Ecliptic

Fig. 1. The ecliptic is red.

In astronomy, the ecliptic is the great circle that is the intersection of the ecliptic plane and the celestial sphere. The ecliptic plane contains the Earth and the apparent orbit of the Sun (as seen from the Earth); see Fig. 1. Equivalently, the ecliptic plane may be defined as the plane of the Sun and the orbit of the Earth; see Fig. 2.

Fig. 2. The Earth in its orbit around the Sun.

The Earth is the center of the celestial sphere and the celestial equator is a circle in the same plane as the equator of the Earth. The plane of the equator makes an angle of approximately 23.43⁰ with the ecliptic plane; this angle is known as the obliquity of the ecliptic. In Fig. 2 one sees that it is the complement of the angle (66.57 ⁰) that the rotation axis (North-South axis) of the Earth makes with the ecliptic plane.

The intersections of the equator and the ecliptic are the vernal equinox (the date in the spring of the Northern hemisphere that day and night are equally long, around March 21) and the autumnal equinox (the date in the fall that day and night are equally long, around September 21). The solstices are the dates that the Sun seems to stand still (around June 21 and December 21).