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Zodiac
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Zodiac 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.
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Zodiac 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 the zodiac of eighteen signs. The Mayans used twenty signs. The term zodiac may come from the Latin zodiacus, from the Greek (zodiakos kyklos), meaning "circle of animals", derived from (zodion), the diminutive of (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.
Zodiac in 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.
Zodiac in astrology
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.
Western zodiac
The modern longitudes for each sign of 30° longitude. In terms of:
- the sidereal zodiac described here in this article the longitudes are calibrated by treating the star Aldebaran as 45° and Antares as 235°
- the tropical zodiac, the longitudinal calibration is set according to the position of the Sun along the ecliptic at the moment of the vernal equinox each year
For the sidereal zodiac, the movement of the Sun through each sign corresponds roughly with the constellation of the same name. For the tropical zodiac, the movement of the Sun through each sign corresponds roughly to the same days of the Gregorian Calendar each year (precisely the same days relative to the time of the vernal equinox).
*Aries (0°) (The Ram)
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, 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.
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".
Table of constellations vs. zodiac 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 observations |
|---|
| 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 | not recognized in astrology | 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
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.
Zodiac celestial coordinate systems
Any spherical celestial coordinate system must define a fundamental plane and designate a prime meridian: in other words an origin or zero degree mark for longitude. From these definitions, longitudinal meridians perpendicular to the fundamental plane meet at the north and south poles of the celestial sphere enabling the specification of precise coordinates for any position on the sphere.
The zodiac coordinate system designates the ecliptic as its fundamental plane. Like the equator in the Earth’s spherical coordinates, the ecliptic serves as the fundamental plane for the zodiac's coordinate system. The ecliptic is aligned with the Earth's orbital plane with the Sun rather than the equator that is perpendicular to Earth’s axis of rotation. The Earth tilts at an angle of approximately 23° with respect to the orbital plane. This tilt is related to the Earth's precession as it gyrates and rotates on its axis — completing a cycle through its four seasons slightly before it has reached the completion of an orbital cycle. This gyration contributes to the divergence between a tropical year and a sidereal year.
Second, the zodiac system of coordinates specifies a different prime meridian for the tropical and the sidereal systems of coordinates. For the tropical system of coordinates the prime meridian is the position of the Sun at the Vernal Equinox in the epoch of Hipparchus. This prime meridian leads to the fixity of the system with respect to the Earth: in other words the stars of the celestial sphere slowly rotate around the earth over the course of thousands of years. For the sidereal system of coordinates, Ptolemy specified zodiac signs using two bright stars near the ecliptic and opposite each other to serve as equatorial nodes: 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. Therefore, even in the sidereal system of coordinates the 12 signs only roughly correspond to the 12 constellations from which they take their name, though they are in the same general region of one another. For the sidereal system the selection of two relatively stable stars (in other words their proper motion is relatively small), leads to a system of coordinates that treats the celestial sphere as fixed and the position of the Sun at Earth’s equinox as moving through the celestial sphere.
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 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.
The equinox moving through the sidereal signs
Below are several images depicting how the vernal equinox precesses through the celestial sphere from 1500 BC through AD 2500 (projected). The brightest star you see in the images is the Sun. For a tropical zodiac the first point in Aries and consequently the beginning of the tropical sign of Aries is defined as the position of the Sun's center at the moment of the astronomical vernal equinox. For comparison the constellations are shown with stick labeled figures. Also the planets are labeled that lie near the vernal equinox for that year. Finally, the stars that lie near the outer boundaries of the sidereal signs 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 start of the sign of Aries in terms of a tropical zodiac) lies in relation to the sidereal sign of Aries.
Click on any image to see a larger view.
Comparison to modern systems
Though perhaps as old as 10,000 years, the zodiac coordinate system boasts advantages over its more common modern counterparts. Since the coordinate system is celestially entered, it is insulated from the many eccentricities of Earth's motion: including its rotation, intricacies of Earth time, precession, nutation and its elliptical and perturbed 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 know the current mean sidereal time, the observer's terrestrial longitude and latitude, and the epoch the observer wishes to utilize, and to account for other peculiarities of Earth's motion. Of course, modern astronomical computers handle most of the tasks for 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 designation of a tropical rather than sidereal coordinate system. One example where this exhibits itself is in the constellation boundaries drawn up by the IAN. The neat constellation boundaries drawn in 1933 exhibit increasingly distorted boundary 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 terrestrial equator for its equatorial plane it is not susceptible to the drifting of stars across the celestial equator as in the commonly used equatorial coordinate system (right ascension, declination). In The Almanack Ptolemy criticizes Hipparchus’ use of an equatorial plane in some of Hipparchus’ variously specified coordinate systems for this very reason (Ptolemy 1999).
These advantages make the zodiac coordinate system a very efficient coordinate system, requiring little labor to use and maintain, issues particularly important to early astronomers, often working in isolation from one another.
The key disadvantage of a zodiac system of coordinates will manifest as a problem, if the nodal stars that serve as its 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 eccentricities compared to the other stars undermines the usefulness of the system of coordinates. For example the inadvertent selection of an asteroid 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 200 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 orient themselves to a geocentric coordinate system (such as ecliptic or equatorial coordinates). Using zodiac coordinates requires an astronomer to locate the correct star, whether Antares 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.
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
External links
Translated and annotated by G.J. Toomer ; with a foreword by Owen Gingerich.
- PhD thesis, entitled "The Definition of the Babylonian Zodiac and the Influence of Babylonian Astronomy on the Subsequent Defining of the Zodiac".
- First published in The Traditional Astrologer magazine, (Ascella), Issue 14, May 1997, pp.23-27
- Peters, Christian Heinrich Friedrich and Edward Ball Knobel. . (1915). Publishers, Carnegie Institution of Washington.
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