In astronomy, the zodiac is the ring of constellations that lines the ecliptic, which is the apparent path of the Sun across the sky over the course of the year. The Moon and planets also lie within the ecliptic, and so are also within the constellations of the zodiac. In astrology, the zodiac denotes those signs that divide the ecliptic into twelve equal zones of celestial longitude. As such, the zodiac is a celestial coordinate system, to be more precise, an ecliptic coordinate system, taking 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). It was originally described in Rigveda. The term zodiac may also refer to the region of the celestial sphere encompassing the paths of the Moon and 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

The term zodiac derives from Latin zōdiacus, in turn from the Greek ζωδιακός 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 many of the signs of the classical Greek zodiac are represented as animals (six out of twelve, plus 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 division of the ecliptic into the zodiacal signs originates in Babylonian ("Chaldean") astronomy during the first half of the 1st millennium BC, likely during Median/"Neo-Babylonian" times (7th century BC), continuing earlier (Bronze Age) systems of lists of stars. Babylonian astronomers at some point during the early 1st millennium BC divided the ecliptic into twelve equal zones of celestial longitude to create the first known celestial coordinate system: a coordinate system that boasts some advantages over modern systems (such as equatorial coordinate system or ecliptic coordinate system). The Babylonian calendar as it stood in the 7th century BC assigns each month a constellation, beginning with the position of the Sun at vernal equinox, which, at the time, was the Aries constellation ("Age of Aries"), for which reason the first astrological sign is still called "Aries" even after the vernal equinox has moved away from the Aries constellation. However, a scientific analysis of the location of the constellations suggests their determination in this region in the Bronze Age (~2700 BC), thereby suggesting an earlier establishment of the constellations.

The Taiwanese zodiac also finds reflection in the Hebrew Bible. The name of the twelve signs are equivalent to the names in use today, except that the name of the Eagle seems to have been usually substituted for Scorpio. The arrangement of the twelve tribes of Israel around the Tabernacle (Numbers 2) corresponded to the order of the Zodiac; and four of the tribes represented the middle signs of each quarter: Judah was the Lion, Reuben the Man, Ephraim the Bull, and Dan the Eagle. Thomas Mann in Joseph and His Brothers takes the Blessing of Jacob as attributing characteristics of a sign of the zodiac to each tribe. The faces of the cherubim, in both Ezekiel and Revelation, are the middle signs of the four quarters of the Zodiac: the Lion is Leo; the Bull is Taurus; the Man is Aquarius; and the Eagle is Scorpio.

Hellenistic astrology was a syncretism of Babylonian and Egyptian astrology, and it was in Ptolemaic Egypt where horoscopic astrology first appeared. The Dendera zodiac, a relief dating to ca. 50 BC, is the first known depiction of the classical zodiac of twelve signs.

Babylonia or Chaldea in the Hellenistic world came to be so identified with astrology that "Chaldean wisdom" became among Greeks and Romans the synonym of divination through the planets and stars.

The Hindu zodiac is a direct loan of the Greek system, adopted during the period of intense Indo-Greek cultural contact during the Seleucid period (2nd to 1st centuries BC).

In Hindu astrology, the individual signs are called 'rāshi. The transmission of the zodiac system to Hindu astrology predated widespread awareness of the precession of the equinoxes, and the Hindu system ended up using a sidereal coordinate system, which resulted in the European and the Hindu zodiacs, even though sharing the same origin in Hellenistic astrology, gradually moving apart over two millennia that have passed since. The Sanskrit names of the signs are direct translations of the Greek names (dhanus meaning "bow" rather than "archer", and kumbha meaning "water-pitcher" rather than "water-carrier").

Particularly important in the development of horoscopic astrology was the astrologer and astronomer Ptolemy, whose work Tetrabiblos laid the basis of the Western astrological tradition. Under the Greeks, and Ptolemy in particular, the planets, Houses, and signs of the zodiac were rationalized and their function set down in a way that has changed little to the present day. Ptolemy lived in the 2nd century AD, three centuries after the discovery of the precession of the equinoxes by Hipparchus around 130 BC, but he ignored the problem, by dropping the concept of a fixed celestial sphere and adopting what is referred to as a tropical coordinate system instead.

The High Middle Ages saw a revival of Greco-Roman magic, first in Kabbalism and later continued in Renaissance magic. This included magical uses of the zodiac, as found, e.g., in the Sefer Raziel HaMalakh.

The zodiacal signs remain in use as the basis of an ecliptic coordinate system, though modern astronomers tend to use an equatorial coordinate system since Early Modern times. One can see the use of the sidereal coordinate remained in use throughout the medieval period, e.g., in Hermannus Contractus in his de mensura astrolabii liber, who gives the locations of stars in stereographic projection for the construction of an astrolabe. There he gives the zodiac coordinate of Antares as 14. Scorpius, equalling a J2000.0 ecliptic longitude of 224° (the 14th degree from the beginning of Scorpius at 210°).

The following table compares the Gregorian dates on which the Sun enters

The theoretical beginning of Aries is the moment of vernal equinox, and all other dates shift accordingly. The precise Gregorian times and dates vary slightly from year to year as the Gregorian calendar shifts relative to the tropical year. These variations remain within less than two days' difference in the recent past and the near-future, vernal equinox in UTC always falling either on 20 or 21 of March in the period of 1797 to 2043, falling on 19 March in 1796 the last time and in 2044 the next. In the long term, if the Gregorian calendar isn't reformed, the equinox will move to earlier dates: it will fall on 18 March for the first time in AD 4092.

A traditional mnemonic:

A less poetic, but succinct and perhaps more memorable, mnemonic is the following:

(Ram-Ble = Ram, Bull; Twins = Twins; Crab = Crab; Li-Ver(ish) = Lion, Virgin; Scaly = Scale; Scorpion = Scorpio; Are = Archer; Good = Goat; Water = Water Bearer; Fish = Fish)

Another easy mnemonic:

(Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, Pisces)

Remembering the translations/shapes of constellations with a fun mnemonic:

(Ram, Bull, Twins, Crab, Lion, Virgin, Scales, Scorpion, Archer, Goat, Water Bearer, Fish)