The zodiac is a belt-shaped region of the sky that extends approximately 8° north or south (as measured in celestial latitude) of the

ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic agains ...

, the apparent path of the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

across the celestial sphere over the course of the year. The paths of the

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

and visible planets are within the belt of the zodiac. In

Western astrology Western astrology is the system of astrology most popular in Western countries. Western astrology is historically based on Ptolemy's ''Tetrabiblos'' (2nd century CE), which in turn was a continuation of Hellenistic and ultimately Babylonian tra ...

, and formerly astronomy, the zodiac is divided into twelve signs, each occupying 30° of

celestial longitude Astronomical coordinate systems are organized arrangements for specifying positions of satellites, planets, stars, galaxies, and other celestial objects relative to physical reference points available to a situated observer (e.g. the true horizon ...

and roughly corresponding to the following star constellations: Aries, Taurus,

Gemini Gemini may refer to: Space * Gemini (constellation), one of the constellations of the zodiac ** Gemini in Chinese astronomy * Project Gemini, the second U.S. crewed spaceflight program * Gemini Observatory, consisting of telescopes in the Norther ...

Cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...

, Leo,

Virgo Virgo may refer to: *Virgo (astrology), the sixth astrological sign of the zodiac * Virgo (constellation), a constellation *Virgo Cluster, a cluster of galaxies in the constellation Virgo *Virgo Stellar Stream, remains of a dwarf galaxy * Virgo Su ...

, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. These

astrological sign In Western astrology, astrological signs are the twelve 30-degree sectors that make up Earth's 360-degree orbit around the Sun. The signs enumerate from the first day of spring, known as the First Point of Aries, which is the vernal equinox. ...

s form a

celestial coordinate system Astronomical coordinate systems are organized arrangements for specifying positions of satellites, planets, stars, galaxies, and other celestial objects relative to physical reference points available to a situated observer (e.g. the true hor ...

, or more specifically an

ecliptic coordinate system The ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent positions, orbits, and pole orientations of Solar System objects. Because most planets (except Mercury) and many small Solar System b ...

, which takes the ecliptic as the origin of latitude and the Sun's position at vernal equinox as the origin of

longitude Longitude (, ) is a geographic coordinate that specifies the east– west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek lette ...

Name

The English word ' derives from , the Latinized form of the

Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...

( ), meaning "cycle or circle of little animals". () is the diminutive of (, "animal"). The name reflects the prominence of animals (and mythological hybrids) among the twelve signs.

Usage

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. The construction of the zodiac is described in Ptolemy's comprehensive 2nd century AD work, the '' Almagest''. Although the zodiac remains the basis of the

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 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.

History

Early history

As early as the 14th century BC a complete list of the 36 Egyptian

decans The decans (; Egyptian ''bꜣktw'' or ''baktiu'', "hoseconnected with work") are 36 groups of stars (small constellations) used in the ancient Egyptian astronomy to conveniently divide the 360 degree ecliptic into 36 parts of 10 degrees each, bo ...

was placed among the hieroglyphs adorning the tomb of Seti I; they figured again in the temple of Ramesses II, and characterize every Egyptian astrological monument. Both the famous zodiacs of Dendera display their symbols, identified by Karl Richard Lepsius. Beit Alpha

The division of the ecliptic into the zodiacal signs originates in Babylonian astronomy during the first half of the

1st millennium BC The 1st millennium BC, also known as the last millennium BC, was the period of time lasting from the years 1000 BC to 1 BC ( 10th to 1st centuries BC; in astronomy: JD – ). It encompasses the Iron Age in the Old World and sees the trans ...

. The zodiac draws on stars in earlier Babylonian star catalogues, such as the

MUL.APIN MUL.APIN () is the conventional title given to a Babylonian compendium that deals with many diverse aspects of Babylonian astronomy and astrology. It is in the tradition of earlier star catalogues, the so-called ''Three Stars Each'' lists, but ...

catalogue, which was compiled around 1000 BC. Some constellations can be traced even further back, to Bronze Age (

First Babylonian dynasty The Old Babylonian Empire, or First Babylonian Empire, is dated to BC – BC, and comes after the end of Sumerian power with the destruction of the Third Dynasty of Ur, and the subsequent Isin-Larsa period. The chronology of the first dynasty ...

) sources, including

"The Twins," from MAŠ.TAB.BA.GAL.GAL "The Great Twins," and

"The Crab," from AL.LUL "The Crayfish," among others. Around the end of the 5th century BC, Babylonian astronomers divided the ecliptic into 12 equal "signs", by analogy to 12 schematic months of 30 days each. Each sign contained 30° of

, thus creating the first known celestial coordinate system. According to calculations by modern astrophysics, the zodiac was introduced between 409 and 398 BC, during Persian rule, and probably within a very few years of 401 BC. Unlike modern astrologers, who place the beginning of the sign of Aries at the position of the Sun at the vernal equinox in the Northern Hemisphere ( March equinox), Babylonian astronomers fixed the zodiac in relation to stars, placing the beginning of

at the "Rear Twin Star" ( β Geminorum) and the beginning of Aquarius at the "Rear Star of the Goat-Fish" ( δ Capricorni). Due to the precession of the equinoxes, the time of year the Sun is in a given constellation has changed since Babylonian times, the point of March equinox has moved from Aries into Pisces. Because the division was made into equal arcs, 30° each, they constituted an ideal system of reference for making predictions about a planet's longitude. However, Babylonian techniques of observational measurements were in a rudimentary stage of evolution. They measured the position of a planet in reference to a set of "normal stars" close to the ecliptic (±9° of latitude) as observational reference points to help positioning a planet within this ecliptic coordinate system. In

Babylonian astronomical diaries The Babylonian astronomical diaries are a collection of Babylonian cuneiform texts that contain systematic records of astronomical observations and political events as well as predictions, based on astronomical observations. They also include othe ...

, a planet position was generally given with respect to a zodiacal sign alone, less often in specific degrees within a sign. When the degrees of longitude were given, they were expressed with reference to the 30° of the zodiacal sign, i.e., not with a reference to the continuous 360° ecliptic. In astronomical

ephemerides In astronomy and celestial navigation, an ephemeris (pl. ephemerides; ) is a book with tables that gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position (and possibly ...

, the positions of significant astronomical phenomena were computed in sexagesimal fractions of a degree (equivalent to minutes and seconds of arc). For daily ephemerides, the daily positions of a planet were not as important as the astrologically significant dates when the planet crossed from one zodiacal sign to the next.

Hebrew astronomy and astrology

Knowledge of the Babylonian zodiac is said to be reflected in the

Hebrew Bible The Hebrew Bible or Tanakh (;"Tanach"
'' E. W. Bullinger interpreted the creatures appearing in the

book of Ezekiel The Book of Ezekiel is the third of the Latter Prophets in the Tanakh and one of the major prophetic books, following Isaiah and Jeremiah. According to the book itself, it records six visions of the prophet Ezekiel, exiled in Babylon, during ...

as the middle signs of the four quarters of the zodiac, with the Lion as Leo, the Bull is Taurus, the Man representing Aquarius and the Eagle representing Scorpio. Some authors have linked the twelve tribes of Israel with the same signs or the lunar Hebrew calendar having twelve

lunar months In lunar calendars, a lunar month is the time between two successive syzygies of the same type: new moons or full moons. The precise definition varies, especially for the beginning of the month. Variations In Shona, Middle Eastern, and Euro ...

in a

lunar year A lunar calendar is a calendar based on the monthly cycles of the Moon's phases ( synodic months, lunations), in contrast to solar calendars, whose annual cycles are based only directly on the solar year. The most commonly used calendar, the Gr ...

. Martin and others have argued that the arrangement of the tribes around the

Tabernacle According to the Hebrew Bible, the tabernacle ( he, מִשְׁכַּן, mīškān, residence, dwelling place), also known as the Tent of the Congregation ( he, link=no, אֹהֶל מוֹעֵד, ’ōhel mō‘ēḏ, also Tent of Meeting, etc.), ...

(reported in the

Book of Numbers The book of Numbers (from Greek Ἀριθμοί, ''Arithmoi''; he, בְּמִדְבַּר, ''Bəmīḏbar'', "In the desert f) is the fourth book of the Hebrew Bible, and the fourth of five books of the Jewish Torah. The book has a long and c ...

) corresponded to the order of the zodiac, with Judah, Reuben, Ephraim, and Dan representing the middle signs of Leo, Aquarius, Taurus, and Scorpio, respectively. Such connections were taken up by Thomas Mann, who in his novel '' Joseph and His Brothers'' attributes characteristics of a sign of the zodiac to each tribe in his rendition of the Blessing of Jacob.

Hellenistic and Roman era

The Babylonian star catalogs entered Greek astronomy in the 4th century BC, via Eudoxus of Cnidus. Babylonia or Chaldea in the Hellenistic world came to be so identified with astrology that "Chaldean wisdom" became among Greeks and

Romans Roman or Romans most often refers to: *Rome, the capital city of Italy * Ancient Rome, Roman civilization from 8th century BC to 5th century AD *Roman people, the people of ancient Rome *''Epistle to the Romans'', shortened to ''Romans'', a lette ...

the synonym of divination through the planets and stars. Hellenistic astrology derived in part from Babylonian and Egyptian astrology. Horoscopic astrology first appeared in Ptolemaic Egypt (305 BC–30 BC). The Dendera zodiac, a relief dating to ca. 50 BC, is the first known depiction of the classical zodiac of twelve signs. The earliest extant Greek text using the Babylonian division of the zodiac into 12 signs of 30 equal degrees each is the ''Anaphoricus'' of

Hypsicles of Alexandria Hypsicles ( grc-gre, Ὑψικλῆς; c. 190 – c. 120 BCE) was an ancient Greek mathematician and astronomer known for authoring ''On Ascensions'' (Ἀναφορικός) and the Book XIV of Euclid's ''Elements''. Hypsicles lived in Alexandria. ...

(fl.190BC). Particularly important in the development of Western 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 Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In oth ...

of the equinoxes by Hipparchus around 130 BC. Hipparchus's lost work on precession never circulated very widely until it was brought to prominence by Ptolemy, and there are few explanations of precession outside the work of Ptolemy until late Antiquity, by which time Ptolemy's influence was widely established. Ptolemy clearly explained the theoretical basis of the western zodiac as being a tropical coordinate system, by which the zodiac is aligned to the equinoxes and solstices, rather than the visible constellations that bear the same names as the zodiac signs.

Hindu zodiac

According to mathematician-historian Montucla, the Hindu zodiac was adopted from the Greek zodiac through communications between ancient India and the Greek empire of Bactria. The

Hindu zodiac Jyotisha or Jyotishya (from Sanskrit ', from ' “light, heavenly body" and ''ish'' - from Isvara or God) is the traditional Hindu system of astrology, also known as Hindu astrology, Indian astrology and more recently Vedic astrology. It is one ...

uses the sidereal coordinate system, which makes reference to the fixed stars. The tropical zodiac (of Mesopotamian origin) is divided by the intersections of the

and equator, which shifts in relation to the backdrop of fixed stars at a rate of 1° every 72 years, creating the phenomenon known as precession of the equinoxes. The Hindu zodiac, being sidereal, does not maintain this seasonal alignment, but there are still similarities between the two systems. The Hindu zodiac signs and corresponding Greek signs sound very different, being in Sanskrit and Greek respectively, but their symbols are nearly identical. For example, ''dhanu'' means "bow" and corresponds to Sagittarius, the "archer", and ''kumbha'' means "water-pitcher" and corresponds to Aquarius, the "water-carrier".

Middle Ages

Angers Cathedral South Rose Window of Christ with Zodiac

During the Abbasid era, Greek reference books were systematically translated into Arabic, and Islamic astronomers then did their own observations, correcting Ptolemy's Almagest. One such book was Al-Sufi's ''

Book of Fixed Stars The ''Book of Fixed Stars'' ( ar, كتاب صور الكواكب ', literally ''The Book of the Shapes of Stars'') is an astronomical text written by Abd al-Rahman al-Sufi (Azophi) around 964. Following the translation movement in the 9th centu ...

'', which has pictorial depictions of 48 constellations. The book was divided into three sections: constellations of the zodiac, constellations north of the zodiac, and southern constellations. When Al-Sufi's book, and other works, were translated in the 11th century, there were mistakes made in the translations. As a result, some stars ended up with the names of the constellation they belong to (e.g.

Hamal Hamal , designation Alpha Arietis (α Arietis, abbreviated Alpha Ari, α Ari), is the brightest star in the northern zodiacal constellation of Aries. With an apparent visual magnitude of 2.0, it is the mean 50th-brightest star in the ...

in Aries). The High Middle Ages saw a revival of interest in Greco-Roman magic, first in Kabbalism and later continued in

Renaissance magic Renaissance magic was a resurgence in Hermeticism and Neo-Platonic varieties of the magical arts which arose along with Renaissance humanism in the 15th and 16th centuries CE. These magical arts (called '' artes magicae'') were divided into sev ...

. This included magical uses of the zodiac, as found, e.g., in the Sefer Raziel HaMalakh. The zodiac is found in medieval stained glass as at

Angers Cathedral Angers Cathedral (french: Cathédrale Saint-Maurice d'Angers) is a Roman Catholic church dedicated to Saint Maurice in Angers, France. It is the seat of the Bishops of Angers. Built between the 11th and 16th centuries, it is known for its mixt ...

, where the master glass maker, André Robin, made the ornate rosettes for the North and South transepts after the fire there in 1451. Mughal king Jahangir issued an attractive series of coins in gold and silver depicting the twelve signs of the zodiac.

Medieval Islamic era

Ottoman Sundial at the Debbane Palace museum

Astrology emerged in the 8th century CE as a distinct discipline in Islam, with a mix of Indian, Hellenistic Iranian and other traditions blended with Greek and Islamic astronomical knowledge, for example Ptolemy's work and Al-Sufi's Book of Fixed Stars. A knowledge of the influence that the stars have on events on the earth was important in Islamic civilization. As a rule, it was believed that the signs of the zodiac and the planets control the destiny not only of people but also of nations, and that the zodiac has the ability to determine a person's physical characteristics as well as intelligence and personal traits. The practice of astrology at this time could be divided into 4 broader categories: Genethlialogy, Catarchic Astrology, Interrogational Astrology and General Astrology. However the most common type of astrology was Genethlialogy, which examined all aspects of a person's life in relation to the planetary positions at their birth; more commonly known as our horoscope. Astrology services were offered widely across the empire, mainly in bazaars, where people could pay for a reading. Astrology was valued in the royal courts, for example, the Abbasid Caliph Al-Mansur used astrology to determine the best date for founding the new capital of Baghdad. Whilst horoscopes were generally widely accepted by society, many scholars condemned the use of astrology and divination, linking it to occult influences. Many theologians and scholars thought that it went against the tenets of Islam; as only God should be able to determine events rather than astrologers looking at the positions of the planets. In order to calculate someone's horoscope, an astrologer would use 3 tools: an astrolabe, ephemeris and a takht. First, the astrologer would use an astrolabe to find the position of the sun, align the rule with the persons time of birth and then align the rete to establish the altitude of the sun on that date. Next, the astrologer would use an Ephemeris, a table denoting the mean position of the planets and stars within the sky at any given time. Finally, the astrologer would add the altitude of the sun taken from the astrolabe, with the mean position of the planets on the person's birthday, and add them together on the takht (also known as the dustboard). The dust board was merely a tablet covered in sand; on which the calculations could be made and erased easily. Once this had been calculated, the astrologer was then able to interpret the horoscope. Most of these interpretations were based on the zodiac in literature. For example, there were several manuals on how to interpret each zodiac sign, the treatise relating to each individual sign and what the characteristics of these zodiacs were.

Early modern

An example of the use of signs as astronomical coordinates may be found in the ''Nautical Almanac and Astronomical Ephemeris for the year 1767''. The "Longitude of the Sun" columns show the sign (represented as a digit from 0 to and including 11), degrees from 0 to 29, minutes, and seconds. The zodiac symbols are Early Modern simplifications of conventional pictorial representations of the signs, attested since Hellenistic times.

Twelve signs

What follows is a list of the signs of the modern zodiac (with the ecliptic longitudes of their first points), where 0° Aries is understood as the vernal equinox, with their Latin, Greek, Sanskrit, and Babylonian names. But note that the Sanskrit and the name equivalents (after c.500 BC) denote the constellations only, not the tropical zodiac signs. The "English translation" isn't usually used by English speakers. The Latin names are standard English usage (except that "Capricorn" is used rather than "Capricornus"). The following table compares the Gregorian dates on which the Sun enters a sign in the Ptolemaic tropical zodiac, and a sign in the sidereal system proposed by

Cyril Fagan Cyril Fagan (born Dublin, Ireland, May 22, 1896, died Tucson, Arizona, United States, January 5, 1970) was an Irish astrologer, Generally considered ''the father'' - alongside Donald A. Bradley, ''the mother'', of Sidereal astrology '' Sid ...

The beginning of Aries is defined as 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 The Gregorian calendar is the calendar used in most parts of the world. It was introduced in October 1582 by Pope Gregory XIII as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years dif ...

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 UT always falling either on 20 or 21 March in the period of 1797 to 2043, falling on 19 March in 1796 the last time and in 2044 the next. The vernal equinox has fallen on 20 March UT since 2008, and will continue to do so until 2043.

As each sign takes up exactly 30 degrees of the zodiac, the average duration of the solar stay in each sign is one twelfth of a sidereal year, or 30.43 standard days. Due to Earth's slight orbital eccentricity, the duration of each sign varies appreciably, between about 29.4 days for Capricorn and about 31.4 days for Cancer (see

Equation of time In mathematics, an equation is a formula that expresses the equality of two expressions, by connecting them with the equals sign . The word ''equation'' and its cognates in other languages may have subtly different meanings; for example, in F ...

). In addition, because the Earth's axis is at an angle, some signs take longer to rise than others, and the farther away from the equator the observer is situated, the greater the difference. Thus, signs are spoken of as "long" or "short" ascension.

Constellations

In tropical astrology, the zodiacal signs are distinct from the constellations associated with them, not only because of their drifting apart due to the precession of equinoxes but because the physical constellations take up varying widths of the ecliptic, so the Sun is not in each constellation for the same amount of time. Thus,

takes up 5 times as much

ecliptic longitude The ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent positions, orbits, and pole orientations of Solar System objects. Because most planets (except Mercury) and many small Solar System b ...

Scorpius Scorpius is a zodiac constellation located in the Southern celestial hemisphere, where it sits near the center of the Milky Way, between Libra to the west and Sagittarius to the east. Scorpius is an ancient constellation that pre-dates the Gr ...

. The zodiacal signs are an abstraction from the physical constellations, and each represent exactly th of the full circle, but the time spent by the Sun in each sign varies slightly due to the eccentricity of the Earth's orbit.

Sidereal astrology '' Sidereal'' and ''tropical'' are terms used to describe two different definitions of a year, applied in sidereal solar calendars or tropical solar calendars. In astrology, they refer to two different systems of ecliptic coordinates used to d ...

remedies this by assigning the zodiac sign approximately to the corresponding constellation. This alignment needs re calibrating every so often to keep the alignment in place. The ecliptic intersects with 13 constellations of Ptolemy's '' Almagest'', as well as of the more precisely delineated

IAU designated constellations In contemporary astronomy, 88 constellations are recognized by the International Astronomical Union (IAU). Each constellation is a region of the sky, bordered by arcs of right ascension Right ascension (abbreviated RA; symbol ) is the an ...

. In addition to the twelve constellations after which the twelve zodiac signs are named, the ecliptic intersects

Ophiuchus Ophiuchus () is a large constellation straddling the celestial equator. Its name comes from the Ancient Greek (), meaning "serpent-bearer", and it is commonly represented as a man grasping a snake. The serpent is represented by the constella ...

, the bottom part of which interjects between Scorpio and Sagittarius. Occasionally this difference between the astronomical constellations and the

s is mistakenly reported in the popular press as a "change" to the list of traditional signs by some astronomical body like the IAU,

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...

, or the Royal Astronomical Society. This happened in a 1995 report of the ''

BBC Nine O'Clock News The ''BBC Nine O'Clock News'' was the flagship BBC News programme. It was launched on 14 September 1970 and ran until 13 October 2000, when it was replaced by the ''BBC Ten O'Clock News''. History The ''Nine O'Clock News'' was the BBC's flags ...

'' and various reports in 2011 and 2016. Some "parazodiacal" constellations are touched by the paths of the planets, leading to counts of up to 25 "constellations of the zodiac". The ancient Babylonian

catalog lists Orion, Perseus, Auriga, and Andromeda. Modern astronomers have noted that planets pass through Crater,

Sextans Sextans is a minor equatorial constellation which was introduced in 1687 by Johannes Hevelius. Its name is Latin for the astronomical sextant, an instrument that Hevelius made frequent use of in his observations. Notable features Sextans as ...

Cetus Cetus () is a constellation, sometimes called 'the whale' in English. The Cetus was a sea monster in Greek mythology which both Perseus and Heracles needed to slay. Cetus is in the region of the sky that contains other water-related constellat ...

, Pegasus,

Corvus ''Corvus'' is a widely distributed genus of medium-sized to large birds in the family Corvidae. It includes species commonly known as crows, ravens and rooks. The species commonly encountered in Europe are the carrion crow, the hooded crow ...

, Hydra, and Scutum, with

Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...

very rarely passing through Aquila, Canis Minor, Auriga, and

Serpens Serpens ( grc, , , the Serpent) is a constellation in the northern celestial hemisphere. One of the 48 constellations listed by the 2nd-century astronomer Ptolemy, it remains one of the 88 modern constellations designated by the International ...

. Some other constellations are mythologically associated with the zodiacal ones:

Piscis Austrinus Piscis Austrinus is a constellation in the southern celestial hemisphere. The name is Latin for "the southern fish", in contrast with the larger constellation Pisces, which represents a pair of fish. Before the 20th century, it was also known a ...

, 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. Hydra in the Early Bronze Age marked the

celestial equator The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. This plane of reference bases the equatorial coordinate system. In other words, the celestial equator is an abstract proj ...

and was associated with Leo, which is shown standing on the serpent on the Dendera zodiac.

is the Crow or Raven mysteriously perched on the tail of Hydra.

Precession of the equinoxes

The zodiac system was developed in Babylonia, some 2,500 years ago, during the " Age of Aries". At the time, it is assumed, the precession of the equinoxes was unknown. Contemporary use of the coordinate system is presented with the choice of interpreting the system either as sidereal, with the signs fixed to the stellar background, or as tropical, with the signs fixed to the point (

vector Vector most often refers to: *Euclidean vector, a quantity with a magnitude and a direction *Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematic ...

of the Sun) at the March equinox.

takes the tropical approach, whereas Hindu astrology takes the sidereal one. This results in the originally unified zodiacal coordinate system drifting apart gradually, with a clockwise (westward) precession of 1.4 degrees per century. For the tropical zodiac used in Western astronomy and astrology, this means that the tropical ''sign'' of Aries currently lies somewhere within the ''constellation'' Pisces ("

Age of Pisces An astrological age is a time period in astrological theory which astrologers say, parallels major changes in the development of Earth's inhabitants, particularly relating to culture, society, and politics. There are twelve astrological ages corr ...

"). The sidereal coordinate system takes into account the ayanamsa, ''ayan'' meaning "transit' or 'movement', and ''amsa'' meaning 'small part', i.e. movement of equinoxes in small parts. It is unclear when Indians became aware of the precession of the equinoxes, but Bhāskara II's 12th-century treatise '' Siddhanta Shiromani'' gives equations for measurement of precession of equinoxes, and says his equations are based on some lost equations of '' Suryasiddhanta'' plus the equation of Munjaala. The discovery of precession is attributed to Hipparchus around 130 BC. Ptolemy quotes from Hipparchus's now-lost work entitled "On the Displacement of the Solstitial and Equinoctial Points" in the seventh book of his 2nd century astronomical text, ''Almagest'', where he describes the phenomenon of precession and estimates its value. Ptolemy clarified that the convention of Greek mathematical astronomy was to commence the zodiac from the point of the vernal equinox and to always refer to this point as "the first degree" of Aries. This is known as the "tropical zodiac" (from the Greek word trópos, turn) because its starting point revolves through the circle of background constellations over time. The principle of the vernal point acting as the first degree of the zodiac for Greek astronomers is described in the 1st century BC astronomical text of

Geminus Geminus of Rhodes ( el, Γεμῖνος ὁ Ῥόδιος), was a Greek astronomer and mathematician, who flourished in the 1st century BC. An astronomy work of his, the ''Introduction to the Phenomena'', still survives; it was intended as an int ...

of Rhodes. Geminus explains that Greek astronomers of his era associate the first degrees of the zodiac signs with the two solstices and the two equinoxes, in contrast to the older Chaldean (Babylonian) system, which placed these points within the zodiac signs. This illustrates that Ptolemy merely clarified the convention of Greek astronomers and did not originate the principle of the tropical zodiac, as is sometimes assumed. Ptolemy demonstrates that the principle of the tropical zodiac was well known to his predecessors within his astrological text, the '' Tetrabiblos'', where he explains why it would be an error to associate the regularly spaced signs of the seasonally aligned zodiac with the irregular boundaries of the visible constellations:

In modern astronomy

Astronomically, the zodiac defines a belt of space extending 8° or 9° in celestial latitude to the north and south of the

, within which the orbits of the Moon and the principal planets remain. It is a feature of the ecliptic coordinate system – a

centered upon the ecliptic (the plane of the Earth's orbit and the Sun's apparent path), by which

is measured in degrees east of the vernal equinox (the ascending intersection of the ecliptic and equator). The zodiac is narrow in angular terms because most of the Sun's planets have orbits that have only a slight

inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Eart ...

to the

orbital plane The orbital plane of a revolving body is the geometric plane in which its orbit lies. Three non-collinear points in space suffice to determine an orbital plane. A common example would be the positions of the centers of a massive body (host) an ...

of the Earth. Stars within the zodiac are subject to occultations by the Moon and other solar system bodies. These events can be useful, for example, to estimate the cross-sectional dimensions of a minor planet, or check a star for a close companion. The Sun's placement upon the vernal equinox, which occurs annually around 21 March, defines the starting point for measurement, the first degree of which is historically known as the " first point of Aries". The first 30° along the ecliptic is nominally designated as the zodiac sign Aries, which no longer falls within the proximity of the constellation Aries since the effect of precession is to move the vernal point through the backdrop of visible constellations (it is currently located near the end of the constellation Pisces, having been within that constellation since the 2nd century AD). The subsequent 30° of the ecliptic is nominally designated the zodiac sign Taurus, and so on through the twelve signs of the zodiac so that each occupies th (30°) of the zodiac's great circle. Zodiac signs have never been used to determine the boundaries of astronomical constellations that lie in the vicinity of the zodiac, which are, and always have been, irregular in their size and shape. The convention of measuring celestial longitude within individual signs was still being used in the mid-19th century, but modern astronomy now numbers degrees of celestial longitude continuously from 0° to 360°, rather than 0° to 30° within each sign. This coordinate system is primary used by astronomers for observations of solar system objects. The use of the zodiac as a means to determine astronomical measurement remained the main method for defining celestial positions by Western astronomers until the Renaissance, at which time preference moved to the equatorial coordinate system, which measures astronomical positions by right ascension and declination rather than the ecliptic-based definitions of celestial longitude and celestial latitude. The orientation of equatorial coordinates are aligned with the Earth's axis of rotation, rather than the plane of the planet's orbit around the Sun. The word "zodiac" is used in reference to the zodiacal cloud of dust grains that move among the planets, and the zodiacal light that originates from their scattering of sunlight. While its name is derived from the zodiac, the zodiacal light covers the entire night sky, with enhancements in certain directions.

Unicode characters

In Unicode, the symbols of zodiac signs are encoded in block "Miscellaneous Symbols". They can be forced to look like text by appending U+FE0E, or like emojis by appending U+FE0F:

References

External links

"A Treatise on Zodiacal Signs and Constellations: Unique Jewels on the Benefits of Keeping Time"
is a manuscript that dates back to 1831 with a focus on Arabic, Coptic and Syriac calendars.
Zodiac Constellations at Constellation Guide
{{DEFAULTSORT:Zodiac, The Z + Ancient astronomy Astrology Astronomical coordinate systems Birthdays Early scientific cosmologies Western astrology