ELLIPTICAL ORBIT

he reasons for this yearly variation in the apparent motion of the Sun are twofold. The first reason has to do with the fact that the Earth's orbit is not a perfect circle, but is elliptical with the Sun being nearer one end of the ellipse. The speed of the Earth in this elliptical orbit varies from a minimum at the farthest distance to a maximum at the closest distance of the Earth to the Sun. The second reason for the yearly variation has to do with the fact that the Earth's equator is inclined to the plane of the Earth's orbit around the Sun. These two effects are explained in the following paragraphs.

1. Elliptical Orbit. While the Earth is rotating upon its axis, it is also moving around the Sun in the same sense, or direction, as its rotation. If we select a spot on the Earth where the Sun is directly overhead, in order for that spot to rotate with the Earth and come back so that the Sun is overhead again, it must turn a little extra because of the Earth's motion around the Sun. The Earth turns a little more than once with respect to the stars in order to complete one rotation with respect to the Sun. The "little extra" is just the angle through which the Earth has moved around the Sun in a day's time. On the average, this angle amounts to a little less than one degree per day (360 degrees/ 365 ¼ days) and is illustrated in Figure 1.

Figure 1. The Earth must rotate 360 degrees plus a, a very small angle, for observer at A to return to the same position relative to the Sun at B.