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A constellation is a group of stars visibly related to each other in a particular configuration. In three-dimensional space, most of the stars we see have little relation to one another, but can appear to be grouped on the imaginary plane of the night sky. People are very good at finding patterns and throughout history have grouped stars that appear close to one another into constellations.

The grouping of stars into contellations 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 International Astronomical Union (IAU) divides the sky into 88 official constellations with precise boundaries, so that every direction belongs to exactly one constellation. These are mostly based upon the constellations of the ancient Greek tradition, passed down through the Middle Ages. The zodiac includes the following 12:

In addition to these 12, Ptolemy listed also the following 36 (which now count as 38, due to the break-up of Argo Navis):

In more recent times this list has been added to, first to fill gaps between Ptolemy's patterns (the Greeks considered the sky as including both constellations and dim spaces between) and second to fill up the southern sky as explorers sailed where they could see it. The 38 new constellations are:

There were also other constellations that didn't make the cut, most notably Quadrans Muralis (now part of Boötes) for which the quadrantid meteors are named. Various other less official patterns have existed alongside the constellations called asterisms, such as the Big Dipper and Little Dipper. Note that in any of these figures the stars rarely have anything to do with one another - they are just along roughly the same lines of sight, and are typically very far apart.

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