ecliptic, inastronomy, the great circle that is the apparent path of theSun among theconstellations in the course of ayear; from another viewpoint, theprojection on thecelestial sphere of theorbit ofEarth around the Sun. The constellations of thezodiac 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 thevernal andautumnal equinoxes.

In the ecliptic system of astronomical coordinates,celestial longitude is measured in degrees east from the vernal equinox along the ecliptic.Celestial latitude is measured in degrees north (positive) or south (negative) from the ecliptic to the ecliptic poles. Each ecliptic pole is 23.44° from the correspondingcelestial pole.