CONSTELLATION

In common usage, a constellation is a group of stars that are connected together to form a 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[1] with exact boundaries, so that every direction or place in the sky belongs within one constellation. In the northern celestial hemisphere, 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 appears to pass through the 12 constellations of the zodiac (plus Ophiuchus) and ancient Greek astronomers believed they had a special significance.[citation needed]

The constellation boundaries were drawn up by Eugène Delporte in 1930, and he drew them along vertical and horizontal lines of right ascension and declination. However, he did so for the epoch B1875.0, the era when Benjamin A. Gould made the proposal on which Delporte based his work. The consequence of the early date is that due to precession of the equinoxes, the borders on a modern star map (eg, for epoch J2000) are already somewhat skewed and no longer perfectly vertical or horizontal. This skew will increase over the years and centuries to come.

In three-dimensional space, most of the stars we see have little or no relation to one another, but can appear to be grouped on the celestial sphere of the night sky.

A star pattern may be widely known but may not be recognized by the International Astronomical Union; such a pattern of stars is called an asterism. An example is the grouping called the Big Dipper (North America) or the Plough (UK).

The stars in a constellation or asterism rarely have any astrophysical relationship to each other; they just happen to appear close together in the sky as viewed from Earth and typically lie many light-years apart in space. However, one exception to this is the Ursa Major moving group.

The grouping of stars into constellations is essentially arbitrary, and different cultures have had different constellations, although a few of the more obvious ones tend to recur frequently, e.g., Orion and Scorpius.

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.