The Ecliptic

The ecliptic is the apparent path of the Sun against the background stars. This apparent movement of the Sun across the sky is an artifact of the Earth's orbital motion around the Sun. In the image below, the narrow dark line curving across the centre of the graph is the ecliptic. It is surrounded by a much broader light grey strip (approximately 9° on either side) known as the band of the zodiac. The ecliptic passes through thirteen constellations: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Ophiuchus, Sagittarius, Capricornus, Aquarius and Pisces. The band of the zodiac passes through the official boundaries of nine additional constellations: Auriga, Cetus, Corvus, Crater, Hydra, Orion, Pegasus, Scutum and Sextans.

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The plane of the ecliptic is an imaginary plane or flat surface defined by the Earth's orbit around the Sun. The planets and many of the small solar system bodies like asteroids have orbits near this plane. For this reason, the planets' movements against the background stars are always within the band of the zodiac. The orbit of the Moon around the Earth is inclined about 5° to the ecliptic so it too remains within the band of the zodiac.

Current theories suggest that the solar system bodies formed from a spinning, flattened, proto-planetary disc billions of years ago. This is why the major bodies (the planets) and many of the smaller objects orbit around the Sun in more or less the same plane. However, some objects, notably long-period comets which may originate in the Oort Cloud, can have very high inclinations to the ecliptic, and can appear in any part of the sky.

The ecliptic is used as a reference plane in some astronomical coordinate systems. The intersection of the ecliptic and the terrestrial equatorial plane defines the zero point for both ecliptic longitude and right ascension. However, the ecliptic is not fixed. Perturbations are caused by other planets, particularly the large gas giants. There are both short- and long-period variations. For this reason, when using the ecliptic as the basis of a coordinate system, it is customary to adopt the instantaneous ecliptic at a specific epoch (time).

There does exist a fixed plane for use in astronomical coordinate systems. This is the invariable plane of the solar system which is defined as the plane normal to the total angular momentum vector of the solar system.