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The Earth in its orbit around the Sun causes the Sun to appear on the celestial sphere moving over the ecliptic (red), which is tilted with respect to the equator (blue).
In both astrology and historical astronomy, the zodiac (Greek: ζῳδιακός, zōdiakos) is a circle of twelve 30° divisions of celestial longitude that are centered upon the ecliptic: the apparent path of the Sun across the celestial sphere over the course of the year. The paths of the Moon and visible planets also remain close to the ecliptic, within the belt of the zodiac, which extends 8-9° north or south of the ecliptic, as measured in celestial latitude. Historically, these twelve divisions are called signs. Essentially, the zodiac is a celestial coordinate system, or more specifically an ecliptic coordinate system, which takes the ecliptic as the origin of latitude, and the position of the sun at vernal equinox as the origin of longitude.
It is known to have been in use by the Roman era, based on concepts inherited by Hellenistic astronomy from Babylonian astronomy of the Chaldean period (mid-1st millennium BC), which, in turn, derived from an earlier system of lists of stars along the ecliptic. The construction of the zodiac is described in Ptolemy’s Almagest (2nd century AD).
The term zodiac derives from Latin zōdiacus, which in its turn comes from the Greek ζῳδιακὸς κύκλος (zōdiakos kuklos), meaning “circle of animals”, derived from ζώδιον (zōdion), the diminutive of ζῶον (zōon) “animal”. The name is motivated by the fact that half of the signs of the classical Greek zodiac are represented as animals (besides two mythological hybrids).
Although the zodiac remains the basis of the ecliptic coordinate system in use in astronomy besides the equatorial one, the term and the names of the twelve signs are today mostly associated with horoscopic astrology.
The term “zodiac” may also refer to the region of the celestial sphere encompassing the paths of the planets corresponding to the band of about eight arc degrees above and below the ecliptic. The zodiac of a given planet is the band that contains the path of that particular body; e.g., the “zodiac of the Moon” is the band of five degrees above and below the ecliptic. By extension, the “zodiac of the comets” may refer to the band encompassing most short-period comets.
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