[Illustration: FIG. 23.--The Changes of the Sun with the Seasons.]
I have as yet only been speaking of the _daily_ movements by which the
sun appears to go across the heavens between morning and evening. We
next consider the annual movements which give rise to the changes of
the seasons. It is now Christmastide, when the days are short and dark,
while six months ago the days were long and glorious in the warmth
and brightness of summer. A similar recurrence of the seasons takes
place every year, and thus we learn that some great changes alter the
relation between the earth and the sun year after year. We must try and
explain this. Why is it that we enjoy warmth at one season, and suffer
from frost and snow at another?
Note first a great difference between the sun in summer and the sun in
winter. I will ask you to look out at noon any day when the clouds are
absent, and you will then find the sun at the highest point it reaches
during the day. All the morning the sun has been gradually climbing
from the east; all the afternoon it will be gradually sinking down to
the west. Let us make the same observation at different parts of the
year. Suppose we take the shortest day in December. You will look out
about twelve o’clock from some situation which affords a view towards
the south, and there, as shown in the adjoining sketch (Fig. 23), is
the midwinter sun.
But now the spring approaches, and the days begin to lengthen. If you
watch the sun you will see it pass higher and higher every noon until
Midsummer Day is reached, and then the sun at noon is found quite high
up in the sky. As autumn draws near, the sun at noon creeps downwards
again until, when the next shortest day has come round, we find that
it passes just where it did at the previous midwinter. With unceasing
regularity year after year the sun goes through these changes. When he
is high at noon we have days both long and warm; when he is low at noon
we have days both short and cold.
Vulcan with his golden boat was naturally expected to give an
explanation of this. As the summer drew on, each day Vulcan shot out
the sun with a stronger impulse, so that it should ascend higher and
higher. His greatest effort was made on Midsummer Day, when, after
rowing but a little way round from the north towards the east, he drove
off the sun with a terrific effort. The sun soared aloft to the utmost
height it could reach, and in the meantime Vulcan returned to the west
to be ready to catch the sun as it descended. On the other hand, in
midwinter, he came round much further through the east to the south,
and then shot up the sun with his feeblest effort, and had to paddle as
hard as ever he could so as to complete his long return voyage during
the brief day.
[Illustration: FIG. 24.--How the Stars are to be seen in broad
Daylight.]
It is evident that there are two quite distinct kinds of motion of the
sun. There is first the daily rising and setting, for which we have
accounted by showing that it is merely an appearance produced by the
fact that the earth is turning round. But now we have been considering
quite a different motion by which the sun seems to creep up and down in
the heavens, and this takes a whole year to go through its changes.
There is still another point which we must consider before we can
understand all these puzzling movements of the sun. We shall ask the
stars to help us by their familiar constellations. You know, perhaps,
the Great Bear, or the Plough as it is often called, and Orion. There
are also Aries the Ram, Taurus the Bull, and other fancifully named
systems. These constellations have been known for countless ages, and
for our present purposes we may think of them as permanent groups
in the heavens, which do not alter either their own shapes or their
positions relatively to each other. These groups of stars extend all
around the sky. They are not only over our heads and on all sides down
to the horizon, but if we could dig a deep hole through the earth,
coming out somewhere near New Zealand, and if we then looked through,
we should see that there was another vault of stars beneath us. We
stand on our comparatively little earth in what seems the centre of
this great universe of stars all around. It is true we do not often
see the stars in broad daylight, but they are there nevertheless. The
blaze of sunlight makes them invisible. A good telescope will always
show the stars, and even without a telescope they can sometimes be
seen in daylight in rather an odd way. If you can obtain a glimpse of
the blue sky on a fine day from the bottom of a coal pit, stars are
often visible. The top of the shaft is, however, generally obstructed
by the machinery for hoisting up the coal, but the stars may be seen
occasionally through the tall chimney attached to a manufactory when an
opportune disuse of the chimney permits of the observation being made
(Fig. 24). The fact is that the long tube has the effect of completely
screening from the eye the direct light of the sun. The eye thus
becomes more sensitive, and the feeble light from the stars can make
its impression, and produce vision. From all these various lines of
reasoning we see that there can be no doubt of the continuous presence
of stars above and around us, and below us, on every side, and at all
times.
[Illustration: FIG. 25.--The Sun seems to revolve around the Earth.]
If you look out at Christmas time, towards the south, you will see the
Belt of Orion and the Dog Star in a splendid portion of the heavens.
These stars you will see every winter in the same place. But you may
look in vain for them in summer. No doubt you can see stars in the
summer evenings, but they will be totally different from those that
adorned the skies in winter. Each season has its own constellations.
This simple fact was known to the ancients, and we shall find
its explanation full of meaning. Let us select four well-known
constellations which will best answer our purpose. They lie in a circle
round the heavens. They are Orion, Virgo, Scorpio, and Pisces. I am
supposing that you are looking out at midnight towards the south. In
December you will see Orion; in March, Virgo; in June, Scorpio; and in
September, Pisces; and then next December you will be looking at Orion
again. See what this proves. At midnight, of course, the sun is at the
other side of the earth, so that if I am looking at Orion in midwinter
the sun must be behind my back. Look at our little picture (Fig. 25).
The earth is in the middle, and the sun must be on the opposite side
to Orion. That is, the sun must be somewhere about the position I have
marked at A. In March we see Virgo in the south at midnight, when, of
course, the sun is at the other side of the earth; so that the sun must
be somewhere at B. In June Scorpio is seen, so that the sun must be at
the other side, at C. That is to say, in midsummer the sun is in that
part of the sky where Orion is situated. If, therefore, on a bright
June day we could see the stars, we should find Orion in the south.
But, of course, the light of the sun makes Orion invisible. We can,
however, see the stars by our telescopes, and on rare occasions an
eclipse of the sun will occur, by which he is temporarily extinguished,
and then we can see the stars without the help of a telescope, even
though it is daytime.
[Illustration: FIG. 26.--The Earth, however, really revolves around the
Sun.]
Thus it would seem as if the sun were first at A and then at B, C, and
D, and then began to go round again. I say it would _seem_ as if the
sun had these movements, and the ancients thought there was no doubt
about the matter. Even after it was plain that the earth turned round
on its axis so as to give the changes of day and night, it was still
thought necessary to suppose that the sun went round the earth once in
the year, in order to explain how the changes in the stars during the
different seasons were produced.
Here is another case in which we must be careful to distinguish between
what appears to be true and what is actually the case. Everything that
we undoubtedly see would be just as well explained by supposing that
the sun remained at rest, and that the earth revolved around it, as
in Fig. 26. If, for instance, the earth were at A in midwinter, then
the sun is on the opposite side to Orion, and of course at midnight we
shall be able to see Orion. So in spring the earth is at B, and we see
Virgo, and similarly in summer we have Scorpio, and in autumn Pisces.
Thus all that is actually visible could be fully accounted for by
regarding the sun as fixed in the centre, and the earth as travelling
round it from A to B, to C and to D respectively, and completing the
journey in a twelvemonth. Which idea are we to adopt? Shall we say that
the earth goes round the sun, or the sun goes round the earth?
I remember an old college story, which I cannot help giving you at this
place. It may serve to lighten what I fear you must otherwise have
thought rather a tedious part of our subject. There were three students
brought up for examination in astronomy, and they showed a lamentable
ignorance of the subject, but the examiner, being a kind-hearted man,
wished, if possible, to pass them; and so he proposed to the three
youths the very simplest question that he could think of. Accordingly,
addressing the first student, he said: “Now tell me, does the earth go
round the sun, or the sun go round the earth?” “It is--the earth--goes
round the sun.” “What do you say?” he inquired, turning rather suddenly
on the next, who gasped out: “Oh, sir--of course--it is the sun goes
round the earth.” “What do you say?” he shouted at the third unhappy
victim. “Oh, sir, it is--sometimes one way, sir, and sometimes the
other!”
But which is it? Well, we must remember that the earth is comparatively
a very little body and the sun a very big one, so it is not at all
surprising to learn that the earth goes round the sun, which remains,
practically speaking, at rest in the centre. Thus our great earth and
all it contains are continually bound in what is very nearly a circular
course round the great luminary. You will find it instructive to work
out this little sum. How fast is the earth moving, or how far do we go
in a second? We are about 93,000,000 miles from the sun, and the great
circle that we go round has a diameter twice as great as this--that is,
about 186,000,000 miles. The circumference of a circle is nearly three
and one-seventh times its diameter, and accordingly the whole length
of the voyage in the year is about 585,000,000 miles. This has to be
accomplished in 365 days, so that the daily run must be about 1,600,000
miles. We divide this by 24, to find the distance journeyed each hour,
which we find to be about 67,000 miles; and we must divide this again
by 60 to find the length covered in a minute, and by 60 again for the
progress made each second. It is truly startling to find that, night
and day, this great earth has to travel more than eighteen miles every
second in order to get round its mighty path in the allotted time.
I began this lecture about forty minutes ago, and I think from what I
have said you will be able to calculate a result that will, I dare say,
astonish you. In these forty minutes we have moved about 45,000 miles.
No doubt my lecture commenced in this hall, and in your presence; but
can I truly say I began it _here_? Well, no; I began it not here, but
at a place 45,000 miles away; but we have all been travelling together,
and the journey has been so very smooth and free from all jolts, that
we never thought anything about the motion.
I am sure many of those to whom I am now speaking have read accounts
of voyages in the Arctic regions. You have been told of the sufferings
of the crews during the long winters, amid the ice and snow; and you
have heard how, during that dismal period, there is total darkness,
for the sun never rises for weeks and months together. On the other
hand, these northern regions often present a more cheerful picture.
During midsummer, the long darkness of winter is atoned for by
perpetual sunshine. At midnight there is still the full brilliance
of day, and the sun, though low, no doubt, has not passed below the
horizon. Even in the northerly parts of Europe we can see the midnight
sun. Lord Dufferin, in his delightful narrative of a cruise, entitled
“Letters from High Latitudes,” gives an interesting illustration
of the perplexities arising from endless daylight. It appears that
everything went on happily until the fatal moment when the yacht
crossed the Arctic circle. Then it was that dire tribulation arose
among the poultry. A fine cock was the cause of the trouble. Knowing
his duty, he always liked to be particular about performing the
important task of crowing at sunrise. This he could do regularly, so
long as the yacht remained in reasonable latitudes, where the sun
behaved properly. But when they crossed the Arctic circle, the cock
was confronted with a wholly new experience. The sun never set in the
evening, and consequently never had to rise in the morning. What was
the distracted bird to do? He did everything. He burst into occasional
fits of terrific crowing at all sorts of hours, then he gave up crowing
altogether, but finding that did not mend matters, he took to crowing
incessantly. Exhaustion was succeeded by delirium, and rather than
live any longer in a universe where the sun was capable of pranks
so heartless, the indignant fowl flung himself from the vessel and
perished in the Arctic Ocean.