This article is about the astrological and astronomical concept of the zodiac. For other uses, see zodiac (disambiguation)

The term zodiac (from Greek zodiakos ^*, "circle of little animals", from zodiaion, the diminutive of zoon "animal") denotes several places where a circle of twelve animals occurs. In Chinese astrology, months and years pass through a cycle of twelve animals that imply certain fortunes or misfortunes related to events occurring within those signs.

Likewise, Indo-European cultures developed a zodiac of twelve signs associated with a yearly cycle and with constellations of stars that lie along the apparent path of the Sun across the heavens (the ecliptic). The ecliptic is the path of the Sun independent of Earth's rotation, so that it is visible as the position of the Sun as seen at the same time each day. Eventually, "ancient astronomers" (Powell 2004) divided those constellations 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 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 diffentiate them from the fixed stars of the celestial sphere (Ptolemy).

Strictly speaking, the eventual culmination of the Indo-European zodiac (originating with Babylonian, Greek, and Indian astronomers) includes signs (also constellations) that are not all represented by animals (e.g., Libra, Virgo, Gemini). However, the term probably derives from earlier conceptions of the constellations along the ecliptic as sculpted animals. As with the Chinese zodiac, Indo-European astrologers understand the movement of the planets and the Sun through the zodiac as a method to explain and predict events on Earth.

The zodiac symbols are early modern simplifications of conventional pictorial representations of the signs, attested since Hellenistic times. The characters are encoded in unicode at positions U+2648 to U+2653 (hexadecimal numbers): . The glyph representation of these characters will depend on the font in which they are displayed.

Zodiac celestial coordinate systems

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For any spherical celestial coordinate system one needs to define an equatorial plane and designate an origin for longitude. From these definitions longitudinal meridians perpendicular to the equatorial plane meet at the north and south poles of the celestial sphere and one can precisely specify a unique position on the sphere.

First, the zodiac coordinate system designates the ecliptic as the equatorial plane. One should not confuse the zodiac's equatorial plane with the Earth's equator which is so named because it serves as the equatorial plane for our terrestrial coordinate system. Instead the ecliptic is aligned with the Earth's orbital plane with the Sun. The Earth tilts at an angle of approximately 23° with respect to the orbital plane. This tilt is partly due to the Earth's precession as it gyrates and rotates on its axis. It contributes to the divergence between a tropical year and a sidereal year and thus contributes to the precession of the equinoxes which marks the difference between tropical and sidereal celestial coordinate systems.

Second, a system of coordinates must normally specify an origin (0° point) for longitude. However, the zodiac system names twelve equal regions (called signs) and technically any sign can serve as the beginning or the end of the sphere's precession. However, the system further divides each sign into thirty degree units. One reason this system of coordinates was particularly useful in antiquity is that writing large numbers was difficult before the introduction of arabic numbers. The zodiac system of coordinates kept every number below ninety (the highest value for zodiac latitudes). Since the constellations along the ecliptic varied in size, defining 12 equal signs of 30° each did require an arbitrary assignment of boundaries roughly corresponding to the ecliptic constellations.

It is believed many of the classical astronomers specified zodiac signs using two bright stars near the ecliptic and opposite each other to serve as equatorial nodes or poles (not longitudinal poles): Aldebaran and Antares in the constellations Taurus and Scorpius respectively. These stars served rather well because not only were they on opposites sides of the ecliptic, but they also fell very near the center of their constellations and were therefore designated as Taurus 15 and Scorpius 15, meaning the middle 15° points within those signs. From these two stars then the remaining equatorial boundaries of the 12 signs of the zodiac follow (see Powell 2004).

Below are the Greek names of the signs of the zodiac (with the ecliptic longitudes of their first pointsThese 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°)
2. ♉ Taurus (30°)
3. ♊ Gemini (60°)
4. ♋ Cancer (90°)
5. ♌ Leo (120°)
6. ♍ Virgo (150°)
7. ♎ Libra (180°)
8. ♏ Scorpius (210°)
9. ♐ Sagittarius (240°)
10. ♑ Capricornus (270°)
11. ♒ Aquarius (300°)
12. ♓ Pisces (330°)

Sidereal versus tropical

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 sitings with sitings of previous astronomers and 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. It is remarkable 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.

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 fot the tropical system of coordinates. The current epoch used is J2000.0 which indicates the position of the stars at the time and day of the vernal equinox in the year 2000 of a Julian calendar (a sidereal calendar) calibrated so that January 1st 2000 at 00:00 TT coincides with the same time on the common Gregorian calendar.

The use of the phrase "first point in Aries" causes some confusion when considering sidereal verses 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 cataloging 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 2000 AD 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 sytem, 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.

The equinox moving through the sidereal signs

Below are several images depicting how the vernal equinox precesses through the celestial sphere from 1,500BC through 2,500AD. The constellations are shown with stick figures and with labels. Also the planets are labeled that lie near the vernal equinox. Finally, stars that lie near the outer boundaries of Aries (HIP16641) and Pisces (14 Piscium/HIP116323) as well as the dividing point between the two (HIP7243) are marked to provide some sense of where the first point in Aries (the vernal equinox in terms of a tropical zodiac, since it's nearly impossible to use the arbitrary constellations for any reference point) is in relation to the sidereal signs.

Click on any image to see a larger view.

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Comparison to modern systems

Though perhaps as old as 4,000 years, the zodiac coordinate system boasts advantages over its more common modern counterparts. Since the coordinate system is celestially centered, it is insulated from the many eccentricities of Earth's motion: including its rotation, intricacies of Earth time, precession, nutation and its eliptical and perterbed orbit around our Sun. To use the zodiac coordinate system all one has to know is where to find one of the nodal constellations that include the fixed reference points for the system: Taurus and Scorpio. Since those constellations are located on opposite sides of the ecliptic along the zodiac, one should always be visible in the night sky. Also these constellations are both located within the prominent band of cloud and dust of the Milky Way. From these constellations astronomers can orient themselves for locating any point in the coordinate system.

The modern, commonly used tropical systems require an observer to input the current mean sidereal time, the observers terrestial longitude and latitude, the epoch the observer wishes to utilize and account for other peculiarities of Earth's motion. Of course, modern astronomical computers handle most of the tasks fot observers, but it involves a large effort by many different astronomers behind the scenes.

In addition, much of the motion of the stars in modern tropical coordinate systems can be attributed wholly to these peculiarities of Earth's motion. Astronomers make the distinction between the proper motion of a star (typically relatively subtle), from the other motion that arises totally from the deisignation of a tropical rather than sidereal coordinate system. One example where this exhibits itself is in the constellation boundaries drawn up by the IAU. The neat constellation boundaries drawn in 1930 exhibit increasingly distorted bondary lines over time. This may seem like something of little consequence, but why bother drawing neat boundaries around constellations if they inherently become erratic in the dominant coordinate system in use then and now.

Finally, since the zodiac system uses the ecliptic rather than the terrestial equator for its equatorial plane it is not succeptible to the drifting of stars across the celestial equator as in the commonly used equatorial coordinate system (right ascension, declination). In The Almagest Ptolemy criticizes Hipparchus’ use of an equatorial plane in some of Hipparchus’ variously specified coordinate systems for this very reason (Ptolemy 1998).

These advantages make the zodiac coordinate system a very efficient system of coordinates in terms of requiring very little human-hours of labor to use and maintain: issues particularly important to early astronomers, typically working in often in isolation from one another.

The key disadvantage of a zodiac system of coordinates is if the nodal stars that serve as the fixed reference points for the system exhibit significant proper motion so that within the system of coordinates every other star appears to move dramatically in unison. In other words selecting a star that has eccentiricities compared to the other stars undermine the usefullness of the system of coordinates. For example the inadvertant selection of an astroid or an entire galaxy outside our Milky Way would lead to this condition. The reason for this is that celestial objects outside our galaxy revolve around our galaxy in a period of about 220 million years, at least in terms of a frame of reference affixed to our Sun and its neighboring stars. The only other sidereal coordinate system in common use today (that shares many of the zodiac’s advantages) is the galactic coordinate system. In galactic coordinates, the plane of the Milky Way and its own axial center serve as the fixed referents. These are fairly logical reference points for a coordinate system, though of course they cannot be located with the naked eye.

Another disadvantage relates to the apparatus required for orienting one to the coordinate system. Using geocentric coordinates astronomers can easily calibrate their instruments to the fixed reference point. As long as astronomers can obtain an accurate compass reading, they can oritent themselves to a geocentric coordiante system (such as ecliptic or equatorial coordinates). Using zodiac coordinates requires an astronomer to locate the correct star (Anteres or Aldebaran) and the correct constellation (Scorpio or Taurus respectively) and make an accurate reading of the position of that star and accurately orient that star to the ecliptic. This may take more skill than a mere compass reading: especially for amateur astronomers.

Zodiac in astrology

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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 versus sidereal coordinate systems is even more fundamental in (see sidereal versus tropical above, tropical astrology and sidereal astrology). At issue in the debate is whether the signs should be defined in terms of signs roughly aligned with the constellations of the same name (for sidereal astrologers) or whether the signs should be defined in terms of zones derived from nodal points defined by Earth's motion during a tropical year. The selection of two different astronomical coordinate systems implies the Sun moves through each sign at different times of the year, leading to different mapping of the signs to the days of the year for each of these systems.

Regardless of whether the tropical or sidereal definition of the zodiac is chosen, astrologers chart the positions of the Sun and planets in relation to the zodiacal signs. The horoscope is used to associate properties of a sign with the properties of planets positioned in that sign. The planet is said to influence events on Earth but the way in which it does this is modified by the influence of the sign in which it is positioned.

Hindu astrology and the Zodiac

Traditional Hindu astrology has a siderial coordinate zodiac system with twelve signs. The names of the Hindu zodiacal signs, or s, are similiar to Graeco-Babylonian signs, apparently as a result of Indo-Greek contact:

1. "ram" (Aries)
2. "bull" (Taurus)
3. "a pair" (Gemini)
4. "crab" (Cancer)
5. "lion" (Leo)
6. "girl" (Virgo)
7. ', from ' "balance" (Libra)
8. "scorpion" (Scorpius), also kaurpi, loaned from the Greek
9. ' "bow, arc", ' "armed with a bow" (Sagittarius)
10. ' "antelope", also ' "sea-monster" (Capricornus)
11. "pitcher, water-pot" (Aquarius)
12. ' "fish", also ' after specific kinds of fish (Pisces)

This "Hindu zodiac" (', ') thus has similarities to Greek zodiac. The Graeco-Babylonian system of twelve signs overlays the native Hindu system of nine grahas or houses. The difference between the timing of Hindu s and the Western zodiacal signs is due to the sidereal constellation system used by the Hindus, linking the signs to constellations rather than the course of the Sun.

Other zodiac astrologies

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. 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, ox, tiger, rabbit (or hare), dragon, snake, horse, sheep (or goat), monkey, rooster (or phoenix), dog, pig (or boar) For a list of how these animals map to the months and years see Chinese astrology.

Beyond the traditional Chinese system, In New Age or Occultist movements there are sometimes claims of even other systems such as a "Celtic zodiac"Celtic calendar, or a "Galactic zodiac" [http://www.geocities.com/astrologyzodiacs/index.htm" target="_blank" >^*. Other evidence suggests Mayan, Incan and Aztec cultures of the Western hemisphere also noted celestial events along the zodiac.

Historical origin

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By 2,000 BC, the Egyptians and Mesopotamians marked the seasons by the constellations we now call Taurus, Leo, Scorpio and Aquarius. But the marking of seasons by constellations may go back to 5,000 BC.WorldBook Encyopedia (2001) 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).

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 geo-centrist view understand 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 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 understand the difference between the relative motions of each. After cataloging 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 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 translators 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 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 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.

Table of zodiac signs and constellations

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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 signs; and especially due to the precession of the equinoxes for the tropical system of coordinates, the zodiacal signs should not be confused with the constellations of the zodiac. 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. This table provides a nice 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, Ophiuchus (the serpent), as already recognized in Ptolemy's Almagest (Ptolemy 1998), without there being any sign corresponding to Ophiuchus. Of note too, Ophiuchus occupies an honored place along the zodiac: admidst the cluster of dust and clouds looking towards the center of the Milky Way galaxy.

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 30 April.
• the astronomical constellation by the same name, 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.

Sign	meaning, symbol	Dates of Sun's presence			Elements of the zodiac	Quality	Ruling Planet
		tropical zodiac	sidereal (Jyotisha) zodiac	Constellation (IAU definition as of 1977)
Aries	March 21 – April 20	April 14 – May 14	Aries, April 19 – May 14	Fire	Cardinal	Mars
Taurus	April 21 – May 21	May 15 – June 14	Taurus, May 14 – June 21	Earth	Fixed	Venus
Gemini	May 22 – June 21	June 15 – July 16	Gemini, June 21 – July 21	Air	Mutable	Mercury
Cancer	June 22 – July 22	July 17 – August 16	Cancer, July 21 – August 11	Water	Cardinal	the Moon
Leo	July 23 – August 23	August 17 – September 16	Leo, August 11 – September 17	Fire	Fixed	the Sun
Virgo	August 24 – September 23	September 17 – October 17	Virgo, September 17 – October 31	Earth	Mutable	Mercury
Libra	September 24 – October 23	October 18 – November 16	Libra, October 31 – November 23	Air	Cardinal	Venus
Scorpio	October 24 – November 22	November 17 – December 15	Scorpius, November 23 – November 30	Water	Fixed	Pluto/Mars
—	—	—	—	Ophiuchus, November 30 – December 18	—	—	—
Sagittarius	November 23 – December 21	December 16 – January 14	Sagittarius, December 18 – January 19	Fire	Mutable	Jupiter
Capricorn	December 22 – January 20	January 15 – February 12	Capricornus, January 19 – February 16	Earth	Cardinal	Saturn
Aquarius	January 21 – February 19	February 13 – March 14	Aquarius, February 16 – March 12	Air	Fixed	Uranus/Saturn
Pisces	February 20 – March 20	March 15 – April 13	Pisces, March 12 – April 19	Water	Mutable	Neptune/Jupiter

Mnemonic for the zodiac

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A traditional mnemonic: Project Gutenberg ebook "An Alphabet Of Old Friends" see Z for Zodiac.

''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.

See also

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• History of astronomy
• History of astrology
• Astrological sign
• Elements of the zodiac
• Astronomical symbols
• Astrology and alchemy
• Esoteric cosmology
• Gnostic circle — The Zodiac superimposed on the Enneagram
• Zodiacal constellation
• Zodiacal light
• Zodiacal dust

Notes

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External links

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• 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 Heingriech Frederiech and Edward Ball Knobel. Ptolemy's Catalogue of Stars: a revision of the Almagest. (1915). Publishers, Carnegie Institution of Washington.
• eMail Horoscopes (nice illustrations of the Earth, Sun, and zodiac constellation relationships)
• Coordinates for the Constellations (provides a comparison between tropical and sidereal systems)

Celestial coordinate system | Ancient astronomy | Astrology | Constellations

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