Zodiac

For other uses, see Zodiac (disambiguation).

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).

Zodiac (from Greek ζῳδιακός [κύκλος], "circle of little animals", from derived from ζῴδιον, the diminutive of ζῷον "animal") denotes an annual cycle of twelve stations along the ecliptic, the apparent path of the Sun across the heavens through the constellations that divide the ecliptic into twelve equal zones of celestial longitude. The zodiac is recognized as the first known celestial coordinate system. Babylonian astronomers developed eighteen signs which are probably the precusor to the modern zodiac in about the fifth century BC. The Mayans used twenty signs. The term zodiac may come from the Latin zōdiacus, from the Greek ζῳδιακός [κύκλος] (zodiakos kyklos), meaning "circle of animals", derived from ζῴδιον (zodion), the diminutive of ζῷον (zoon) "animal". The American Heritage Dictionary(1970) derives the word further from Indo-European 'gwei-', 'to live'. 'zoe', 'life' is listed as the suffixed form of this Indo-European word. However, the classical Greek zodiac also includes signs (also constellations) that are not represented by animals (e.g., Aquarius, Virgo, Gemini, and—for some—Libra). Another suggested etymology is that the Greek term is cognate with the Sanskrit sodi, denoting "a path", i.e., the path through which the Sun travels.

The zodiac also means a region of the celestial sphere that includes a band of eight arc degrees above and below the ecliptic, and therefore encompasses the paths of the Moon and the naked eye planets (Mercury, Venus, Mars, Jupiter, and Saturn). The classical astronomers called these planets wandering stars to differentiate them from the fixed stars of the celestial sphere (Ptolemy). Astrologers understood the movement of the planets and the Sun through the zodiac as a means of explaining and predicting events on Earth.

Classical zodiac

The zodiac signs as shown in a 16th century woodcut

Further information: History of astrology and History of 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. The construction of the zodiac is described in Ptolemy's Almagest. The zodiac is also understood as a region of the celestial sphere that includes a band of eight arc degrees above and below the ecliptic, and therefore encompasses the paths of the Moon and the naked eye planets (Mercury, Venus, Mars, Jupiter, and Saturn). The classical astronomers called these planets wandering stars to differentiate them from the fixed stars of the celestial sphere (Ptolemy).

History

The division of the ecliptic into the zodiacal signs originates perhaps in Babylonian ("Chaldean") astronomy as early as the 1st millennium BC (likely during Median/"Neo-Babylonian" times) (Powell 2004).

The zodiacal signs remain in use as the basis of an ecliptic coordinate system, though modern astronomers tend to use an equatorial coordinate systems since Early Modern times. One can see the use of the sidereal coordinate system as late as 1,000 AD from 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 Babylonian calendar assigns each month a constellation, beginning with the position of the Sun at vernal equinox. Babylonian astronomers at some point during the 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).

Signs

Below are the Roman names of the signs of the zodiac (with the ecliptic longitudes of their first points). These figures represent the equivalent J2000.0 ecliptic longitudes for the sidereal zodiac described here in this article. The figures also represent ecliptic longitudes for a tropical zodiac where 0° Aries is understood as the vernal equinox:

1. ♈ Aries (0°) (The Ram)
2. ♉ Taurus (30°) (The Bull)
3. ♊ Gemini (60°) (The Twins)
4. ♋ Cancer (90°) (The Crab)
5. ♌ Leo (120°) (The Lion)
6. ♍ Virgo (150°) (The Virgin)
7. ♎ Libra (180°) (The Scales)
8. ♏ Scorpio (210°) (The Scorpion)
9. ♐ Sagittarius (240°) (The Archer)
10. ♑ Capricornus (270°) (The Sea-goat)
11. ♒ Aquarius (300°) (The Water Carrier)
12. ♓ Pisces (330°) (The Fish)

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.

Sidereal versus tropical

Astrologers use astronomical observations of the movements of the night sky for divinatory purposes. The zodiac remains in use in modern astrology, though the issue of tropical astrology (used mainly by Western astrologers) and sidereal astrology (used mainly by Indian astrologers) is central. At issue in the debate is whether the signs should be defined in terms of zones derived from nodal points defined by Earth's motion during a tropical year, or whether the signs should be defined in terms of signs roughly aligned with the constellations of the same name (for sidereal astrologers). This matters because of an astronomical phenomenon called the precession of the equinoxes, whereby the position of the stars in the sky has changed over time. The axis of rotation of the Earth slowly changes direction, making one complete turn approximately every 26,000 years. Originally, Aries corresponded to the summer equinox for the Northern hemisphere, but after about 1/4 of a cycle since the zodiac was invented, Aries now corresponds to 1/4 of the year, roughly April. Likewise, over the centuries the twelve zodiacal signs in Western astrology no longer correspond to the same part of the sky as their original constellations, or their Indian counterparts. In effect, in Western astrology the link between sign and constellation has been broken, whereas in Indian astrology it remains of paramount importance.

The celestial coordinate system described above is what astronomers call a sidereal system of coordinates. In other words it defines the coordinates in relation to what ancient astronomers called the fixed stars (as opposed to the planets other than Earth which were called wandering stars). One could also call this a celestially centered system of coordinates. In time the ancient astronomers such as Hipparchus discovered these fixed stars were not fixed relative to the Earth's tropical year. Due, in part, to the precession of the Earth discussed above, the Earth completed its orbit after it has already completed the tropical cycle: for example: for the cycle of the Sun starting directly over the Tropic of Cancer then to the Tropic of Capricorn and return to the Tropic of Cancer again. Or as another example consider the motion of the Sun from one vernal equinox to the next; the Earth would complete such a cycle shortly before it completed an entire orbit around the Sun. (24 minutes and 20 seconds before). The difference is very subtle, but as astronomers found archival records to compare their sightings with sightings of previous astronomers, the discrepancy thus became apparent. Some estimates of the rate of precession suggest that over a period of 27,000 tropical years, the Earth will have orbited the Sun only 26,999 times. That Hipparchus in the second century BC could recognize and document such a subtle process which is now known as the precession of the equinoxes could be considered remarkable.

Some modern astronomers began to mark the stars according to a tropical zodiac (or other tropical coordinate systems such as the equatorial coordinate system). This tropical zodiac system of coordinates designates the origin of the longitude of the celestial sphere as the first point in Aries. The term may be derived from the constellation of Aries, but this point instead marks the position of the Sun at the time of the vernal equinox for a specified epoch. Among other things, this epoch specifies the first point in Aries and establishes a unique fixed reference point for the tropical system of coordinates. The use of the phrase "first point in Aries" causes some confusion when considering sidereal versus tropical systems of coordinates. The first point in Aries in the sidereal system of coordinates, would be the first star in the Aries sign or perhaps the boundary of that sign. Whereas in tropical coordinates, the vernal equinox defines this point. During the time of Ptolemy's observations and cataloguing of stars the sidereal and tropical longitudinal origins differed by a magnitude of perhaps less than 2°. The close convergence of these two systems of coordinates — combined with the varied interpretations of the phrase "first point in Aries" — makes it difficult to discern Ptolemy's longitudinal origin (see Peters and Knobel 1915).

More recently, in AD 2000 for example, the first point in Aries and the boundary of the sign of Aries — based on the specification of zodiac signs above — diverged by about 25°. In terms of the tropical system, this places the first point in Aries (in other words, the vernal equinox) in the Pisces constellation, near the projection of the NGC 7787 spiral galaxy. Other specifications of zodiac signs (whether sidereal or tropical) choosing different fixed points (in the celestial sphere for sidereal or in relation to Earth's seasonal cycle for tropical) would result in a different divergence either greater than or less than 25°. For example Cyril Fagan's sidereal zodiac is offset from the J2000.0 tropical zodiac by greater than 39° (as of 1977). This difference between the position of fixed stars in the tropical and sidereal coordinate systems is called the Ayanamsa.

Table of constellations vs. zodiac signs

The symbols used in Western astrology to represent the astrological signs

This table provides a comparison between the dates the Sun enters and passes away from the zodiac signs and constellations as defined by various specifications.

Note the ecliptic passes through a thirteenth constellation (or more, depending upon the opinions of astronomers of any given century), Ophiuchus (the serpent bearer), as already recognized in Ptolemy's Almagest, often represented by the rod of Asclepius. Notably, Ophiuchus occupies an honored place along the zodiac: amidst the cluster of dust and clouds looking toward the center of the Milky Way galaxy; although not part of the constellation, Barnard's Star is located within Ophiuchus (this is one of the nearest stars to the Solar System, and it has the largest known proper motion of any star relative to the Sun).

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; note that the sidereal system of Cyril Fagan, introduced in 1944, is again different, with Aries beginning on April 30.
• the astronomical constellation of the same name as the sign, with constellation boundaries as defined in 1930 by the International Astronomical Union.

The dates can vary by as much as 2 days from year to year, depending on the cycle of leap years.

Constellation	Astrological sign	Dates of Sun's presence				Solar stay in constellation	Brightest star in constellation	Astrological birthstone
		Astrological tropical zodiac	Astrological duration (in days)	Astrological sidereal (Jyotisha) zodiac	Astronomical constellations
Aries	Aries	March 21 – April 20	30	April 14 – May 14	Aries, April 18 – May 13	25.5 days	Alpha Arietis	diamond
Taurus	Taurus	April 21 – May 22	31	May 15 – June 14	Taurus, May 13 – June 21	38.2 days	Aldebaran	emerald
Gemini	Gemini	May 22 – June 23	31	June 15 – July 16	Gemini, June 21 – July 20	29.3 days	Pollux	alexandrite
Cancer	Cancer	June 23 – July 23	31	July 17 – August 16	Cancer, July 20 – August 10	21.1 days	Beta Cancri	pearl
Leo	Leo	July 23 – August 23	32	August 17 – September 16	Leo, August 10 – September 16	36.9 days	Regulus	peridot, ruby
Virgo	Virgo	August 23 – September 23	31	September 17 – October 17	Virgo, September 16 – October 30	44.5 days	Spica	sapphire
Libra	Libra	September 23 – October 23	30	October 18 – November 16	Libra, October 30 – November 20	21.1 days	Beta Librae	opal
Scorpius	Scorpio	October 24 – November 22	30	November 17 – December 15	Scorpius, November 20 – November 29	8.4 days	Antares	topaz
Ophiuchus	no sign	n/a	n/a	n/a	Ophiuchus, November 29 – December 17	18.4 days	Alpha Ophiuchi	n/a
Sagittarius	Sagittarius	November 23 – December 21	30	December 16 – January 14	Sagittarius, December 17 – January 20	33.6 days	Epsilon Sagittarii	turquoise
Capricornus	Capricorn	December 22 – January 20	29	January 15 – February 12	Capricornus, January 20 – February 16	27.4 days	Delta Capricorni	garnet
Aquarius	Aquarius	January 20 – February 19	30	February 13 – March 14	Aquarius, February 16 – March 11	23.9 days	Beta Aquarii	amethyst
Pisces	Pisces	February 20 – March 21	30	March 15 – April 13	Pisces, March 11 – April 18	37.7 days	Eta Piscium	Aquamarine

Precession of the equinoxes

Main article: Precession of the equinoxes

File:Jewellery fameo pendant.jpg

Reversible pendant representing a star map of the zodiac signs.

The signs of the zodiac do not necessarily coincide with the actual constellations for which they are named. Because of the division of the zodiac into 12 signs of 30° each; due to various specifications for the boundaries of the constellations; and especially due to the precession of the equinoxes for the tropical system of coordinates, the constellations should not be confused with zodiac signs. As described above, due to precession the tropical signs have moved away from their corresponding constellations, so that today, the beginning of the tropical sign of Aries (defined as the position of the Sun on the vernal equinox) lies somewhere within the constellation Pisces.

It is not entirely clear how ancient astronomers responded to this phenomenon of precession once they discovered it. 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 cycle of the Earth's seasonal cycle rather than its orbital cycle. Such a view is consistent with the reading of Ptolemy as a geocentrist. The geocentrist view understands the motion of celestial objects in strict relation to the Earth as a fixed frame of reference. This view understands the celestial sphere as rotating around the Earth like the spheres of the other planets and the moon: only more slowly. The Earth is the center of everything and is fixed in the same frame of reference as the Universe. The stars precess in relation to the Earth not the other way around. Modern astronomers typically read such a view in Ptolemy who writes: "the sphere of the fixed stars also performs a motion of its own in the opposite direction to the revolution of the universe, that is [the motion of] the great circle through both poles, that of the equator and that of the ecliptic." By "revolution of the universe", Ptolemy refers to the daily cycle that heliocentrists understand as the rotation of the Earth. However, one also finds evidence in Ptolemy's The Almagest that he expresses a view of a fixed celestial sphere; or at least that he understands the difference between the relative motions of each. After cataloguing over 1,000 stars he describes a method for constructing a model of the stars:"Since it is not reasonable to mark the solstitial and equinoctial points on the actual zodiac of the globe (for the stars depicted [on the globe] do not retain a constant distance with respect to these points), we need to take some fixed starting-point in the delineated fixed stars" (emphasis added; brackets are translator's insertions). So Ptolemy's response to the issue of precession is that the zodiac moves through the equinox and also he makes it clear he understands that the equinox moves through the zodiac.

The zodiacal signs remain in use as the basis of an ecliptic coordinate system, though modern astronomers tend to use an equatorial coordinate systems since Early Modern times. One can see the use of the sidereal coordinate system as late as AD 1000 from 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, equaling a J2000.0 ecliptic longitude of 224° (the 14th degree from the beginning of Scorpius at 210°).

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 modern astronomy

In astronomy, the zodiacal constellations are a convenient way of marking the ecliptic (the Sun's path across the sky). The zodiac is also a way for astronomers to mark the path of the moon and planets , as their movements also remain within these constellations. Apart from this role, the zodiacal constellations have no extra significance to astronomers than any other constellation.

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.

Similar systems in other cultures

The term "zodiac" is sometimes also extended to other systems of astrology, both related and unrelated to the Babylonian zodiac.

Hindu zodiac

Further information: Jyotisha

The Greek zodiac was directly adopted into Hindu astrology, where the individual signs are called 'rāshi. Whereas Western astrology uses the tropical zodiac (where the signs are measured from the point of the Spring Equinox on the ecliptic), the Hindu system favors the sidereal zodiac (where the signs are aligned with their eponymous constellations). 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"). Template:Rashi table

Chinese zodiac

Main article: Chinese Zodiac

Chinese astrology also has a system of twelve signs sometimes also referred to as "zodiac". This does not necessarily imply a common origin, since the number of twelve naturally suggests itself from the number of synodic months in a year; in other words, the extent of a zodiacal sign corresponds to the path covered by the Sun between two new moons. Like its Western counterpart, the Chinese zodiac features animals. However, the Chinese zodiac associates each animal with both one month and one solar year. Thus the signs repeat themselves every twelve year cycle. The animals of the Chinese Zodiac are: rat (or mouse), ox (or cow), tiger, rabbit (or hare), dragon (the best), snake, horse, sheep (or goat), monkey, rooster, dog, and pig (or boar). For a list of how these animals map to the months and years see Chinese astrology. Formerly, these animals were also used in the naming of the Chinese hour, where the day is divided into 12 hours. There is also a Chinese lunar zodiac composed of twenty-eight lunar "mansions", each corresponding to a Chinese constellation.

Maya zodiac

The Maya, who once had a highly developed astrology, also possessed a zodiac of some kind, which has been preserved in the Paris Codex (around AD 1200), and includes constellations such as 'Peccary' (possibly corresponding to Gemini) and 'Turtle'. Other evidence suggests Incan and Aztec cultures of the Western hemisphere also noted celestial events along the zodiac.

Hebrew zodiac

The Biblical writers were familiar with the same system of constellations as that which we know today, except that the name of the Eagle seems to have been usually substituted for the Scorpion.

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, the Waterer; and the Eagle is Scorpio. St. John lists them here in counter-clockwise order, backward around the Zodiac; but when he uses them in the structure of his prophecy itself, he lists them in the direct order of the seasons. After the Preamble (chapter 1), the Revelation is divided into four quarters, each "ruled" by one of these creatures. The first quarter (Chapters 2-3) was ruled by Taurus; thus the emphasis on the Seven Stars, on the shoulder of the Bull. The second quarter (Chapters 4-7) is ruled by the figure of "the Lion of the Tribe of Judah," who has conquered to open the sealed Book. The Eagle flies in midheaven with cries of woe throughout the third quarter (Chapters 8-14). And the fourth quarter (Chapters 15-22) is governed by the Man, Aquarius the "Water-Pourer" (cf. the pouring out of the Chalices of wrath, and the River of Life flowing out from the Throne).

The arrangement of the twelve tribes of Israel around the Tabernacle (Numbers 2) corresponded to the order of the Zodiac; and, like the cherubim, 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.

Mesopotamian Zodiac

The creation myth of the Enuma Elish starts its ending by creating the Zodiac as a home for gods, in Tablet 5. It references Nibiru-(as Jupiter) as the prominent planet in the sky.

Other zodiacs

In New Age or Occultist movements there are sometimes claims of even other systems such as a "Celtic zodiac" based on the lunisolar Celtic calendar, or a "Galactic zodiac".

Mnemonics for the zodiac

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 and perhaps more memorable, mnemonic is the following:

The Ramble Twins Crab Liverish;

Scaly Scorpions Are Good Water Fish.

(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)

See also

• Alexander A. Gurshtein
• Astrological sign
• Astrology and alchemy
• Astronomical symbols
• Babylonian influence on Greek astronomy
• Chinese Zodiac
• Circle of stars
• Cusp (astrology)
• Elements of the zodiac
• Esoteric cosmology
• History of astrology
• History of astronomy
• Sefer Raziel HaMalakh
• The Zodiac : Cosmic Sounds, concept album based on the signs of the Zodiac
• Zodiacal cloud
• Zodiacal light

External links

• Template:Harvard reference Translated and annotated by G.J. Toomer ; with a foreword by Owen Gingerich.
• Powell, Robert (2004). The Definition of the Babylonian Zodiac PhD thesis, entitled "The Definition of the Babylonian Zodiac and the Influence of Babylonian Astronomy on the Subsequent Defining of the Zodiac".
• Tropical v Sidereal debate First published in 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.
• Enuma Elish: The full surviving text of the Enûma Elish.