The Zodiac (from Greek zoon, "animal") is an imaginary belt in the heavens extending approximately 8 degrees on either side of the Sun's apparent path, that includes the apparent paths of the Moon and the planets Mercury, Venus, Mars, Jupiter, and Saturn. Divisions of the zodiac represent, in astronomy: constellations, and in astrology: signs. There is a zodiac in western astrology, a different one in Vedic astrology, and a very different one in Chinese astrology.
The origins of the zodiac lie in Sumer in Mesopotania.
In astronomy, the zodiac is a certain part of the sky which has no intrinsic physical significance, representing simply the region of the sky close to the circle on which the randomly oriented plane of our solar system intersects the celestial sphere. It includes the ecliptic. It is, however, a useful region of the sky to define, because it has practical implications for observations from the earth's surface. A naked-eye observer knows that a bright object lying outside of the zodiacal region cannot be a planet. Polar observatories cannot easily observe the planets, because the zodiac is too close to the horizon.
Dating back to the time when there was no clear distinction between astronomy and astrology, the zodiac is traditionally thought of as comprising a certain set of constellations. The constellations of both zodiacs are shown in the table below, including Ophiuchus, which was added to the astronomical zodiac by the International Astronomical Union in 1930 when it based its zodiac on the 1875 equinox. (See the external link below.) In modern astronomy, these, like all constellations, are recognized as chance groupings of stars, with no natural significance. In fact, they are not even true groupings of stars in three-dimensional space. We see the sky without any perception of its depth, so two stars that appear to be neighbors in the same constellation may actually be separated by vast distances.
In western astrology the zodiac is a band on the celestial sphere which contains the perceived paths of the sun, moon, and principal planets and is divided into twelve equal parts, called "signs of the zodiac", each named for a constellation. At the center of this band is the plane of the ecliptic. The width of the zodiac allows for the fact that the orbits of the other bodies are inclined relative to plane of the ecliptic, and thus extend about 8? beyond the ecliptic.
The zodiacal year begins at the point where the plane of the ecliptic intersects with the earth's equatorial plane at the vernal equinox when the sun moves into the northern hemisphere of the earth's equatorial plane. In western tropical astrology, although the signs derive their names from the constellations, they are not the same thing. For example, although the sun always enters the sign of Aries at the vernal equinox about March 20, it will not cross into the astronomical constellation of the same name until nearly a month later. In sidereal astrology the signs are aligned to their correct stellar constellations.
The zodiac includes twelve of the constellations that the ecliptic crosses. It actually crosses a thirteenth, Ophiuchus, but this constellation is not considered part of the zodiac. Because the ecliptic lies in the general plane of the solar system, the Sun and planets seem to move through the zodiacal constellations.
The "tropical" zodiacal dates (those that divide the zodiac starting with the vernal equinox) are used primarily in the United States and western Europe. The "sidereal" dates (those that divide the zodiac starting with the alignment of the Sun with a distant star in Aries) are used everywhere else.
Many people who don't know anything about constellations have heard about the signs of the Zodiac in an astrological context.
The astronomers/astrologers (originally the observations and magic/religious applications were made by the same people), used the movements of the night sky for divinatory purposes. Some of these applications were founded on correspondences between practical knowledge and celestial observations (for example, the relationship between solar position and stellar positions depends on the season, which has practical implications for agriculture), whilst others were completely unfounded.
The familiar "star sign", or more precisely the "sun sign", under which a person is born refers to the position of the sun in the signs on the tropical ecliptic at the time of his or her birth. Because of the precession of the equinoxes over the last three thousand years or so, the signs are out of phase with the astronomical constellations for which they are named by about a month. A few modern western astrologers, in common with those of India, cast horoscopes with reference to the actual constellations, rather than the signs. This is known as sidereal astrology.
The idea of astrological birth sign is that the person would have some characteristics of the mythic symbolism that the ancients identified with that constellation; so, for example, a "Libra" (the scales) will be balanced and stable. Also, since the planets are all found in the zodiac. The position of the Moon or a planet in a particular sign would have an effect on the life of that person. For example: A person may be born on June 1st. This is near the center of the sign of Gemini, and so Gemini would be his sun sign. Any planets also observed near the center of Gemini, would be in "conjunction" with the sun, and said to have a particularly strong effect on the destiny and personality of the person. At the same time, other planets are in other signs of the zodiac, and their effects would be felt on the portions of a person's life "ruled" by that sign. Significance is also associated to the angular positions of planets and signs relative to each other at the moment of a birth or other significant event.
The zodiac as a calendar:
The concept of the zodiac was originated by the Babylonians certainly before 2000 BC as a method of visualizing the passage of time. The zodiac worked as a symbolic calendar. It was divided into twelve parts as suggested by the appearance of 12 moons in a year. The signs are geometric divisions of the celestial sphere, each corresponding to one twelfth of a year.
The signs of the zodiac, as enumerated by Egyptian astronomer, Ptolemy, in the 2nd-century AD, are the ones we know today. The same names are used for both signs in astrology and for constellations in astronomy, but it's important to make a distinction between signs and constellations. Signs are geometric sections, each 30° wide, corresponding with particular periods of time of the year, but which don't necessarily physically correspond with the constellations of the same name.
By the time of Ptolemy the zodiac was already at least two thousand years old. But together with its burgeoning astrological use the basic function and structure of the "calendar of the zodiac" remained. The sign of Aries marks the beginning of the year at the vernal equinox. The retreating crab in Cancer represents the retreat of the Sun from its farthest northern point at the time of the summer solstice. Leo, the symbol of fire, represents summer heat. The scales of Libra signify the balance between day and night at the autumnal equinox. The decline of the sun's power is represented in Scorpio by the scorpion, the symbol of darkness. The water-bearer, Aquarius, represents the rainy season which, in Egypt, meant the yearly flooding of the Nile.
The fishes of Pisces, symbolize the return of life and the resumption of agriculture.
The concept of the zodiac spread form Babylonia to Greece and, from there, to Egypt where the Egyptians substituted their own symbolism. Aries became the Fleece. Two Sprouting Plants replaced the twins of Gemini. Cancer was re-named Scarabaeus. Leo became the Knife and Libra the Mountain of the Sun. Sagittarius was reduced to just an arrow. Capricornus became the image of life, represented by a mirror. Scorpio became a serpent. Aquarius became simply water, while Taurus, Virgo and Pisces were not changed.