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.
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 8º 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. 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.
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.
A traditional mnemonic:
|Sign||Symbol||Birthdates||Actual Astronomical Dates|
(as of AD 2000)
|Aries||♈||March 21 - April 19||April 14 - May 14||April 19 - May 13|
|Taurus||♉||April 20 - May 20||May 15 - June 14||May 14 - June 19|
|Gemini||♊||May 21 - June 20||June 15 - July 16||June 20 - July 20|
|Cancer||♋||June 21 - July 22||July 17 - August 16||July 21 - August 9|
|Leo||♌||July 23 - August 22||August 17 - September 16||August 10 - September 15|
|Virgo||♍||August 23 - September 22||September 17 - October 17||September 16 - October 30|
|Libra||♎||September 23 - October 22||October 18 - November 16||October 31 - November 22|
|Scorpius||♏||October 23 - November 21||November 17 - December 15||November 23 - November 29|
|Ophiuchus||November 30 - December 17|
|Sagittarius||♐||November 22 - December 21||December 16 - January 14||December 18 - January 18|
|Capricornus||♑||December 22 - January 19||January 15 - February 12||January 19 - February 15|
|Aquarius||♒||January 20 - February 18||February 13 - March 14||February 16 - March 11|
|Pisces||♓||February 19 - March 20||March 15 - April 13||March 12 - April 18|
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.
Even 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), while some others were completely unfounded.
The familiar "sign", more precisely the "sun sign", under which a person is born usually refers to the apparent 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 astrologers cast horoscopes with reference to the actual constellations, rather than the signs.
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.
Babylonians certainly before 2000 BC as a method of visualizing the passage of time. While the zodiac has come to be associated primarily with astrology, the zodiac originated 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, 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, which don't necessarily correspond to constellations.
By the time of Ptolemy the zodiac was already at least two thousand years old. But the basic structure of the "calendar of the zodiac" remained. Aries marks the 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.
See also: Chinese astrology