Page:Astronomy for Everybody.djvu/63

clock keeps time with the apparent rotation of the celestial sphere, so that the astronomer has only to look at his clock to see, by day or by night, what stars are on the meridian and what the positions of the constellations are.

If the earth's axis were perpendicular to the plane of the ecliptic, the latter would coincide with the equator, and we should have no difference of seasons the year round. The sun would always rise in the exact east and set in the exact west. There would be only a very slight change in the temperature arising from the fact that the earth is a little nearer the sun in January than in July. Owing to the obliquity of the ecliptic it follows that, while the sun is north of the equator, which is the case from March to September, the sun shines upon the northern hemisphere during a greater time of each day and at a greater angle, than on the southern hemisphere. In the southern hemisphere the opposite is the case. The sun shines longer from September till March than it does on the northern hemisphere. Thus we have winter in the northern hemisphere when it is summer in the southern, and vice versa.

Before going farther let us recapitulate the phenomena we have described from the two points of view: one that of the real motions of the earth; the other that of the apparent motions of the heavens, to which the real motions give rise.