Page:Astronomy for Everybody.djvu/59

tre on the level of the platform; then, if the earth's axis were vertical, its equator would be horizontal and on a level with the platform and therefore would always be directed toward the sun in its centre, as the earth made its annual course around the platform. Then, on the celestial sphere, the ecliptic determined by the course of the sun would be the same circle as the equator. The obliquity of the ecliptic arises from the fact that the earth's orbit is not vertical, as just supposed, but is in-

clined twenty-three and a half degrees. The ecliptic has the same inclination to the plane of the platform; thus the obliquity is the result of the inclination of the earth's axis. An important fact connected with the subject is that, as the earth makes its revolutions around the sun, the direction of its axis remains unchanged in space; hence its north pole is tipped away from the sun or toward it, according to its position in the orbit. This is shown in Figure 6, which represents the platform we have supposed, with the axis tipped toward the right hand. The north pole will always be tipped in this