Cairo, in connection with the
station at Alexandria. Furthermore, two
stations have been established in India.
In addition to these preparations, the
transit will not £ail to receive due
attention at the observatories of Mel-
bourne, Sydney, the Cape of Good Hope,
and Madras. Some of the Colonial
Governments of Australia have voted
special grants of money for the observation
of the phenomenon. Then there is the
very complete expedition fitted out by
Lord Lindsay, with the view of observing
the transit at the Mauritius. Colonel
Campbell, of Blythswood, has also under-
taken to observe the phenomenon
TRANSIT OF VENUS, 91
at Thebes. The various observing
parties despatched from Greenwich have
been furnished with an admirable
equipment of instruments with the use
of which the several observers have
undergone a course of training at the
Royal Observatory during the last two or
three years, under the guidance of Captain
Tupman, R.M.A. Photography will be
used in connection with the observatories
at all the stations. Mr. Warren De La
Rue has liberally undertaken to superin-
tend this part of the Greenwich arrange-
ments. Much ability has been displayed
by Proctor in the discussions generally
relating to the two transits.
\
92 TRANSIT OF VENUS,
The observers connected with the
various Greenwich expeditions are chiefly
naval officers, with the addition of some
officers of the engineers and artillery, and
a few private observers. The following
plan of arrangements, relative to the
appointment of the different observers,
was drawn up and issued some months ago
by the Astronomer Royal.
Aiypointmenta of Observers to the sevefi'ctl Dis-
tricts of Obsei^ution, and Subordination of
Observers.
"1. Captain G. L. Tupman, KM. A.,
is head of the entire enterprise, and is
responsible, through the Astronomer
TRANSIT OF VENUS, 93
Royal, to the Government for every part.
Every observer is responsible to Captain
Tupman.
^'2. When the different expeditions are
separated, the observers in each district of
observation are responsible to the local
chief of the district, and the chief to the
Astronomer Royal. The districts of ob-
servation and the observers will be the
following, the name first following that of
the local chief being that of the deputy,
who will, if necessary, take his place ; —
'' 3. District A. Egypt: Chief, Capt. C. .
O. Browne, R.A., astronomer; Observers,
Capt. W. de W. Abney, R.E., astronomer
and photographer; S. Hunter, astronomer.
I
94 TRAXSIT OF VEXUS,
" 4. District B. Sandwich Islands : Gene-
ral Chief, Capt. G. L. Tupman, R.M. A. :
Deputy, if necessary. Prof. G. Forbes.
" Subdivisions of the Sandwich Islands :
— Honolulu : Chief, Capt. G. L. Tupman,
astronomer; Observers, J. W. Nichol,
astronomer and photographer; Lieut. F»
E. Ramsden, R.N., astronomer and photo-
grapher. Hawaii : Chief, Prof. G. Forbes,
astronomer; Observer, H. G. Barnacle,
astronomer. Kauai : Chief, R. Johnson,
astronomer; Observer, Lieut. E. J. W.
Noble, R.E.M., astronomer.
" 5. District C. Rodriguez : Chief,
Lieut. C. B. Neate, R.N., astronomer;
Observers, C. E. Burton, astronomer and
TRANSIT OF VENUS, 95
photographer; Lieut. R. Hoggan, R.N.,
astronomer and photographer.
'' 6. District D. Christchurch (New
Zealand): Chief, Major H. Palmer, R.E.;
Observers, Lieut. L. Darwin, R.E.,
astronomer and photographer ; Lieut. H*
Crawford, R.N., astronomer.
** 7. District E. Kerguelen Island :
General Chief, Rev. S. J. Perry; Deputy,
if necessary, Lieut. C. Corbet, R.N.
" Sub-divisions of the Kerguelen Is-
land : — Christmas Harbour : Chief, Rev.
S. J. Perry, astronomer and photographer ;
Observers, Eevs. W. Sidgreaves, astrono-
mer ; Lieut. S. Goodridge, R.N., astrono-
mer ; J. B. Smith, astronomer and photo-
I
96 TRANSIT OF VENUS,
grapher. Port Paliser : Chief, Lieut. C.
Corbet, R.N. ; Observer, Lieut. G. E.
Coke, R.N.
" 8. In addition to these gentle-
men, three non-commissioned officers or
privates of the Corps of Royal Engineers
will be attached to each of the five dis-
tricts, and will be under the direction of
the chief of each district.'*
Expeditions for observing the transit
have also been sent to various parts of
the world by the Governments of France,
Germany, Italy, Holland, Russia, and the
United States of America.
TRANSIT OF VENUS, 97
CondvAing Remarks.
It has been stated that the solar paral-
lax, as generally adopted by astronomers
in the present day, would place the Sun
at a distance of ninety-one and a half
millions of miles from the Earth. This
element puts us at once in possession of
the distances of all the planets from the
Sun. In this manner we arrive at a
knowledge of the dimensions of the
system of which the Earth forms one of the
constituent bodies. We find the extreme
planet of the system revolving round
the Sun to be situated at a distance
of nearly three thousand millions of
98 TRANSIT OF VENUS,
miles. We discover that the planets
are bodies of immense size, several of
them vastly exceeding the Earth in mag-
nitude. We obtain from the same source
a knowledge of the magnitude of the
orbits of the satellites which are found to
accompany the larger planets of the sys-
tem. Once in possession of a knowledge
of the solar parallax, we are enabled to
determine the masses of the planets by
comparing them with the suns mass.
The same important element gives us
information respecting the amazing velocity
with which the planets travel in their
orbits, and the consequent enormous in-
tensity of the sun's attraction, which pre-
TRAXSIT OF VENUS, 99
vents them from flying off into space. If
we direct our attention to the system
of comets, we are equally struck with
the light thrown upon the movements of
those mysterious bodies by our know-
ledge of the solar parallax. We obtain
an instructive insight into the amazing
.velocity with which in many instances
they travel in their orbits, we measure
the dimensions of their orbits, and com-
pute with precision the distances to which,
when travelling to their aphelia, they
recede into the illimitable depths of space*
The planet Neptune revoWes round the
sun at a distance of nearly three thousand
millions of miles from the Earth. The
lOO TRANSIT OF VENUS,
great comet of 1858, when passing
through the aphelion of its orbit, recedes
to a distance of thirty thousand millions
of miles from the Earth, and yet this
enormous distance amounts to only a
seven hundredth part of the distance of
the nearest of the fixed stars. It is to be
remembered furthermore, that whatever
knowledge we possess respecting the dis-
tances, magnitudes, and masses of the fixed
stars, is dependent on the same element.
As soon as we have determined the radius
of the Earth's annual orbit round the Sun,
we can make use of the diameter as a new
base-line, and upon this immensely im-
proved vantage ground proceed to deter-
I
TRANSIT OF VENUS. loi
mine the distances of the stars. When
Copernicus propounded the true system of
the universe, it was argued by his oppon-
ents that according to his views the
stars ought to present an annual varia-
tion of aspect and position depending upon
the motion of the Earth in its orbit.
Copernicus met this objection by remark-
ing that the whole solar system was a
mere point in comparison with the sphere
of the fixed stars. It is worthy of note,
that until a comparatively recent period,
this was the only reply which could be
given to the opponents of the Copemican
theory. In the present day several stars
have been detected, presenting parallaxes
i
102 TRANSIT OF VENUS.
of such undoubted magnitude, that we are
enabled with perfect confidence, to assign
their distances from the Earth. The re-
sults which have been arrived at by re-
searches in this branch of astronomy,
are calculated to inspire with awe all
who devote their attention to the subject.
Even the masses of the stars have been,
in some instances, determined; and we
arrive in this manner at the startling
conclusion that the luminaries of the
stellar vault are bodies of immense size,
rivalling the Sun in magnitude and splen-
dour. It has been an opinion generally
held by astronomers, thsit the stars
are distant Suns. Our knowledge of
TRANSIT OF VENUS. 103
the value of the solar parallax, com-
bined with the results of recent researches
in stellar astronomy, assure us beyond all
doubt of the reality of this fact.
We shall conclude with a statement
of the steps by which the human mind
is enabled to ascend in succession to the
contemplation of these lofty truths.
First, the astronomer measures a base
line seven or eight miles in length upon
the Earth's surface. Combining this
result with the solar parallax, he deter-
mines the distances of the planets from
the sun, their magnitudes and masses, and
the velocities of their orbital movements.
He computes the dimensions of the orbits
104 TRANSIT OF VENUS,
of comets and meteor streams, and assigns
with precision the distances to which they
recede into space when they have reached
the aphelia of their orbits. Finally,
assuming as a new base line for his re-
searches the diameter of the Earth's orbit,
a line measuring a little more than a
hundred and eighty millions of miles in
length, he determines the distances and
masses of the stars. He computes
the velocities with which they travel
in space, and compares them in this
respect with the movement of the solar
system in space. Nay, the spectroscope
informs him respecting the materials
of which those remote bodies consist, and
TRANSIT OF VENUS, 105
thus teaches him another important fact
in support of the grand doctrine that the
Sun is no other than a star, and that the
innumerable bodies of the stellar vault
are magnificent globes of light, rivalling
the Sun in magnitude and splendour.
We have here presented to us a striking
instance of the sublimity of the views
respecting the immensity of the physical
universe which the science of astronomy
has disclosed to the researches of the
human mind.
eLASoow:
PRINTED AT THB UNiySRSITT PRESS.
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