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In both astrology and historical astronomy, the zodiac (Greek: ????????, zoidiakos) 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.

Usage
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 zodiacus, which in its turn comes from the Greek ???????? ?????? (zoidiakos kuklos), meaning "circle of animals", derived from ?????? (zoidion), the diminutive of ???? (zoion) "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.

Zodiacal constellations
It is important to distinguish the zodiacal signs from the constellations associated with them, not only because of their drifting apart due to the precession of equinoxes but also because the physical constellations by nature of their varying shapes and forms take up varying widths of the ecliptic. Thus, Virgo takes up fully five times as much ecliptic longitude as Scorpius. The zodiacal signs, on the other hand, are an abstraction from the physical constellations designed to represent exactly one twelfth of the full circle each, or the longitude traversed by the Sun in about 30.4 days.
 
There have always been a number of "parazodiacal" constellations that are also touched by the paths of the planets. The MUL.APIN lists Orion, Perseus, Auriga, and Andromeda. Furthermore, there are a number of constellations mythologically associated with the zodiacal ones : Piscis Austrinus, The Southern Fish, is attached to Aquarius. In classical maps, it swallows the stream poured out of Aquarius' pitcher, but perhaps it formerly just swam in it. Aquila, The Eagle, was possibly associated with the zodiac by virtue of its main star, Altair.[citation needed] Hydra in the Early Bronze Age marked the celestial equator and was associated with Leo, which is shown standing on the serpent on the Dendera zodiac. Corvus is the Crow or Raven mysteriously perched on the tail of Hydra.
 
Due to the constellation boundaries being redefined in 1930 by the International Astronomical Union, the path of the ecliptic now officially passes through thirteen constellations: the twelve traditional 'zodiac constellations' plus Ophiuchus, the bottom part of which interjects between Scorpio and Sagittarius. Ophiuchus is an anciently recognized constellation, catalogued along with many others in Ptolemy's Almagest, but not historically referred to as a zodiac constellation.
 
The technically inaccurate description of Ophiuchus as a sign of the zodiac dates to the 1970s. This drew prominent media attention on 20 January 1985, following the BBC's opening 'Nine o'clock News' announcement that "an extra sign of the zodiac has been announced by the Royal Astronomical Society". Investigation into the source of the story revealed there had been no such announcement, and that the report had merely sensationalized the 67-year-old 'news' of the IAU's decision to alter the number of designated ecliptic constellations. This was done for the purposes of promoting a forthcoming BBC astronomy program, presented by the RAS's Public Relation Officer, which touched upon the topic of precession. The false assumption that Ophiuchus constitutes an astrological sign periodically resurfaces in the media, due to public misconception and failure to appreciate that the irregular astronomical demarcation of visible constellations does not relate to the separate frame of reference provided by the equally spaced twelve-fold longitude division of the ecliptic into zodiacal signs.

In modern astronomy
Epoch (astronomy)
The zodiac is a spherical celestial coordinate system. It designates the ecliptic as its fundamental plane and the position of the Sun at Vernal equinox as its prime meridian.
 
In astronomy, the zodiacal constellations are a convenient way of marking the ecliptic (the Sun's path across the sky) and the path of the moon and planets along the ecliptic. Modern astronomy still uses tropical coordinates for predicting the positions the Sun, Moon, and planets, except longitude in the ecliptic coordinate system is numbered from 0° to 360°, not 0° to 30° within each sign. Longitude within individual signs was still being used as late as 1740 by Jacques Cassini in his Tables astronomiques.
 
Zodiac is also used to refer to the zodiacal cloud of dust grains that move among the planets and the zodiacal light that originates from their scattering of sunlight.
 
Unlike the zodiac signs in astrology, which are all thirty degrees in length, the astronomical constellations vary widely in size. The boundaries of all the constellations in the sky were set by the International Astronomical Union (IAU) in 1930. This was, in essence, a mapping exercise to make the work of astronomers more efficient, and the boundaries of the constellations are not therefore in any meaningful sense an 'equivalent' to the zodiac signs. Along with the twelve original constellations, the boundaries of a thirteenth constellation, Ophiuchus (the serpent bearer), were set by astronomers within the bounds of the zodiac.

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