Astronomy for Young Australians. James Bonwick
is, it seems to me that we have been going up a hill, if the stars do not move.
That will account for your Great Bear going down; but what should cause these new stars to appear in the south?
Why, that looks as though we were getting up to the top of the hill, and should have to go down on the other side.
What shape, then, do you suppose the world ought to be, to account for these curious apparent motions of the stars?
I fancy it ought to be round, and that is what I have always been told it is, father.
I want to ask you whether you have noticed anything peculiar about the apparent daily motion of the stars. Do they all rise in the east and pass over to the west?
This is just the subject I wanted to speak about. I thought at first that they did so, and ought to do so. But I have since found out that this is not the case. They do not all describe the same sized circle.
What have you observed?
Some make a regular sweep across—others only rise to pop down again. Yet there is a more wonderful thing. The stars near the North Polar star perform their circle without going under at all. Then the greatest puzzle is, that the Polar star always keeps its place without any turning.
You do not think that the stars describe circles each day of different sizes according to their distances from the Polar star?
No. I feel sure that it is all a cheat of the eye, though I don’t see where the cheat is.
I thought you were quite settled in your mind that the apparent motion of the stars was owing to the revolution of the earth.
True, father; yet I see no connection between the movement of the earth and the stationary place of the Polar star.
What appearance would the heavenly bodies present, were the earth like a big drum rolling over?
Why, I should fancy that then they would all rise in the east and pass over, in an orderly, proper manner, to the west. Though I have been told that the earth is round I do not understand how that should affect the motion of all the stars, as we see them.
Come, let us see. If the world be round, then some parts would describe a less circle than others in the daily revolution.
Of course, father.
Would there be any portion that would not turn at all?
Yes; the poles, or extremities.
Where would be the most motion?
At the equator.
Then, if the stars be still, and the earth move and be round, would not—
O, I know all about it now. The stars over the pole would appear still, because the pole is still. The stars over England would appear to describe a larger circle than those of the Great Bear over Sweden. Where we are now, the stars overhead describe a very large circle.
And at the equator the greatest circle of all?
Yes, yes—and I can now see why the Great Bear seems to go away from us, as we get up the hill toward the equator. That constellation is opposite to a part of the earth a good way to the north of us. As we rise higher from that place, we must appear to rise higher than the stars over it. How I do long to stand under that bright Belt of Orion, and under Sirius! And how I should like to see the pretty Cross stretched over the mast-head!
Wait a bit, my lad, and you may see all these wonders in time.
How delighted I am to make out that all those funny motions among the stars are simply owing to the world being round.”
SIZE OF THE WORLD
It was while half tired of ship life already, and longing for the excitements of shore in a new country, that the boy turned round to his father, and said, with a sort of yawn—
“What a long while we shall be getting to Australia, father.
It is a great distance to go, James.
Well, I think the earth must be a good big ball to roll about among the stars.
You ought to know its size.
I have been told that it is 25,000 miles round; but who has measured it.
Why as to that, you and I might measure it some evening by the stars. You know that the great space between the Belt and Polar Star is one-quarter of the great circle of the heavens.
Of course.
Every circle is reckoned to be divided into spaces called degrees, of which 360 go to form the circumference; so that one-fourth will be ninety degrees. You can imagine that space divided into ninety of these portions, called degrees.
What rare compasses that would take!
Those two bright stars overhead, which are about twice the same distance apart as the apparent diameter of the Sun, would be nearly equal to one of these ninety portions. Now, you know that if you were at the pole, you would have the Polar Star above you; and, if at the equator, Orion’s Belt would be over your head.
I understand all that, father.
You know, then, that you might be in a place where one of those two bright stars would be overhead, and you might journey on to another place further south, where the other one would be at the zenith or overhead.
This is all clear to me.
Then if you measured the ground you had gone over, it would be equal to one-ninetieth part of the space between the pole and the equator, or about seventy miles. Can you tell how many miles it would be from the pole to the equator?
That would be seventy times ninety—which is 6300.
But that is only one-quarter of the way round—is it not?
Yes; the whole distance round would be four times 6300, or about 25,000 miles.
Well done—we have measured the circumference of the world.”
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