The mean plane of the Earth's orbit around the Sun. The name arises because eclipses of the Sun or Moon can occur only when the Moon passes through this plane.
From the point of view of an observer on Earth, the relative orbital motion of the Earth and Sun makes it look as if the Sun were travelling around the Earth once a year. The Sun's path on the celestial sphere traces out the ecliptic plane, and is often marked as the ecliptic on celestial charts.
Since the orbits of the other planets are inclined to the ecliptic plane by only very small angles, their observed positions in the sky are always close to the ecliptic. The band of constellations through which the ecliptic passes defines the traditional zodiac, though the effects of precession and the precise definition of constellation boundaries mean that the ecliptic now passes through an additional constellation, Ophiuchus.
