Ursa Major

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Ursa Major is a constellation visible throughout the year in the northern hemisphere. Its name means "Great Bear" in Latin, and is derived from the legend of Callisto. It is one of the 88 modern constellations, and was also one of the 48 listed by Ptolemy. It contains a famous asterism of seven bright stars known in the United States as the Big Dipper, because the major stars can be seen to follow the rough outline of a large ladle, or dipper. The Big Dipper is recognized as a grouping of stars in many cultures and eras. In the United Kingdom it is commonly known as the Plough, and was sometimes formerly called King Charles's Wain (where wain means "wagon").

From the bowl to the handle, the stars in the Big Dipper are called Dubhe, Merak, Phecda, Megrez, Alioth, Mizar, and Alkaid (or Benetnash), and are given Bayer designations of Alpha to Eta Ursae Majoris. Mizar has a companion star called Alcor, just visible to the naked eye, that served as a traditional test of sight. Both stars are actually multiple in and of themselves, including the first telescopic and spectroscopic binaries.

The star Polaris, the Pole Star, can be found by measuring a line five times the angular distance between the two pointer stars Dubhe and Merak forming the end of the dipper cup, through those stars and up and away from the dipper. The dipper also points the way to other stars, for instance by sweeping down from the handle one reaches Arcturus and Spica.

In 1869, R. A. Proctor noticed that, except for Dubhe and Alkaid, the stars of the Big Dipper all have proper motions heading towards a common point in Sagittarius. This group, of which a few other members have been identified, formed an open cluster at some distant point in the past. Since then the sparse group has been scattered over a region about 30 by 18 light-years, centered some 75 light-years away, making it the closest cluster-like object. About 100 other stars, including Sirius, form a stream sharing approximately the same proper motion as the ex-cluster, but the exact relationship is unclear. Our solar system is in the outskirts of this stream, but is not a member, being about 40 times older.