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Definition: zodiac from The Penguin Dictionary of Science

Twelve ancient constellations on the ➤ecliptic. Viewed from the Earth, the Sun appears to pass through each of these once per year.


Summary Article: zodiac from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

Zone of the heavens containing the paths of the Sun, Moon, and planets. When this was devised by the ancient Greeks, only five planets were known, making the zodiac about 16° wide. In astrology, the zodiac is divided into 12 signs, each 30° in extent: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. These do not cover the same areas of sky as the astronomical constellations.

The 12 astronomical constellations are uneven in size and do not between them cover the whole zodiac, or even the line of the ecliptic, much of which lies in the constellation of Ophiuchus.

The idea of such a zone is ancient, but the present name is of Greek origin. At first various asterisms were chosen along the zodiac to serve as calendar reference points, but as these were unequally spaced the zone was eventually divided into 12 equal signs, each 30° wide. The sequence of the signs is eastwards, following the motions of the Sun and Moon through the constellations, and is regarded as beginning at the point that marks the position of the Sun at the time of the March equinox. This point is sometimes called ‘The First Point of Aries’.

The equinoxes occur as the Sun enters the signs of Aries and Libra, at which times the Sun passes directly above the Equator. The solstices occur as the Sun enters the signs of Cancer and Capricorn and is passing directly over the Tropic of Cancer and Tropic of Capricorn, circles of latitude 23° 27' north and south of the Equator.

Because of precession (the slow wobble of the Earth on its axis), the equinoctial point, and with it the zodiacal signs, moves westwards through the constellations at a rate of one sign in 2,150 years. The First Point of Aries, which is now in Pisces, would have been in Aries at the time of Greek astronomer Hipparchus, in the second century BC. At the time the present constellation groupings were first made, 2,000 years earlier, the First Point of Aries would have been in Taurus and the initial asterism of the zodiac would have been the Pleiades.

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