File:Orion constellation map.png|320px|thumb|The constellation Orion is one of the most recognized in the sky. The name is associated both with a set of stars in the sky, and a bounded region of the sky, marked in yellow. In modern astronomy the word 'constellation' refers to an area of the celestial sphere, defined by exact boundaries.

Definitions

In colloquial usage, a constellation is what astronomers call an asterism: a group of celestial bodies (usually stars) that appear to form a pattern in the sky or appear visibly related to each other. Examples are Orion (which appears like a human figure with a belt, often referred to as "The Hunter"), Leo (which contains bright stars that outline the form of a lion), Scorpius (which can seem reminiscent of a scorpion), and Crux (a cross). Fact: date=April 2009

In astronomy, however, a constellation is an area of the sky, and contains all the stars and other celestial objects within that area. 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. Most of these constellations are centred on the traditional constellations of Western culture.

Human perception versus reality

Constellations are normally the product of human perception rather than astronomical realities. 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, there are some exceptions. The famous star pattern known as the Big Dipper in North America or the Plough in the UK is almost entirely created by stars that are genuinely close together in astronomical terms; they are known as the Ursa Major moving group.

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

Official constellations

The 88 official constellations defined by the IAU (International Astronomical Union) are mostly based upon those of the ancient Greek tradition, passed down through the Middle Ages, which includes the 'signs of the zodiac,' twelve constellations through which the sun passes and which thus have had special cultural significance. The rest consist of constellations which were defined in the early modern era by astronomers who studied the southern hemisphere's skies, which were invisible to the Greeks.

Boundaries

The constellation boundaries now used by the International Astronomical Union were drawn up in 1930 by Eugène Delporte. 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 this early date is that due to precession of the equinoxes, the borders on a modern star map (e.g., 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.