Читать книгу Astronomy for Young Australians - James Bonwick - Страница 7

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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.”