Zodiac

For other uses, see Zodiac (disambiguation).

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 with respect to the equator (blue-white).

In both astrology and historical astronomy, the zodiac (Greek: ζῳδιακός, zōdiakos) 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 zōdiacus, which in its turn comes from the Greek ζῳδιακὸς κύκλος (zōdiakos kyklos), meaning "circle of animals", derived from ζῴδιον (zōdion), the diminutive of ζῷον (zōon) "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.

History

Further information: Former constellations and Star Names: Their Lore and Meaning

Early history

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

Further information: Babylonian star catalogues and MUL.APIN

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), The classical zodiac is a modification of the MUL.APIN catalogue, which was compiled around 1000 BC. Some of the constellations can be traced even further back, to Bronze Age (Old Babylonian) sources, including Gemini "The Twins", from MAŠ.TAB.BA.GAL.GAL "The Great Twins", and Cancer "The Crab", from AL.LUL "The Crayfish", among others.

Babylonian astronomers at some stage 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). The Babylonian calendar as it stood in the 7th century BC assigned each month to a sign, beginning with the position of the Sun at vernal equinox, which, at the time, was depicted as the Aries constellation ("Age of Aries"), for which reason the first sign is still called "Aries" even after the vernal equinox has moved away from the Aries constellation due to the slow precession of the Earth's axis of rotation.

Because the division was made into equal arcs, 30º 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 and it was probably beyond their capacity to define in a precise way the boundary lines between the zodiacal signs in the sky. Thus, the need to use stars close to the ecliptic (±9º of latitude) as a set of observational reference points to help positioning a planet within this ecliptic coordinate system. Constellations were given the names of the signs and asterisms could be connected in a way that would resemble the sign's name. Therefore, in spite of its conceptual origin, the Babylonian zodiac became sidereal.

In Babylonian astronomical diaries, 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 30º of the zodiacal sign, i.e., not with a reference to the continuous 360º ecliptic. To the construction of their mathematical ephemerides, daily positions of a planet were not as important as the dates when the planet crossed from one zodiacal sign to the next.

Knowledge of the Babylonian zodiac is also reflected in the Hebrew Bible. E. W. Bullinger interpreted the creatures appearing in the books of Ezekiel and Revelation 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 twelve signs. Martin and others have argued that the arrangement of the tribes around the Tabernacle (reported in the Book of Numbers) 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 1st century BC Dendera zodiac (19th-century engraving)

The Babylonian star catalogs entered Greek astronomy in the 4th century BC, via Eudoxus of Cnidus and others . 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. Hellenistic astrology originated from Babylonian and Egyptian astrology . Horoscopic astrology first appeared in Ptolemaic Egypt. The Dendera zodiac, a relief dating to ca. 50 BC, is the first known depiction of the classical zodiac of twelve signs.

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 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.

Hindu zodiac

The Hindu zodiac 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 ecliptic 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 and early modern period

The zodiac signs as shown in a 16th-century woodcut

A 17th-century fresco from the Cathedral of Living Pillar in Georgia depicting Christ within the Zodiac circle

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.

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 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 in the Miscellaneous Symbols block.

The twelve signs

Main article: Astrological sign

What follows is a list of the twelve 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 Babylonian name equivalents denote the constellations only, not the tropical zodiac signs). Also, the "English translation" is not usually used by English speakers. The Latin names are standard English usage.

№	Symbol	Long.	Latin name	English translation	Greek name	Sanskrit name	Sumero-Babylonian name
1	♈	0°	Aries	The Ram	Κριός (Krios)	Meṣha (मेष)	MUL LU.ḪUŊ.GA "The Agrarian Worker", Dumuzi
2	♉	30°	Taurus	The Bull	Ταῦρος (Tavros)	Vṛiṣhabha (वृषभ)	MULGU4.AN.NA "The Steer of Heaven"
3	♊	60°	Gemini	The Twins	Δίδυμοι (Didymoi)	Mithuna (मिथुन)	MULMAŠ.TAB.BA.GAL.GAL "The Great Twins" (Castor and Pollux)
4	♋	90°	Cancer	The Crab	Καρκῖνος (Karkinos)	Karkaṭa (कर्कट)	MULAL.LUL "The Crayfish"
5	♌	120°	Leo	The Lion	Λέων (Leōn)	Siṃha (सिंह)	MULUR.GU.LA "The Lion"
6	♍	150°	Virgo	The Maiden	Παρθένος (Parthenos)	Kanyā (कन्या)	MULAB.SIN "The Furrow"; "The Furrow, the goddess Shala's ear of corn"
7	♎	180°	Libra	The Scales	Ζυγός (Zygos)	Tulā (तुला)	MULZIB.BA.AN.NA "The Scales"
8	♏	210°	Scorpio	The Scorpion	Σκoρπιός (Skorpios)	Vṛśhchika (वृश्चिक)	MULGIR.TAB "The Scorpion"
9	♐	240°	Sagittarius	The (Centaur) Archer	Τοξότης (Toxotēs)	Dhanuṣha (धनुष)	MULPA.BIL.SAG, Nedu "soldier"
10	♑	270°	Capricorn	"Goat-horned" (The Sea-Goat)	Αἰγόκερως (Aigokerōs)	Makara (मकर)	MULSUḪUR.MAŠ "The Goat-Fish" of Enki
11	♒	300°	Aquarius	The Water-Bearer	Ὑδροχόος (Hydrokhoos)	Kumbha (कुम्भ)	MULGU.LA "The Great One", later qâ "pitcher"
12	♓	330°	Pisces	The Fishes	Ἰχθύες (Ikhthyes)	Mīna (मीन)	MULSIM.MAḪ "The Tail of the Swallow", later DU.NU.NU "fish-cord"

18th century star map illustrating how the feet of Ophiuchus cross the ecliptic

Constellations

Equirectangular plot of declination vs right ascension of the modern constellations with a dotted line denoting the ecliptic. Constellations are colour-coded by family and year established. (detailed view)

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, and so the sun is not in each constellation for the same amount of time. 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. 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 1995, following an announcement on the BBC Nine O'Clock News 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 (perhaps for the purposes of promoting a forthcoming BBC astronomy program) the 67-year-old 'news' of the IAU's decision to alter the number of designated ecliptic constellations. The assertion that Ophiuchus constitutes an astrological sign periodically resurfaces in the media, due to a failure to appreciate that the irregular astronomical demarcation of the thirteen ecliptical 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.

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. 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.

Sign				Constellation
Name	Symbol	Tropical zodiac (2011)	Sidereal zodiac (2011)	Name	IAU boundaries	Solar stay	Brightest star
Aries		21st March – 19th April	15 April - 15 May	Aries	19 April – 13 May	25 days	Hamal
Taurus		20th April – 20th May	16 May - 15 June	Taurus	14 May – 19 June	37 days	Aldebaran
Gemini		21st May – 20th June	16 June - 15 July	Gemini	20 June – 20 July	31 days	Pollux
Cancer		21st June – 22nd July	16 July - 15 August	Cancer	21 July – 9 August	20 days	Al Tarf
Leo		23rd July – 21st August	16 August - 15 September	Leo	10 August – 15 September	37 days	Regulus
Virgo		22nd August – 22nd September	16 September - 15 October	Virgo	16 September – 30 October	45 days	Spica
Libra		23rd September – 22nd October	16 October - 15 November	Libra	31 October – 22 November	23 days	Zubeneschamali
Scorpio		23rd October – 21st November	16 November - 15 December	Scorpius	23 November – 29 November	7 days	Antares
n/a				Ophiuchus	30 November – 17 December	18 days	Rasalhague
Sagittarius		22nd November – 21st December	16 December - 14 January	Sagittarius	18 December – 18 January	32 days	Kaus Australis
Capricorn		22nd December – 19th January	15 January – 14 February	Capricornus	19 January – 15 February	28 days	Deneb Algedi
Aquarius		20th January – 19th February	15 February - 14 March	Aquarius	16 February – 11 March	24 days	Sadalsuud
Pisces		20th February – 20th March	15 March - 14 April	Pisces	12 March – 18 April	38 days	Eta Piscium

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.

Precession of the equinoxes

Further information: Axial precession, Epoch (astronomy), Sidereal and tropical astrology, Astrological age, and Ayanamsa

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, it is assumed, the precession of the equinoxes was unknown, as the system made no allowance for it. 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 of vernal equinox.

Western astrology 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").

The sidereal coordinate system takes into account the ayanamsa, a Sanskrit word where literally ayan means transit or movement and amsa means small part i.e. movement of equinoxes in small parts. It is unclear when Indians became aware of the precession of the equinoxes, but Bhaskar-ii in 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.

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

Further information: 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.

Determining Zodiac signs of planets and the Sun

Further information: Astrological sign

Location of the planets on 30 January 1980

Location of the planets on 15 July 1980

In astrology, each planet and the sun have a corresponding zodiac sign that is determined by their location relative to Earth at the time of one's birth.

In the first image to the right, the blue circle is Earth. The yellow lines represent the division of the 12 Zodiac signs, and each planet falls within one. For example, on 30 January 1980, Mercury and the Sun were in Aquarius, Venus was in Pisces, and Mars was in Scorpio.

Six months and 15 days later, all of the planets have continued along their orbits and the Zodiac signs changed. For someone born on 15 July 1980, Venus falls in Gemini, the Sun and Mercury are in Cancer, and Mars is in Libra.

Mnemonics

There are many mnemonics for remembering the 12 signs of the zodiac in order. A traditional mnemonic:

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 mnemonic is the following:

The Ramble Twins Crab Liverish;
Scaly Scorpions Are Good Water Fish.

"Ramble" is for Ram (Aries), Bull (Taurus). "Twins" is for Gemini and "Crab" for Cancer. "Liverish" recalls Lion, Virgin (Leo and Virgo). "Scaly" recalls Scale (Libra). "Scorpion" is for Scorpio, and "Are" for Archer (Sagittarius). "Good" is for Goat (Capricorn), "Water" for the Water Bearer (Aquarius) and "Fish" for Pisces.

Mnemonics in which the initials of the words correspond to the initials of the star signs (Latin, English, or mixed):

All The Great Constellations Live Very Long Since Stars Can't Alter Physics.

As The Great Cook Likes Very Little Salt, She Compensates Adding Pepper.

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

All That Gold Can Load Very Lazy Students Since Children Are at Play

Unicode characters

In Unicode, the symbols are encoded in block Miscellaneous Symbols:

1. U+2648 ♈ ARIES
2. U+2649 ♉ TAURUS
3. U+264A ♊ GEMINI
4. U+264B ♋ CANCER
5. U+264C ♌ LEO
6. U+264D ♍ VIRGO
7. U+264E ♎ LIBRA
8. U+264F ♏ SCORPIUS
9. U+2650 ♐ SAGITTARIUS
10. U+2651 ♑ CAPRICORN
11. U+2652 ♒ AQUARIUS
12. U+2653 ♓ PISCES

See also

• Alexander A. Gurshtein
• Astronomical symbols
• Circle of stars
• Cusp (astrology)
• Elements of the zodiac

References

• B. L. van der Waerden, "History of the zodiac", Archiv für Orientforschung 16 (1953) 216–230.
• Emelianov, Vladimir. Nippurskij kalendar' i rannjaja istorija Zodiaka (Nippur Calendar and the Early History of Zodiac). St.-Petersburg, Peterburgskoje vostokovedenje, 1999.
• Nautical Almanac and Astronomical Ephemeris for the year 1767. London: Board of Longitude, 1766. Facsimile, Google Books.
• Ptolemy, Claudius (1998). The Almagest. Princeton, New Jersey: Princeton University Press. ISBN 0-691-00260-6. 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, summarized by anonymous editor, Wayback version.
• 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.
• Rochberg, Francesca (1998), "Babylonian Horoscopes", American Philosophical Society, New Series, Vol. 88, No. 1, pp i-164
• Sachs, Abraham (1948), "A Classification of the Babylonian Astronomical Tablets of the Seleucid Period", Journal of Cuneiform Studies, Vol. 2, No. 4, pp. 271–290
• Shapiro, Lee T. (2011 April 27) "Constellations in the zodiac." NASA.

External links

Media related to Zodiac at Wikimedia Commons

• "A Treatise on Zodiacal Signs and Constallations: 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.