The zodiac is a belt-shaped region of the sky that extends approximately 8° north and south (as measured in celestial latitude) of the ecliptic, the apparent path of the Sun across the celestial sphere over the course of the year. Also within this zodiac belt appear the Moon and the brightest planets, along the their orbital planes.[1] The zodiac is divided along the ecliptic into 12 equal parts ("signs"), each occupying 30° of celestial longitude. These signs roughly correspond to the astronomical constellations with the following modern names:[2][3] Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces.
The signs have been used to determine the time of the year by identifying each sign with the days of the year the Sun is in the respective sign. In Western astrology, and formerly astronomy, the time of each sign is associated with different attributes. Though the zodiacal system and its angular measurement in 360 sexagesimal degree (°) originated with Babylonian astronomy during the 1st millennium BC. It was only then communicated into Greek astronomy by the 2nd century BC, as well as into developing the Hindu zodiac. Due to the precession of the equinoxes, the time of year the Sun is in a given constellation has changed since Babylonian times, and the point of March equinox has moved from Aries into Pisces.
The zodiac form a celestial coordinate system, or more specifically an ecliptic coordinate system, which takes the ecliptic as the origin of latitude and the Sun's position at vernal equinox as the origin of longitude.[4] In modern astronomy, the ecliptic coordinate system is still used for tracking Solar System objects.
The English word zodiac derives from zōdiacus,[5] the Latinized form of the Ancient Greek zōdiakòs kýklos (ζῳδιακός κύκλος),[6] meaning "cycle or circle of little animals". Zōdion (ζῴδιον) is the diminutive of zōon (ζῷον, "animal").[7] The name reflects the prominence of animals (and mythological hybrids) among the twelve signs.
The zodiac was 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.[8] The construction of the zodiac is described in Ptolemy's comprehensive 2nd century AD work, the Almagest.[9]
Although the zodiac remains the basis of the ecliptic coordinate system in use in astronomy besides the equatorial one,[10][11] the term and the names of the twelve signs are today mostly associated with horoscopic astrology.[12] 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 8 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 5° above and below the ecliptic. By extension, the "zodiac of the comets" may refer to the band encompassing most short-period comets.[13]