The astrological signs are not defined by, nor do they have anything to do with, the constellations you see in the sky. Approximately 2,000 years ago, when the foundations of astrological thinking were formulated, the names of the constellations happened to be paired with the astrological signs. Today, those pairings are no longer in sync: Astrological signs do not line up with the constellations in the same way they did way back then, due to the phenomena known as the precession of the equinoxes. The equinoxes move backwards, or precess, with respect to the constellations by about one degree every 72 years.
However, modern Western astrologers understand that the raw material of their work does not involve a study of distant stars. Rather, their relevant data have to do with the interweavings of the planets in our own solar system within a zone of influence defined by the relationship between the Earth and Sun. The key demarcation points in that relationship are the equinoxes, the points in time and space at which the Earth, with its tilted axis, is positioned with respect to the Sun in such a way that the length of day and night are equal. At the vernal equinox, which occurs on about March 20th of each year, the Sun enters into the sign of Aries.
This zodiac, positioned with respect to the equinoxes, is called the "tropical zodiac." Most Western astrologers, who use this system, are called tropical astrologers. There is also a zodiac based loosely on the constellations. It's called the "sidereal zodiac," and is used primarily by Vedic (or Hindu) astrologers. For an excellent discussion of the differences between sidereal and tropical astrology, and the merits of each, pick up the April/May 2002 issue of The Mountain Astrologer.
Astrologers using the tropical zodiac do not do so out of ignorance of the precession of the equinoxes. Knowledge of the precession is very ancient, and possibly predates the creation of the tropical zodiac. Precession was discovered thousands of years ago in Bharat (also known as India). Sir Norman Lockyer found that many very early temples in Egypt had been moved at different periods in history so that they lined up with a particular star as it precessed across the sky.
The ancient Egyptians also had a succession of cults that adopted symbols (e.g., the bull, the ram) associated with the concurrent precessional age. (See, for e.g., E. C. Krupp, In Search of Ancient
Astronomies, Garden City, NY: Doubleday, 1977.)
Early Christian symbolism was dominated by fishes, the symbol associated with Pisces, which is the constellation that defines the precessional age that began around the birth of Christ. (See C. G. Jung, Aion, translated by R. F. C. Hull, 2nd edition, Princeton: University Press, 1959.) Modern-day Christians in America sometimes use a fish symbol to signal their Christianity. So perhaps an attraction to a symbol associated with the astrological age in which one lives need not be accompanied by conscious knowledge about the age and the precession that defines it.
The ancient Greek astrological writers (e.g., Ptolemy) were very explicit in distinguishing the tropical zodiac they used from the fixed stars and constellations. This distinction is still made among modern Western astrologers who use the tropical zodiac.
Some people who are not knowledgeable about the history or practice of astrology may ask, "Why then do the tropical signs have the same names and symbols as the constellations with which they were aligned 2000 years ago? Aren't the zodiacal constellations the source of the meanings of the tropical signs? And so shouldn't astrologers take the meaning of a tropical sign from the constellation most closely aligned with it now?"
This specious argument is based on the presupposition that the meanings of the signs come from the natures of the symbols in the heavens that we call constellations. But clearly this is not the case. Some of the traditionally dominant traits of Virgo are obsession with detail and an analytical and critical nature. How could these traits be derived from a picture of a virgin? How could the Piscean qualities of spirituality, selflessness, imaginativeness, capacity for inspiration, femininity, and idealism be derive from a picture of two fish? Few traits of each sign can easily be related to the symbol assigned to the constellation of the same name. [montalk: however, Aries (ramming), Taurus (plodding), Scorpio (stinging), Sagittarius (physical), Leo (pompous), and Libra (fair) do relate.]
There is no necessity, given current knowledge, for the tropical signs to have received their meanings from the zodiacal constellations; it is possible that the nature of the tropical signs suggested a symbol to associate with a constellation (since most of the symbols look very little like the pattern of stars we associate with them).
Much depends on which was established first, the tropical signs or the zodiacal constellations. When did the tropical zodiac and constellations appear? The tropical zodiac may have been around a long time. The Sumerians and the Egyptians had a tropical (lunar-solar) calendar by the early part of the third millennium B.C.E.; given the direct and transparent relationship between the signs of the tropical zodiac and the months of the solar year, they may have had a tropical zodiac as well, although we have no direct evidence of this.
Tropical calendars in the form of standing stones (e.g., Stonehenge) date from 1000-5000 B.C.E. in Northwest Europe, so a tropical zodiac might have existed there. Unfortunately, the preliterate people of these cultures left no records behind. Martin Seymour-Smith (The New Astrologer, New York: Collier, 1981) claims that some sort of zodiac, possibly sidereal, with 12 equal signs of 30 degrees, existed in India in 3000 B.C.E. He claims that a manuscript in Sanskrit from that period shows that astrologers then used a zodiac, an equal house system, and aspects counted sign to sign (as in much modern-day Hindu astrology, and as in classical Greek astrology). Unfortunately, Seymour-Smith does not cite any references or explain how the dating of the manuscript was arrived at.
The origin of the modern constellations is somewhat obscure, so it is very difficult to decide whether all of the zodiacal constellations were around to lend meaning to the tropical signs at the time when the tropical zodiac was created (especially because we cannot be certain when the tropical zodiac appeared). Noonan (1976; Journal of Geocosmic Research, Vol. 2, No. 1, pp. 6-7) claims that the first zodiac of the constellations appeared around 500 B.C.E.
The constellations are believed to have been assigned symbols by the Babylonians, but there were originally 36 constellations, and only some of them coincide with the modern sidereal signs. We know that some of the symbols used for the modern signs are recent, because the original symbols were all animals (the word "zodiac," derived from the Greek zoidiako's, means "circle of animals"). We can be certain that the modern constellations of the zodiac existed by about 30 B.C. because they appear very clearly on the ceiling of the Temple of Hathor at Dendera in Upper Egypt. So was the tropical zodiac in use by then?
It might have been. The precession of the equinoxes was certainly common knowledge at that time. Precession was discovered at the very latest in 200 B.C., when Hipparchus wrote about it. But Sir Norman Lockyer found that many very early temples in Egypt had been moved at different periods in history so that they lined up with a particular star as it precessed across the sky.