It was with no little impatience that James had waited for the first
appearance of the Southern Cross, the emblem of Australia. He first
noticed it when a long way north of the equator, and each evening
afterwards bestowed a considerable attention upon it. As he made
southing on his voyage the glorious constellation rose higher and
higher in the heavens. Hour after hour would he sit, marking its
progress, wondering much to see it more or less upright according to
the time of night, and more or less declining from the perpendicular at
the same returning hour of evening upon different days.
James could understand the Cross seeming to perform a circle round the
south pole of the heavens, as he had seen the Bear round the north
pole, from the rotundity of the earth and its daily motion.
It was quite natural, then, that the lad should be so enthusiastic when
he spoke of the constellation to his father in these terms:
“O that pretty Southern Cross! I like it more and more as I get down
to the south. I did feel sad to see the old Bear drop gradually, night
after night, till it seemed to sink for ever in the northern Atlantic;
but I have a new friend here, that keeps rising higher and higher each
night, as if to welcome us to our southern home. How it would please
many boys in Europe to see the Cross!
But it was seen in Europe formerly.
Another puzzle, dear father. Did it run away from the south pole to
have a look at the north one?
It was not a run, but a gradual slide out of its place, and as gradual
a slide back into it again.
What a queer trick! Is it on its rambles now, I wonder?
It is always on the move.
Yes; I know it describes a circle in the sky once in twenty-four hours,
but that is owing to the world rolling over to get its daily light.
Yet it has another motion; not really its own any more than the other.
And this leads it further away from the central point over our south
pole, and then brings it to its place again.
Well, I am glad it is here now for me. Will there be time for my London
cousins to catch a glance at it when it wanders northward again?
O no, no; it does not move quite so fast as that. But if Abraham had
been sojourning in southern Europe, instead of Asia, he could have seen
it, though it was then making its way back to the south.
How could the Cross slip away from the other southern stars to go on
such migrations?
Not so. All the stars keep their places relatively to each other, as
you see them do in the nightly progress from east to west. In the daily
motion none get before the other, nor did the Cross get before its
neighbours.
Well, I am fairly done. That is a riddle.
Look at the question, boy. What makes the apparent daily motion of the
stars?
The real daily motion of the earth.
And if, then, you observe any other peculiarity of movement among your
bright friends up there, to what may you reasonably ascribe it?
I should imagine some peculiar twist, roll, or slipping of this world
of ours.
True. And if there be seen among the polar stars, north as well as
south, a slight but regular movement, a sort of a swing round, so that
some stars get farther off the spot we call the south pole of the
heavens, while others approached it nearer, and yet so swung round that
at last all find their old places again, how could you get a motion of
the earth to make up for all that?
Ah! I must contrive that the world should do three things. It must roll
over once a day, roll round the sun once a year, and yet wriggle about
in another way at the same time.--Stop a bit--I have it. My top turns
round itself, and pretty quickly, too; it shifts about on the floor
almost in a circle as it turns round; and I see it, especially when it
is quietly spinning, have a slow roll of the head, like a sailor when
walking ashore, as if its head were a little giddy.
You have hit it exactly, my good fellow. The three motions of your
top are much like the three motions of the world. That top-heavy slow
swinging of your top while it is spinning in full force is like a sort
of head-rolling of the earth. The poles seem to have a roll of their
own, independent of the regular roll.
Yes; and that would make the Polar stars seem as though they were
swinging. But how long are they before they are in their regular places
again?
Astronomers calculate 25,850 years.
But will not this changing make the star-charts of the ancients all
wrong?
Indeed it does. Aries the Ram, for instance, as a _sign_, is the first
thirty degrees on the celestial globe; but as a _constellation_ it has
shifted to between the thirtieth and sixtieth degree on the ecliptic.
This is called the precession of the Equinoxes.
How is that? I know when the equinoxes are--March 21st and September
23rd--when it is equal night all over the world.
The European vernal equinox took place at the first point of Aries. But
by this precession, or more properly recession, or going backward of
the stars, the Equinox takes place when the sun is in Taurus the Bull,
two thousand years after it was in the Ram.
I see. The stars not only get earlier four minutes a-day to accommodate
the earth in its annual motion, but make a change to accommodate the
swinging of its pole.
Can you tell me, James, how much the stars slip back in a year?
Let me calculate it. There are 360 degrees in a circle, and sixty
times as many minutes; that is, 21,600. As it is 25,000 years in the
circle, the stars would not shift one minute of distance, and the sun’s
apparent size in the heavens is thirty minutes, or half-a-degree.
By this Precession a star comes to the same spot about twenty minutes
later every year. It is, as it were, sliding back. In Europe, James,
one effect has been to bring what we call the Polar star within one
degree of the north pole of the heavens; whereas, a few thousand
years ago it was twelve degrees off it. The bright star of the Lyre
Constellation will some day be the North Polar star.
Ah, but in 25,000 years the present Polar star will be in its old
place. The end of the little Bear’s tail seems now fastened on the
north pole; but it must be a sort of greasy pole for the stars to slip
off it as soon as they get near it.”