Ecliptic, in astronomy, the great circle that is the apparent path of the Sun among the constellations in the course of a year; from another viewpoint, the projection on the celestial sphere of the orbit of Earth around the Sun. The constellations of the zodiac are arranged along the ecliptic. The ecliptic is inclined at 23.44° to the plane of the celestial equator; this inclination is called the obliquity of the ecliptic. The two points of intersection of the ecliptic and the plane mark the vernal and autumnal equinoxes. In the ecliptic system of astronomical coordinates, celestial longitude is measured ....