VENUS.
Interest attaching to this Planet--The Unexpectedness of its
Appearance--The Evening Star--Visibility in Daylight--Lighted only
by the Sun--The Phases of Venus--Why the Crescent is not Visible to
the Unaided Eye--Variations in the Apparent Size of the Planet--The
Rotation of Venus--Resemblance of Venus to the Earth--The Transit
of Venus--Why of such Especial Interest--The Scale of the Solar
System--Orbits of the Earth and Venus not in the same
Plane--Recurrence of the Transits in Pairs--Appearance of Venus in
Transit--Transits of 1874 and 1882--The Early Transits of 1631 and
1639--The Observations of Horrocks and Crabtree--The Announcement
of Halley--How the Track of the Planet differs from Different
Places--Illustrations of Parallax--Voyage to Otaheite--The Result
of Encke--Probable Value of the Sun's Distance--Observations at
Dunsink of the Last Transit of Venus--The Question of an Atmosphere
to Venus--Other Determinations of the Sun's Distance--Statistics
about Venus.
It might, for one reason, have been not inappropriate to have commenced
our review of the planetary system by the description of the planet
Venus. This body is not especially remarkable for its size, for there
are other planets hundreds of times larger. The orbit of Venus is no
doubt larger than that of Mercury, but it is much smaller than that of
the outer planets. Venus has not even the splendid retinue of minor
attendants which gives such dignity and such interest to the mighty
planets of our system. Yet the fact still remains that Venus is peerless
among the planetary host. We speak not now of celestial bodies only seen
in the telescope; we refer to the ordinary observation which detected
Venus ages before telescopes were invented.
Who has not been delighted with the view of this glorious object? It is
not to be seen at all times. For months together the star of evening is
hidden from mortal gaze. Its beauties are even enhanced by the caprice
and the uncertainty which attend its appearance. We do not say that
there is any caprice in the movements of Venus, as known to those who
diligently consult their almanacs. The movements of the lovely planet
are there prescribed with a prosaic detail hardly in harmony with the
character usually ascribed to the Goddess of Love. But to those who do
not devote particular attention to the stars, the very unexpectedness of
its appearance is one of its greatest charms. Venus has not been
noticed, not been thought of, for many months. It is a beautifully clear
evening; the sun has just set. The lover of nature turns to admire the
sunset, as every lover of nature will. In the golden glory of the west a
beauteous gem is seen to glitter; it is the evening star--the planet
Venus. A few weeks later another beautiful sunset is seen, and now the
planet is no longer a point low down in the western glow; it has risen
high above the horizon, and continues a brilliant object long after the
shades of night have descended. Again, a little later, and Venus has
gained its full brilliancy and splendour. All the heavenly host--even
Sirius and even Jupiter--must pale before the splendid lustre of Venus,
the unrivalled queen of the firmament.
After weeks of splendour, the height of Venus at sunset diminishes, and
its lustre begins gradually to decline. It sinks to invisibility, and is
forgotten by the great majority of mankind; but the capricious goddess
has only moved from one side of the sky to the other. Ere the sun rises,
the morning star will be seen in the east. Its splendour gradually
augments until it rivals the beauty of the evening star. Then again the
planet draws near to the sun, and remains lost to view for many months,
until the same cycle of changes recommences, after an interval of a year
and seven months.
When Venus is at its brightest it can be easily seen in broad daylight
with the unaided eye. This striking spectacle proclaims in an
unmistakable manner the unrivalled supremacy of this planet as compared
with its fellow-planets and with the fixed stars. Indeed, at this time
Venus is from forty to sixty times more brilliant than any stellar
object in the northern heavens.
The beautiful evening star is often such a very conspicuous object that
it may seem difficult at first to realise that the body is not
self-luminous. Yet it is impossible to doubt that the planet is really
only a dark globe, and to that extent resembles our own earth. The
brilliance of the planet is not so very much greater than that of the
earth on a sunshiny day. The splendour of Venus entirely arises from the
reflected light of the sun, in the manner already explained with respect
to the moon.
We cannot distinguish the characteristic crescent shape of the planet
with the unaided eye, which merely shows a brilliant point too small to
possess sensible form. This is to be explained on physiological grounds.
The optical contrivances in the eye form an image of the planet on the
retina which is necessarily very small. Even when Venus is nearest to
the earth the diameter of the planet subtends an angle not much more
than one minute of arc. On the delicate membrane a picture of Venus is
thus drawn about one six-thousandth part of an inch in diameter. Great
as may be the delicacy of the retina, it is not adequate to the
perception of form in a picture so minute. The nervous structure, which
has been described as the source of vision, forms too coarse a canvas
for the reception of the details of this tiny picture. Hence it is that
to the unaided eye the brilliant Venus appears merely as a bright spot.
Ordinary vision cannot tell what shape it has; still less can it reveal
the true beauty of the crescent.
If the diameter of Venus were several times as great as it actually is;
were this body, for instance, as large as Jupiter or some of the other
great planets, then its crescent could be readily discerned by the
unaided eye. It is curious to speculate on what might have been the
history of astronomy had Venus only been as large as Jupiter. Were
everyone able to see the crescent form without a telescope, it would
then have been an elementary and almost obvious truth that Venus must be
a dark body revolving round the sun. The analogy between Venus and our
earth would have been at once perceived; and the doctrine which was left
to be discovered by Copernicus in comparatively modern times might not
improbably have been handed down to us with the other discoveries which
have come from the ancient nations of the East.
[Illustration: Fig. 43. Venus, May 29th, 1889.]
Perhaps the most perfect drawing of Venus that has been hitherto
obtained is that made (Fig. 43) by Professor E.E. Barnard, on 29th May,
1889, with a 12-inch equatorial, at the Lick Observatory, which for this
purpose and on this occasion Professor Barnard found to be superior to
the 36-inch. The markings shown seem undoubtedly to exist on the planet,
and in 1897 Professor Barnard writes: "The circumstances under which
this drawing was made are memorable with me, for I never afterwards had
such perfect conditions to observe Venus."
In Fig. 44 we show three views of Venus under different aspects. The
planet is so much closer to the earth when the crescent is seen, that it
appears to be part of a much larger circle than that made by Venus when
more nearly full. This drawing shows the different aspects of the globe
in their true relative proportions. It is very difficult to perceive
distinctly any markings on the brilliantly lighted surface. Sometimes
observers have seen spots or other features, and occasionally the
pointed extremities of the horns have been irregular, as if to show that
the surface of Venus is not smooth. Some observers report having seen
white spots at the poles of Venus, in some degree resembling the more
conspicuous features of the same character to be seen on Mars.
[Illustration: Fig. 44.--Different Aspects of Venus in the Telescope.]
As it is so very difficult to see any markings on Venus, we are hardly
yet able to give a definite answer to the important question as to the
period of rotation of this planet round its axis. Various observers
during the last two hundred years have from very insufficient data
concluded that Venus rotated in about twenty-three hours. Schiaparelli,
of Milan, turned his attention to this planet in 1877 and noticed a dark
shade and two bright spots, all situated not far from the southern end
of the crescent. This most painstaking astronomer watched these
markings for three months, and found that there was no change
perceptible in the position which they occupied. This was particularly
the case when he continued his watch for some consecutive hours. This
fact seemed to show conclusively that Venus could not rotate in
twenty-three hours nor in any other short period. Week after week the
spots remained unaltered, until Schiaparelli felt convinced that his
observations could only be reconciled with a period of rotation between
six and nine months. He naturally concluded that the period was 225
days--that is to say, the period which Venus takes to complete one
revolution round the sun; in other words, Venus always turns the same
face to the sun.
This remarkable result was confirmed by observations made at Nice; but
it has been vigorously assailed by several observers, who maintain that
their own drawings can only agree with a period about equal to that of
the rotation of our own earth. Schiaparelli's result is, however, well
supported by the letters of Mr. Lowell. He has published a number of
drawings of Venus made with his 24-inch refractor, and he finds that the
rotation is performed in the same time as the planet's orbital
revolution, the axis of rotation being perpendicular to the plane of the
orbit. The markings seen by Mr. Lowell were long and streaky, and they
were always visible whenever his own atmospheric conditions were fairly
good.
We have seen that the moon revolves so as to keep the same face always
turned towards the earth. We have now seen that the planets Venus and
Mercury each appear to revolve in such a way that they keep the same
face towards the sun. All these phenomena are of profound interest in
the higher departments of astronomical research. They are not mere
coincidences. They arise from the operation of the tides, in a manner
that will be explained in a later chapter.
It happens that our earth and Venus are very nearly equal in bulk. The
difference is hardly perceptible, but the earth has a diameter a few
miles greater than that of Venus. There are indications of the existence
of an atmosphere around Venus, and the evidence of the spectroscope
shows that water vapour is there present.
If there be oxygen in the atmosphere of Venus, then it would seem
possible that there might be life on that globe not essentially
different in character from some forms of life on the earth. No doubt
the sun's heat on Venus is greatly in excess of the sun's heat with
which we are acquainted, but this is not an insuperable difficulty. We
see at present on the earth, life in very hot regions and life in very
cold regions. Indeed, with each approach to the Equator we find life
more and more exuberant; so that, if water be present on the surface of
Venus and if oxygen be a constituent of its atmosphere, we might expect
to find in that planet a luxuriant tropical life, of a kind perhaps
analogous in some respects to life on the earth.
In our account of the planet Mercury, as well as in the brief
description of the hypothetical planet Vulcan, it has been necessary to
allude to the phenomena presented by the transit of a planet over the
face of the sun. Such an event is always of interest to astronomers, and
especially so in the case of Venus. We have in recent years had the
opportunity of witnessing two of these rare occurrences. It is perhaps
not too much to assert that the transits of 1874 and 1882 have received
a degree of attention never before accorded to any astronomical
phenomenon.
The transit of Venus cannot be described as a very striking or beautiful
spectacle. It is not nearly so fine a sight as a great comet or a shower
of shooting stars. Why is it, then, that it is regarded as of so much
scientific importance? It is because the phenomenon helps us to solve
one of the greatest problems which has ever engaged the mind of man. By
the transit of Venus we may determine the scale on which our solar
system is constructed. Truly this is a noble problem. Let us dwell upon
it for a moment. In the centre of our system we have the sun--a majestic
globe more than a million times as large as the earth. Circling round
the sun we have the planets, of which our earth is but one. There are
hundreds of small planets. There are a few comparable with our earth;
there are others vastly surpassing the earth. Besides the planets there
are other bodies in our system. Many of the planets are accompanied by
systems of revolving moons. There are hundreds, perhaps thousands, of
comets. Each member of this stupendous host moves in a prescribed orbit
around the sun, and collectively they form the solar system.
It is comparatively easy to learn the proportions of this system, to
measure the relative distances of the planets from the sun, and even the
relative sizes of the planets themselves. Peculiar difficulties are,
however, experienced when we seek to ascertain the actual _size_ of the
system as well as its shape. It is this latter question which the
transit of Venus offers us a method of solving.
Look, for instance, at an ordinary map of Europe. We see the various
countries laid down with precision; we can tell the courses of the
rivers; we can say that France is larger than England, and Russia larger
than France; but no matter how perfectly the map be constructed,
something else is necessary before we can have a complete conception of
the dimensions of the country. We must know _the scale on which the map
is drawn_. The map contains a reference line with certain marks upon it.
This line is to give the scale of the map. Its duty is to tell us that
an inch on the map corresponds with so many miles on the actual surface.
Unless it be supplemented by the scale, the map would be quite useless
for many purposes. Suppose that we consulted it in order to choose a
route from London to Vienna, we can see at once the direction to be
taken and the various towns and countries to be traversed; but unless we
refer to the little scale in the corner, the map will not tell how many
miles long the journey is to be.
A map of the solar system can be readily constructed. We can draw on it
the orbits of some of the planets and of their satellites, and we can
include many of the comets. We can assign to the planets and to the
orbits their proper proportions. But to render the map quite efficient
something more is necessary. We must have the scale which is to tell us
how many millions of miles on the heavens correspond to one inch of the
map. It is at this point we encounter a difficulty. There are, however,
several ways of solving the problem, though they are all difficult and
laborious. The most celebrated method (though far from the best) is that
presented on an occasion of the transit of Venus. Herein, then, lies the
importance of this rare event. It is one of the best-known means of
finding the actual scale on which our system is constructed. Observe the
full importance of the problem. Once the scale has been determined, then
all is known. We know the size of the sun; we know his distance; we know
the bulk of Jupiter, and the distances at which his satellites revolve;
we know the dimensions of the comets, and the number of miles to which
they recede in their wanderings; we know the velocity of the shooting
stars; and we learn the important lesson that our earth is but one of
the minor members of the sun's family.
As the path of Venus lies inside that of the earth, and as Venus moves
more quickly than the earth, it follows that the earth is frequently
passed by the planet, and just at the critical moment it will sometimes
happen that the earth, the planet, and the sun lie in the same straight
line. We can then see Venus on the face of the sun, and this is the
phenomenon which we call the _transit of Venus_. It is, indeed, quite
plain that if the three bodies were exactly in a line, an observer on
the earth, looking at the planet, would see it brought out vividly
against the brilliant background of the sun.
Considering that the earth is overtaken by Venus once every nineteen
months, it might seem that the transits of the planet should occur with
corresponding frequency. This is not the case; the transit of Venus is
an exceedingly rare occurrence, and a hundred years or more will often
elapse without a single one taking place. The rarity of these phenomena
arises from the fact that the path of the planet is inclined to the
plane of the earth's orbit; so that for half of its path Venus is above
the plane of the earth's orbit, and in the other half it is below. When
Venus overtakes the earth, the line from the earth to Venus will
therefore usually pass over or under the sun. If, however, it should
happen that Venus overtakes the earth at or near either of the points in
which the plane of the orbit of Venus passes through that of the earth,
then the three bodies will be in line, and a transit of Venus will be
the consequence. The rarity of the occurrence of a transit need no
longer be a mystery. The earth passes through one of the critical parts
every December, and through the other every June. If it happens that the
conjunction of Venus occurs on, or close to, June 6th or December 7th,
then a transit of Venus will occur at that conjunction, but in no other
circumstances.
The most remarkable law with reference to the repetition of the
phenomenon is the well-known eight-year interval. The transits may be
all grouped together into pairs, the two transits of any single pair
being separated by an interval of eight years. For instance, a transit
of Venus took place in 1761, and again in 1769. No further transits
occurred until those witnessed in 1874 and in 1882. Then, again, comes a
long interval, for another transit will not occur until 2004, but it
will be followed by another in 2012.
This arrangement of the transits in pairs admits of a very simple
explanation. It happens that the periodic time of Venus bears a
remarkable relation to the periodic time of the earth. The planet
accomplishes thirteen revolutions around the sun in very nearly the same
time that the earth requires for eight revolutions. If, therefore, Venus
and the earth were in line with the sun in 1874, then in eight years
more the earth will again be found in the same place; and so will Venus,
for it has just been able to accomplish thirteen revolutions. A transit
of Venus having occurred on the first occasion, a transit must also
occur on the second.
It is not, however, to be supposed that every eight years the planets
will again resume the same position with sufficient precision for a
regular eight-year transit interval. It is only approximately true that
thirteen revolutions of Venus are coincident with eight revolutions of
the earth. Each recurrence of conjunction takes place at a slightly
different position of the planets, so that when the two planets came
together again in the year 1890 the point of conjunction was so far
removed from the critical point that the line from the earth to Venus
did not intersect the sun, and thus, although Venus passed very near the
sun, yet no transit took place.
[Illustration: Fig. 45.--Venus on the Sun at the Transit of 1874.]
Fig. 45 represents the transit of Venus in 1874. It is taken from a
photograph obtained, during the occurrence, by M. Janssen. His telescope
was directed towards the sun during the eventful minutes while it
lasted, and thus an image of the sun was depicted on the photographic
plate placed in the telescope. The lighter circle represents the disc of
the sun. On that disc we see the round, sharp image of Venus, showing
the characteristic appearance of the planet during the progress of the
transit. The only other features to be noticed are a few of the solar
spots, rather dimly shown, and a network of lines which were marked on a
glass plate across the field of view of the telescope to facilitate
measurements.
The adjoining sketch (Fig. 46) exhibits the course which the planet
pursued in its passage across the sun on the two occasions in 1874 and
1882. Our generation has had the good fortune to witness the two
occurrences indicated on this picture. The white circle denotes the disc
of the sun; the planet encroaches on the white surface, and at first is
like a bite out of the sun's margin. Gradually the black spot steals in
front of the sun, until, after nearly half an hour, the black disc is
entirely visible. Slowly the planet wends its way across, followed by
hundreds of telescopes from every accessible part of the globe whence
the phenomenon is visible, until at length, in the course of a few
hours, it emerges at the other side.
It will be useful to take a brief retrospect of the different transits
of Venus of which there is any historical record. They are not numerous.
Hundreds of such phenomena have occurred since man first came on the
earth. It was not until the approach of the year 1631 that attention
began to be directed to the matter, though the transit which undoubtedly
occurred in that year was not noticed by anyone. The success of Gassendi
in observing the transit of Mercury, to which we have referred in the
last chapter, led him to hope that he would be equally fortunate in
observing the transit of Venus, which Kepler had also foretold. Gassendi
looked at the sun on the 4th, 5th, and 6th December. He looked at it
again on the 7th, but he saw no sign of the planet. We now know the
reason. The transit of Venus took place during the night, between the
6th and the 7th, and must therefore have been invisible to European
observers.
Kepler had not noticed that another transit would occur in 1639. This
discovery was made by another astronomer, and it is the one with which
the history of the subject may be said to commence. It was the first
occasion on which the phenomenon was ever actually witnessed; nor was it
then seen by many. So far as is known, it was witnessed by only two
persons.
[Illustration: Fig. 46.--The Path of Venus across the Sun in the
Transits of 1874 and 1882.]
A young and ardent English astronomer, named Horrocks, had undertaken
some computations about the motions of Venus. He made the discovery that
the transit of Venus would be repeated in 1639, and he prepared to
verify the fact. The sun rose bright on the morning of the day--which
happened to be a Sunday. The clerical profession, which Horrocks
followed, here came into collision with his desires as an astronomer. He
tells us that at nine he was called away by business of the highest
importance--referring, no doubt, to his official duties; but the service
was quickly performed, and a little before ten he was again on the
watch, only to find the brilliant face of the sun without any unusual
feature. It was marked with a spot, but nothing that could be mistaken
for a planet. Again, at noon, came an interruption; he went to church,
but he was back by one. Nor were these the only impediments to his
observations. The sun was also more or less clouded over during part of
the day. However, at a quarter past three in the afternoon his clerical
work was over; the clouds had dispersed, and he once more resumed his
observations. To his intense delight he then saw on the sun the round,
dark spot, which was at once identified as the planet Venus. The
observations could not last long; it was the depth of winter, and the
sun was rapidly setting. Only half an hour was available, but he had
made such careful preparations beforehand that it sufficed to enable him
to secure some valuable measurements.
Horrocks had previously acquainted his friend, William Crabtree, with
the impending occurrence. Crabtree was therefore on the watch, and
succeeded in seeing the transit; a striking picture of Crabtree's famous
observation is shown in one of the beautiful frescoes in the Town Hall
at Manchester. But to no one else had Horrocks communicated the
intelligence; as he says, "I hope to be excused for not informing other
of my friends of the expected phenomenon, but most of them care little
for trifles of this kind, rather preferring their hawks and hounds, to
say no worse; and although England is not without votaries of astronomy,
with some of whom I am acquainted, I was unable to convey to them the
agreeable tidings, having myself had so little notice."
It was not till long afterwards that the full importance of the transit
of Venus was appreciated. Nearly a century had rolled away when the
great astronomer, Halley (1656-1742), drew attention to the subject. The
next transit was to occur in 1761, and forty-five years before that
event Halley explained his celebrated method of finding the distance of
the sun by means of the transit of Venus.[15] He was then a man sixty
years of age; he could have no expectation that he would live to witness
the event; but in noble language he commends the problem to the notice
of the learned, and thus addresses the Royal Society of London:--"And
this is what I am now desirous to lay before this illustrious Society,
which I foretell will continue for ages, that I may explain beforehand
to young astronomers, who may, perhaps, live to observe these things, a
method by which the immense distance of the sun may be truly
obtained.... I recommend it, therefore, again and again to those curious
astronomers who, when I am dead, will have an opportunity of observing
these things, that they would remember this my admonition, and
diligently apply themselves with all their might in making the
observations, and I earnestly wish them all imaginable success--in the
first place, that they may not by the unseasonable obscurity of a cloudy
sky be deprived of this most desirable sight, and then that, having
ascertained with more exactness the magnitudes of the planetary orbits,
it may redound to their immortal fame and glory." Halley lived to a good
old age, but he died nineteen years before the transit occurred.
The student of astronomy who desires to learn how the transit of Venus
will tell the distance from the sun must prepare to encounter a
geometrical problem of no little complexity. We cannot give to the
subject the detail that would be requisite for a full explanation. All
we can attempt is to render a general account of the method, sufficient
to enable the reader to see that the transit of Venus really does
contain all the elements necessary for the solution of the problem.
We must first explain clearly the conception which is known to
astronomers by the name of _parallax_; for it is by parallax that the
distance of the sun, or, indeed, the distance of any other celestial
body, must be determined. Let us take a simple illustration. Stand near
a window whence you can look at buildings, or the trees, the clouds, or
any distant objects. Place on the glass a thin strip of paper vertically
in the middle of one of the panes. Close the right eye, and note with
the left eye the position of the strip of paper relatively to the
objects in the background. Then, while still remaining in the same
position, close the left eye and again observe the position of the
strip of paper with the right eye. You will find that the position of
the paper on the background has changed. As I sit in my study and look
out of the window I see a strip of paper, with my right eye, in front of
a certain bough on a tree a couple of hundred yards away; with my left
eye the paper is no longer in front of that bough, it has moved to a
position near the outline of the tree. This apparent displacement of the
strip of paper, relatively to the distant background, is what is called
parallax.
Move closer to the window, and repeat the observation, and you find that
_the apparent displacement of the strip increases_. Move away from the
window, and the displacement decreases. Move to the other side of the
room, the displacement is much less, though probably still visible. We
thus see that the change in the apparent place of the strip of paper, as
viewed with the right eye or the left eye, varies in amount as the
distance changes; but it varies in the opposite way to the distance, for
as either becomes greater the other becomes less. We can thus associate
with each particular distance a corresponding particular displacement.
From this it will be easy to infer that if we have the means of
measuring the amount of displacement, then we have the means of
calculating the distance from the observer to the window.
It is this principle, applied on a gigantic scale, which enables us to
measure the distances of the heavenly bodies. Look, for instance, at the
planet Venus; let this correspond to the strip of paper, and let the
sun, on which Venus is seen in the act of transit, be the background.
Instead of the two eyes of the observer, we now place two observatories
in distant regions of the earth; we look at Venus from one observatory,
we look at it from the other; we measure the amount of the displacement,
and from that we calculate the distance of the planet. All depends,
then, on the means which we have of measuring the displacement of Venus
as viewed from the two different stations. There are various ways of
accomplishing this, but the most simple is that originally proposed by
Halley.
From the observatory at A Venus seems to pursue the upper of the two
tracks shown in the adjoining figure (Fig. 47). From the observatory at
B it follows the lower track, and it is for us to measure the distance
between the two tracks. This can be accomplished in several ways.
Suppose the observer at A notes the time that Venus has occupied in
crossing the disc, and that similar observations be made at B. As the
track seen from B is the larger, it must follow that the time observed
at B will be greater than that at A. When the observations from the
different hemispheres are compared, the _times_ observed will enable the
lengths of the tracks to be calculated. The lengths being known, their
places on the circular disc of the sun are determined, and hence the
amount of displacement of Venus in transit is ascertained. Thus it is
that the distance of Venus is measured, and the scale of the solar
system is known.
[Illustration: Fig. 47.--To Illustrate the Observation of the Transit of
Venus from Two Localities, A and B, on the Earth.]
The two transits to which Halley's memorable researches referred
occurred in the years 1761 and 1769. The results of the first were not
very successful, in spite of the arduous labours of those who undertook
the observations. The transit of 1769 is of particular interest, not
only for the determination of the sun's distance, but also because it
gave rise to the first of the celebrated voyages of Captain Cook. It was
to see the transit of Venus that Captain Cook was commissioned to sail
to Otaheite, and there, on the 3rd of June, on a splendid day in that
exquisite climate, the phenomenon was carefully observed and measured by
different observers. Simultaneously with these observations others were
obtained in Europe and elsewhere, and from the combination of all the
observations an approximate knowledge of the sun's distance was gained.
The most complete discussion of these observations did not, however,
take place for some time. It was not until the year 1824 that the
illustrious Encke computed the distance of the sun, and gave as the
definite result 95,000,000 miles.
For many years this number was invariably adopted, and many of the
present generation will remember how they were taught in their
school-days that the sun was 95,000,000 miles away. At length doubts
began to be whispered as to the accuracy of this result. The doubts
arose in different quarters, and were presented with different degrees
of importance; but they all pointed in one direction, they all indicated
that the distance of the sun was not really so great as the result which
Encke had obtained. It must be remembered that there are several ways of
finding the distance of the sun, and it will be our duty to allude to
some other methods later on. It has been ascertained that the result
obtained by Encke from the observations made in 1761 and 1769, with
instruments inferior to our modern ones, was too great, and that the
distance of the sun may probably be now stated at 92,000,000 miles.
I venture to record our personal experience of the last transit of
Venus, which we had the good fortune to view from Dunsink Observatory on
the afternoon of the 6th of December, 1882.
The morning of the eventful day appeared to be about as unfavourable
for a grand astronomical spectacle as could well be imagined. Snow, a
couple of inches thick, covered the ground, and more was falling, with
but little intermission, all the forenoon. It seemed almost hopeless
that a view of the phenomenon could be obtained from that observatory;
but it is well in such cases to bear in mind the injunction given to the
observers on a celebrated eclipse expedition. They were instructed, no
matter what the day should be like, that they were to make all their
preparations precisely as they would have done were the sun shining with
undimmed splendour. By this advice no doubt many observers have
profited; and we acted upon it with very considerable success.
There were at that time at the observatory two equatorials, one of them
an old, but tolerably good, instrument, of about six inches aperture;
the other the great South equatorial, of twelve inches aperture, already
referred to. At eleven o'clock the day looked worse than ever; but we at
once proceeded to make all ready. I stationed Mr. Rambaut at the small
equatorial, while I myself took charge of the South instrument. The snow
was still falling when the domes were opened; but, according to our
prearranged scheme, the telescopes were directed, not indeed upon the
sun, but to the place where we knew the sun was, and the clockwork was
set in motion which carried round the telescopes, still constantly
pointing towards the invisible sun. The predicted time of the transit
had not yet arrived.
The eye-piece employed on the South equatorial must also receive a brief
notice. It will, of course, be obvious that the full glare of the sun
has to be greatly mitigated before the eye can view it with impunity.
The light from the sun falls upon a piece of transparent glass inclined
at a certain angle, and the chief portion of the sun's heat, as well as
a certain amount of its light, pass through the glass and are lost. A
certain fraction of the light is, however, reflected from the glass, and
enters the eye-piece. This light is already much reduced in intensity,
but it undergoes as much further reduction as we please by an ingenious
contrivance. The glass which reflects the light does so at what is
called the polarising angle, and between the eye-piece and the eye is a
plate of tourmaline. This plate of tourmaline can be turned round by the
observer. In one position it hardly interferes with the polarised light
at all, while in the position at right angles thereto it cuts off nearly
the whole of it. By simply adjusting the position of the tourmaline, the
observer has it in his power to render the image of any brightness that
may be convenient, and thus the observations of the sun can be conducted
with the appropriate degree of illumination.
But such appliances seemed on this occasion to be a mere mockery. The
tourmaline was all ready, but up to one o'clock not a trace of the sun
could be seen. Shortly after one o'clock, however, we noticed that the
day was getting lighter; and, on looking to the north, whence the wind
and the snow were coming, we saw, to our inexpressible delight, that the
clouds were clearing. At length, the sky towards the south began to
improve, and at last, as the critical moment approached, we could detect
the spot where the sun was becoming visible. But the predicted moment
arrived and passed, and still the sun had not broken through the clouds,
though every moment the certainty that it would do so became more
apparent. The external contact was therefore missed. We tried to console
ourselves by the reflection that this was not, after all, a very
important phase, and hoped that the internal contact would be more
successful.
At length the struggling beams pierced the obstruction, and I saw the
round, sharp disc of the sun in the finder, and eagerly glanced at the
point on which attention was concentrated. Some minutes had now elapsed
since the predicted moment of first contact, and, to my delight, I saw
the small notch in the margin of the sun showing that the transit had
commenced, and that the planet was then one-third on the sun. But the
critical moment had not yet arrived. By the expression "first internal
contact" we are to understand the moment when the planet has completely
entered _on_ the sun. This first contact was timed to occur twenty-one
minutes later than the external contact already referred to. But the
clouds again disappointed our hope of seeing the internal contact. While
steadily looking at the exquisitely beautiful sight of the gradual
advance of the planet, I became aware that there were other objects
besides Venus between me and the sun. They were the snowflakes, which
again began to fall rapidly. I must admit the phenomenon was singularly
beautiful. The telescopic effect of a snowstorm with the sun as a
background I had never before seen. It reminded me of the golden rain
which is sometimes seen falling from a flight of sky-rockets during
pyrotechnic displays; I would gladly have dispensed with the spectacle,
for it necessarily followed that the sun and Venus again disappeared
from view. The clouds gathered, the snowstorm descended as heavily as
ever, and we hardly dared to hope that we should see anything more; 1
hr. 57 min. came and passed, the first internal contact was over, and
Venus had fully entered on the sun. We had only obtained a brief view,
and we had not yet been able to make any measurements or other
observations that could be of service. Still, to have seen even a part
of a transit of Venus is an event to remember for a lifetime, and we
felt more delight than can be easily expressed at even this slight gleam
of success.
But better things were in store. My assistant came over with the report
that he had also been successful in seeing Venus in the same phase as I
had. We both resumed our posts, and at half-past two the clouds began to
disperse, and the prospect of seeing the sun began to improve. It was
now no question of the observations of contact. Venus by this time was
well on the sun, and we therefore prepared to make observations with the
micrometer attached to the eye-piece. The clouds at length dispersed,
and at this time Venus had so completely entered on the sun that the
distance from the edge of the planet to the edge of the sun was about
twice the diameter of the planet. We measured the distance of the inner
edge of Venus from the nearest limb of the sun. These observations were
repeated as frequently as possible, but it should be added that they
were only made with very considerable difficulty. The sun was now very
low, and the edges of the sun and of Venus were by no means of that
steady character which is suitable for micrometrical measurement. The
margin of the luminary was quivering, and Venus, though no doubt it was
sometimes circular, was very often distorted to such a degree as to make
the measures very uncertain.
We succeeded in obtaining sixteen measures altogether; but the sun was
now getting low, the clouds began again to interfere, and we saw that
the pursuit of the transit must be left to the thousands of astronomers
in happier climes who had been eagerly awaiting it. But before the
phenomena had ceased I spared a few minutes from the somewhat mechanical
work at the micrometer to take a view of the transit in the more
picturesque form which the large field of the finder presented. The sun
was already beginning to put on the ruddy hues of sunset, and there, far
in on its face, was the sharp, round, black disc of Venus. It was then
easy to sympathise with the supreme joy of Horrocks, when, in 1639, he
for the first time witnessed this spectacle. The intrinsic interest of
the phenomenon, its rarity, the fulfilment of the prediction, the noble
problem which the transit of Venus helps us to solve, are all present to
our thoughts when we look at this pleasing picture, a repetition of
which will not occur again until the flowers are blooming in the June of
A.D. 2004.
The occasion of a transit of Venus also affords an opportunity of
studying the physical nature of the planet, and we may here briefly
indicate the results that have been obtained. In the first place, a
transit will throw some light on the question as to whether Venus is
accompanied by a satellite. If Venus were attended by a small body in
close proximity, it would be conceivable that in ordinary circumstances
the brilliancy of the planet would obliterate the feeble beam of rays
from the minute companion, and thus the satellite would remain
undiscovered. It was therefore a matter of great interest to scrutinise
the vicinity of the planet while in the act of transit. If a satellite
existed--and the existence of one or more of such bodies has often been
suspected--then it would be capable of detection against the brilliant
background of the sun. Special attention was directed to this point
during the recent transits, but no satellite of Venus was to be found.
It seems, therefore, to be very unlikely that Venus can be attended by
any companion globe of appreciable dimensions.
The observations directed to the investigation of the atmosphere
surrounding Venus have been more successful. If the planet were devoid
of an atmosphere, then it would be totally invisible just before
commencing to enter on the sun, and would relapse into total
invisibility as soon as it had left the sun. The observations made
during the transits are not in conformity with such suppositions.
Special attention has been directed to this point during the recent
transits. The result has been very remarkable, and has proved in the
most conclusive manner the existence of an atmosphere around Venus. As
the planet gradually moved off the sun, the circular edge of the planet
extending out into the darkness was seen to be bounded by a circular arc
of light, and Dr. Copeland, who observed this transit in very favourable
circumstances, was actually able to follow the planet until it had
passed entirely away from the sun, at which time the globe, though
itself invisible, was distinctly marked by the girdle of light by which
it was surrounded. This luminous circle is inexplicable save by the
supposition that the globe of Venus is surrounded by an atmospheric
shell in the same way as the earth.
It may be asked, what is the advantage of devoting so much time and
labour to a celestial phenomenon like the transit of Venus which has so
little bearing on practical affairs? What does it matter whether the sun
be 95,000,000 miles off, or whether it be only 93,000,000, or any other
distance? We must admit at once that the enquiry has but a slender
bearing on matters of practical utility. No doubt a fanciful person
might contend that to compute our nautical almanacs with perfect
accuracy we require a precise knowledge of the distance of the sun. Our
vast commerce depends on skilful navigation, and one factor necessary
for success is the reliability of the "Nautical Almanac." The increased
perfection of the almanac must therefore bear some relation to increased
perfection in navigation. Now, as good authorities tell us that in
running for a harbour on a tempestuous night, or in other critical
emergencies, even a yard of sea-room is often of great consequence, so
it may conceivably happen that to the infinitesimal influence of the
transit of Venus on the "Nautical Almanac" is due the safety of a
gallant vessel.
But the time, the labour, and the money expended in observing the
transit of Venus are really to be defended on quite different grounds.
We see in it a fruitful source of information. It tells us the distance
of the sun, which is the foundation of all the great measurements of the
universe. It gratifies the intellectual curiosity of man by a view of
the true dimensions of the majestic solar system, in which the earth is
seen to play a dignified, though still subordinate, part; and it leads
us to a conception of the stupendous scale on which the universe is
constructed.
It is not possible for us, with a due regard to the limits of this
volume, to protract any longer our discussion of the transit of Venus.
When we begin to study the details of the observations, we are
immediately confronted with a multitude of technical and intricate
matters. Unfortunately, there are very great difficulties in making the
observations with the necessary precision. The moments when Venus enters
on and leaves the solar disc cannot be very accurately observed, partly
owing to a peculiar optical illusion known as "the black drop," whereby
Venus seems to cling to the sun's limb for many seconds, partly owing to
the influence of the planet's atmosphere, which helps to make the
observed time of contact uncertain. These circumstances make it
difficult to determine the distance of the sun from observations of
transits of Venus with the accuracy which modern science requires. It
seems therefore likely that the final determination of the sun's
distance will be obtained in quite a different manner. This will be
explained in Chapter XI., and hence we feel the less reluctance in
passing any from the consideration of the transit of Venus as a method
of celestial surveying.
We must now close our description of this lovely planet; but before
doing so, let us add--or in some cases repeat--a few statistical facts
as to the size and the dimensions of the planet and its orbit.
The diameter of Venus is about 7,660 miles, and the planet shows no
measurable departure from the globular form, though we can hardly doubt
that its polar diameter must really be somewhat shorter than the
equatorial diameter. This diameter is only about 258 miles less than
that of the earth. The mass of Venus is about three-quarters of the mass
of the earth; or if, as is more usual, we compare the mass of Venus with
the sun, it is to be represented by the fraction 1 divided by 425,000.
It is to be observed that the mass of Venus is not quite so great in
comparison with its bulk as might have been expected. The density of
this planet is about 0.850 of that of the earth. Venus would weigh 4.81
times as much as a globe of water of equal size. The gravitation at its
surface will, to a slight extent, be less than the gravitation at the
surface of the earth. A body here falls sixteen feet in a second; a body
let fall at the surface of Venus would fall about three feet less. It
seems not unlikely that the time of rotation of Venus may be equal to
the period of its revolution around the sun.
The orbit of Venus is remarkable for the close approach which it makes
to a circle. The greatest distance of this planet from the sun does not
exceed the least distance by one per cent. Its mean distance from the
sun is about 67,000,000 miles, and the movement in the orbit amounts to
a mean velocity of nearly 22 miles per second, the entire journey being
accomplished in 224.70 days.