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The ecliptic (or ecliptic plane) is the plane of the Earth's orbit around the Sun. From Earth, over the course of a year, the Sun appears to be moving in a line (a great circle) around the celestial sphere and the term ecliptic refers to that line around the sky, which constitutes a plane viewed from a point on it. The line can be traced by watching which stars are behind the Sun as it sets. This line passes through the well-known list of constellations used in astrology. The orbits of the solar system planets and many of the minor planets are not far from the ecliptic, with long-period comets less so. The ecliptic coordinate system is a spherical celestial coordinate system based upon the ecliptic.
Other reference planes used in astronomy include the celestial equator (a basis for the equatorial coordinate system), which is the plane through Earth's equator; the galactic plane, which is the plane through the Milky Way's galactic disk; and the supergalactic plane, which is a plane suggested by the Local Group and nearby galaxy clusters.