ECLIPTIC, in astronomy, is the great circle on the celestial sphere which forms the apparent path of the sun in the course of the year. The twelve constellations or signs of the zodiac are arranged along the ecliptic. The plane of the ecliptic is the plane of the earth's orbit, or more strictly the plane in which the centre of gravity of the earth and moon revolves round the sun ; it meets the celestial sphere in the great circle above mentioned.