Page:Astronomy for Everybody.djvu/53

equator the degree of longitude is about 69½ statute miles, but at the latitude of 45° it is only about 42 miles. At 60° it is less than 35 miles, at the pole it comes down to nothing, because there the meridians meet.

We may see that the speed of the rotation of the earth follows the same law of diminution. At the equator, 15° is about 1,000 miles. We may therefore see that, in that part of the earth, the latter revolves at the rate of 1,000 miles an hour. This is about 1,500 feet per second. But in latitude 45° the speed is diminished to little more than 1,000 feet per second. At 60°, north, it is only half that at the equator; at the poles it goes down to nothing.

In applying this system the only trouble arises from the earth's rotation. As long as we do not travel, we remain on the same circle of longitude on the earth. But by the rotation of the earth, the right ascension of any point in the sky which seems to us fixed, is continually changing. The only difference between the celestial meridian and an hour circle is that the former travels round with the earth, while the latter is fixed on the celestial sphere.

There is a strict resemblance in almost every point between the earth and the celestial sphere. As the former revolves on its axis from west to east, the latter seems to revolve from east to west. If we imagine the earth centred inside the celestial sphere with a common axis passing through them, as shown in the figure, we shall have a clear idea of the relations we wish to set forth.

If the sun, like the stars, seemed fixed on the celestial