Page:Popular Astronomy - Airy - 1881.djvu/101

Rh through the Poles must be about 7899 miles; that passing through the equator about 7926 miles. Therefore we may consider it as established, that the form of the earth is spheroidal.

The next thing we have to consider is, what inference we are to draw from that, in reference to the movement of the earth. By a rough experiment it was shown, that if we take any circular substance that is susceptible of a change of shape, and whirl it round an axis, it will change from a circle into an oval; we think, therefore, that even supposing we had nothing else to guide us, there is good reason to infer, from the oval shape of the earth, that it does turn upon its axis. But, in addition to this, we see the sun, moon, and stars, every day turning from east to west. We know (by the duration of the lunar eclipses) that the distance of the moon is considerable; and (by the fact, that solar eclipses and occultations of stars do not extend over all the earth) that the distances of the sun and stars are very much greater; we also see that the system of stars appears to move all in a piece; we judge it unlikely that these distant bodies should thus revolve round the earth every day. And we conclude that the apparent movement is caused by the earth's turning from west to east. It is worth mentioning, that the planet Jupiter, the largest planet of the system, turns visibly round its axis in a shorter time than the earth. You may suppose, then, that Jupiter is much more flattened by the velocity of his rapid rotation than the earth; and indeed you can see it at once with a telescope without the aid of a micrometer, the equatoreal being to the Polar diameter as 16 to 15, nearly—a proportion which makes the former measure 5000 miles greater than the latter.