zodiac

Wheel of the zodiac: This 6th century mosaic pavement in a synagogue incorporates Greek-Byzantine elements, Beit Alpha, Israel

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 on the equator (blue).

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 which divide the ecliptic into twelve equal zones of celestial longitude. As such, the zodiac is a celestial coordinate system, more precisely 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.^[1] The construction of the zodiac is described in Ptolemy's Almagest (2nd century AD). It was originally described in Rigveda which is the oldest book of the world. 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 which 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 ^[2]

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.

History

The 1st century BC Denderah Zodiac (19th-century engraving)

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)^[3], 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 suggest their determination in this region in the Bronze Age (~2700 BC),^[4][5] 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. ^[6][7] 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. ^[8][9][10] 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.^{[citation needed]}

Cathedral of Living Pillar) in Georgia depicting Christ within the Zodiac circle

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, the 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.^[11] 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, apparently by dropping the concept of a fixed celestial sphere and adopting what is referred to as a tropical coordinate system instead.

The zodiac signs as shown in a 16th century woodcut

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 twelve signs

The symbols used in Western astrology to represent the astrological signs

What follows is a list of the twelve signs of the 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 Babylonian name equivalents denote the constellations only, not the tropical zodiac signs):

The zodiacal symbols are Early Modern simplifications of conventional pictorial representations of the signs, attested since Hellenistic times. The symbols are encoded in Unicode at positions U+2648 to U+2653.

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.^[13]

There have always been a number of "parazodiacal" constellations which 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. 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. The MUL.APIN glosses Hydra as "the Snake Ningizzida, lord of the Netherworld". Ningizzida together with Dumuzi (Aries) and Pabilsag (Sagittarius) governed the household of the queen of the underworld.

Taking the current constellation boundaries as defined in 1930 by the International Astronomical Union, the ecliptic itself passes through an additional thirteenth constellation, Ophiuchus, situated between Scorpius and Sagittarius. This is already recognized in Ptolemy's Almagest.^{[citation needed]}

Table of dates

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

• a sign in the Ptolemaic tropical zodiac;
• a sign in the Hindu sidereal system
• the astronomical constellation of the same name as the sign, with constellation boundaries as defined in 1930 by the International Astronomical Union.

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.^[14] 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.^[15] 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.

Sign				Constellation[16][17]
Name	Symbol	Tropical zodiac (2009, UTC)	Sidereal zodiac (Jyotisha) (2009, UTC)[18]	Name	IAU constellation boundaries (2009)	Solar stay	Brightest star
Aries		20 March – 19 April	14 April – 14 May	Aries	18 April – 14 May	25.5 days	Hamal
Taurus		19 April – 20 May	14 May – 13 June	Taurus	14 May – 21 June	38.2 days	Aldebaran
Gemini		20 May – 21 June	13 June – 14 July	Gemini	21 June – 20 July	29.3 days	Pollux
Cancer		21 June – 22 July	14 July – 13 August	Cancer	20 July – 10 August	21.1 days	Al Tarf
Leo		22 July – 23 August	13 August – 13 September	Leo	10 August – 16 September	36.9 days	Regulus
Virgo		23 August – 22 September	13 September – 13 October	Virgo	16 September – 31 October	44.5 days	Spica
Libra		22 September – 23 October	13 October – 12 November	Libra	31 October – 21 November	21.1 days	Zubeneschamali
Scorpio		23 October – 22 November	13 November – 13 December	Scorpius	21 November – 29 November	8.4 days	Antares
Serpentarius		n/a		Ophiuchus	29 November – 18 December	18.4 days	Rasalhague
Sagittarius		22 November – 21 December	13 December – 12 January	Sagittarius	18 December – 20 January	33.6 days	Kaus Australis
Capricorn		21 December – 20 January	12 January – 12 February	Capricornus	20 January – 17 February	27.4 days	Deneb Algedi
Aquarius		20 January – 18 February	12 February – 14 March	Aquarius	17 February – 13 March	23.9 days	Sadalsuud
Pisces		18 February – 20 March	14 March – 14 April	Pisces	13 March – 19 April	37.7 days	Eta Piscium

Precession of the equinoxes

Path taken by the point of vernal equinox along the ecliptic over the past 6000 years

The zodiac system was developed in Babylonia, some 2,500 years ago, during the "Age of Aries". At the time, the precession of the equinoxes was unknown, and the system made no allowance for it. Contemporary use of the coordinate system is presented with the choice of either interpreting the system as sidereal, with the signs fixed to the stellar background, or as tropical, with the signs fixed to the point of vernal equinox.

Western astrology takes the tropical approach, while Hindu astrology takes the sidereal one. This results in the originally unified zodiacal coordinate system drifting apart gradually, with an angular velocity of about 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"). The choice of origin for the sidereal coordinate system is known as the ayanamsa, a Sanskrit word.

It is not entirely clear how the Hellenistic astronomers responded to this phenomenon of precession once it had been discovered by Hipparchus around 130 BC. Today, some read Ptolemy as dropping the concept of a fixed celestial sphere and adopting what is referred to as a tropical coordinate system instead: in other words, one fixed to the Earth's seasonal cycle rather than the distant stars.

Some modern Western astrologers, such as Cyril Fagan, have advocated abandoning the tropical system in favour of a sidereal one.

In modern 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.

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 essentially 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.

Mnemonics for the zodiac

A traditional mnemonic:^[19]

The Ram, the Bull, the Heavenly Twins,

And next the Crab, the Lion shines,

The Virgin and the Scales.

The Scorpion, Archer, and the Goat,

The Man who holds the Watering Pot,

And Fish with glittering scales.

A less poetic, but succinct and perhaps more memorable, mnemonic is the following:^[20]

The Ramble Twins Crab Liverish;

Scaly Scorpions Are Good Water Fish.

(Ram-Ble = Ox, 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:

All The Great Constellations Live Very Long Since Stars Can't Alter Physics.^[21]

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

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

Really Boring Teachers Can Live Very Sadly Since Apples Give Worthless Feelings.

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

References

• B. L. van der Waerden, "History of the zodiac", Archiv für Orientforschung 16 (1953) 216–230.
• Ptolemy, Claudius (1998), written at Princeton, NJ, The Almagest, Princeton University Press, ISBN 069100260 Translated and annotated by G.J. Toomer ; with a foreword by Owen Gingerich.
• Powell, Robert, Influence of Babylonian Astronomy on the Subsequent Defining of the Zodiac (2004), PhD thesis.
• Tropical v Sidereal debate, The Traditional Astrologer magazine, (Ascella), Issue 14, May 1997, pp.23–27
• Peters, Christian Heinrich Friedrich and Edward Ball Knobel. Ptolemy's Catalogue of Stars: a revision of the Almagest. (1915). Publishers, Carnegie Institution of Washington.

See also

Wikimedia Commons has media related to: Zodiac

Astrology portal

• Ecliptic
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v • d • e The Zodiac
Astronomy | Constellations of the Zodiac
Aries	Taurus	Gemini	Cancer	Leo	Virgo	Libra	Scorpius	Ophiuchus	Sagittarius	Capricornus	Aquarius	Pisces
Astrology | Signs of the Zodiac
Aries	Taurus	Gemini	Cancer	Leo	Virgo	Libra	Scorpio	Sagittarius	Capricorn	Aquarius	Pisces
See also: Ecliptic  · Zodiacal light

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