Indo-European zodiac

Zodiac celestial coordinate systems

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.

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 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.

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 5,000 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 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.

Zodiac in astrology

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 rāśis, are similar to Graeco-Babylonian signs, apparently as a result of Indo-Greek contact:

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) and cat. For a list of how these animals map to the months and years see Chinese astrology. There is also a Chinese lunar zodiac comprised of twenty-eight lunar "mansions", each corresponding to a Chinese constellation.

Historical origin

Table of constellations vs. 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. 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.

Mnemonic for the zodiac

A traditional mnemonic: ^[6]

Notes

1. ^ 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.
2. ^ http://www.shadowdrake.com/zodiac.html
3. ^ http://www.shadowdrake.com/zodiac.html
4. ^ http://www.geocities.com/astrologyzodiacs/index.htm
5. ^ WorldBook Encyclopedia (2001)
6. ^ Project Gutenberg ebook "An Alphabet Of Old Friends" see Z for Zodiac.

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