Page:Astronomy for Everybody.djvu/58

This page has been proofread, but needs to be validated.
34
THE CELESTIAL MOTIONS

bore the same names as the constellations among which they were situated. This is not the case at present, owing to the slow motion of precession soon to be described.

It will be seen that the two great circles we have described spanning the entire celestial sphere are fixed in entirely different ways. The equator is determined by the direction in which the axis of the earth points, and spans the sphere midway between the two celestial poles. The ecliptic is determined by the earth's motion around the sun.

These two circles do not coincide, but intersect each other at two opposite points, at an angle of twenty-three and a half degrees, or nearly one quarter of a right angle. This angle is called the obliquity of the ecliptic. To understand exactly how it arises we must mention a fact about the celestial poles; from what we have said of them it will be seen that they are not determined by anything in the heavens, but by the direction of the earth's axis only; they are nothing but the two opposite points in the heavens which lie exactly in the line of the earth's axis. The celestial equator, being the great circle halfway between the poles, is also fixed by the direction of the earth's axis and by nothing else.

Let us now suppose that the earth's orbit around the sun is horizontal. We may in imagination represent it by the circumference of a round level platform with the sun in its centre. We suppose the earth to move around the circumference of the platform with its cen-

This article is issued from Wikisource. The text is available under Creative Commons Attribution-Share Alike 4.0 unless otherwise noted. Additional terms may apply for the media files.