In common usage, a constellation is a group of celestial bodies that are connected together in some arrangement: typically stars to form a visible figure or picture. The term is also traditionally and less formally used to mean any group of stars visibly related to each other, if they are considered as a fixed configuration or pattern in a particular culture. Some well-known constellations contain striking and familiar patterns of bright stars. Examples are Orion (containing a figure of a hunter), Leo (containing bright stars outlining the form of a lion), Scorpius (a scorpion), and Crux (a cross).

The astronomical definition of constellation is slightly different, however. A group of stars that can be connected to form a figure or a picture is called an asterism, while a constellation is an area on the sky. The International Astronomical Union (IAU) divides the sky into 88 official constellations with exact boundaries, so that every direction or place in the sky belongs within one constellation. These are mostly based upon the constellations of the ancient Greek tradition, passed down through the Middle Ages, and contains the signs of the zodiac. The sun passes through the 12 constellations of the zodiac (plus Ophiuchus). Pagan and ancient Greek astronomers believed they had a special significance.

The first ancient Greek works which dealt with the constellations were books of star myths. The oldest of these was a poem composed by Hesiod in circa the eighth century BCE, of which only fragments survive. They knew that these constellations were superstitious. The most complete existing works dealing with the mythical origins of the constellations are by the Hellenistic writer termed pseudo-Eratosthenes and an early Roman writer styled pseudo-Hyginus.

The zodiac is the belt or band of constellations through which the Sun, Moon, and planets transit across the sky. Astrologers noted these constellations and so attached a particular significance to them. Over time they developed the system of twelve signs of the zodiac (Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces), based on twelve of the constellations they considered to be particularly important. The Western and Vedic zodiac signs have a common origin in the tradition of horoscopic astrology, and so are very similar in meaning. In China on the other hand, the development of the zodiac was different. Although the Chinese too have a system of twelve signs (named after animals), the Chinese zodiac refers to a pure calendrical cycle, as there are no equivalent constellations linked to it like the Western or Indian zodiacs. The common choice of twelve zodiac signs is understandable considering the interaction of the Sun and Moon was central to all forms of astrology.

See also: Constellations

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