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I'm doing an astronomy exam tomorrow and in the practice paper it asks for the difference between constellation and asterism. It seems asterism is a group of recognizable stars; however I thought that is what a constellation is. So what exactly is the difference?

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These answers are pretty unsatisfying: OK, so constellations are these patches of sky decided upon by some group of people. Presumably, they had some rationale in picking the current set of patches, and not some other set. Is it just based on Greek tradition? I think that's the sort of answer that people are generally looking for. –  allyourcode Apr 2 at 3:19

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The constellations are the 88 internationally recognized stellar groupings in the sky that toghether cover the entire celestial sphere. They typcially correspond to a recognziable pattern and many are named from mythology. However, in modern usage, a specified constellation tecnically referres to the entire region of the sky, not just the recognizable star pattern.

An asterism is also a group of stars that don't correspond to the recognized constellations. Some, like the Big Dipper, are a subset of the stars in a larger constellation (in the case of the Big Dipper, the constellation is Ursa Major). Others are made up of stars from multiple constellations. An example of this type is the Summer Triangle, which is composed of the three bright stars Vega, Deneb, and Altair, which are the brightest stars in the Constellations Vega, Cyngus and Aquila, respectively. These are all first magnitude stars and when the triangle is up, it is summer time in the Northern Hemisphere.

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An asterism is a group of recognizable stars, like your definition says. However, it's usually a group within a constellation.

A good example of an asterism is the Big Dipper (or the Plough, if you're in the UK). It's a familiar group of stars, but by itself it's not a constellation. It's a group of stars within the Ursa Major constellation.

Another asterism is the Pleiades, which is a small cluster of stars found in Taurus.

Edit: And to be precise, a constellation actually refers to an area of the sky and not the pattern of stars. The constellation Orion is made up of a specific area of the sky and not just the stars that look like a hunter.

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But there are asterisms that span multiple constellations (e.g. the summer triangle). –  Euro Micelli Aug 1 '11 at 20:50

The difference is minor. Constellations are well known groups of "recognizeable" stars.

An Asterism is any group of stars that seems visually related, but may not be one of the generally recognized constellations.

An Asterism might be a sub group of a larger constellation, or it could span across multiple constellations.

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"Constellations are well known groups of "recognizeable" stars."

This is the popular and historical answer but not the modern astronomical answer.

The modern answer is that constellations are regions of the sky. Meanwhile, asterisms are groups of stars that form some pattern. Most asterisms are within a single region or constellation ("Big Dipper") but many are not ("Summer Triangle").

Subtle but significant difference. It's important because the "region" definition defines every part of the sky. Even the spaces between the famous patterns are included in constellations.

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An asterism is a Pattern of stars that are not part of the constellation. Their speed is different from the constellation. Their position drifts and will individually be varying in position in the sky. Any assumption that an asterism has one fixed position in the constellation will cause unacceptable discrepancies / errors during computation of their spatial positions.

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protected by Qmechanic Apr 16 '13 at 6:48

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