mardi 23 septembre 2008

Ursa Minor

The Little Bear was said by the Greeks to have been first named by the astronomer Thales of Miletus, who lived from about 625 BC to 545 BC. The earliest reference to it seems to have been made by the poet Callimachus of the third century BC, who reported that Thales ‘measured out the little stars of the Wain by which the Phoenicians sail’. Certainly Homer, two centuries before Thales, wrote only of the Great Bear, never mentioning its smaller counterpart. However, it is not clear whether Thales actually invented the constellation or merely introduced it to the Greeks, for Thales was reputedly descended from a Phoenician family and, as Callimachus said, the Phoenicians navigated by reference to Ursa Minor rather than Ursa Major. Aratus points out that although the Little Bear is smaller and fainter than the Great Bear, it lies closer to the pole and hence provides a better guide to true north. We have the word of Eratosthenes that the Greeks also knew Ursa Minor as the Phoenician.

Aratus called the constellation Cynosura, Greek for ‘dog’s tail’. This is the origin of the English word cynosure, meaning ‘guiding star’. According to Aratus the Little Bear represents one of the two nymphs who nursed the infant Zeus in the cave of Dicte on Crete. Apollodorus tells us that the nurses’ names were Adrasteia and Ida. Ursa Minor commemorates Ida while Adrasteia, the senior of the two, is Ursa Major.
Ursa Minor has a similar ladle shape to Ursa Major, and so it is popularly termed the Little Dipper. At the end of the Little Bear’s tail (or the dipper’s handle) is the star Alpha Ursae Minoris, commonly known by the Latin name Polaris because it is the nearest bright star to the north celestial pole. Contrary to common belief, the north pole star is not particularly bright. Polaris is a second-magnitude star, currently lying less than a degree away from the exact north celestial pole, close enough to make it an excellent guide star for navigators.
The second star in the Little Bear’s tail, Delta Ursae Minoris, is called Yildun, a mis-spelling of the Turkish word yildiz meaning ‘star’. According to the German star-name authority Paul Kunitzsch this was wrongly thought to be a Turkish name for the pole star in Renaissance times, and it has since been arbitrarily applied to the star nearest to the true pole star. According to Kunitzsch, an Arab tradition saw the arc of stars forming the handle of the Little Dipper as representing one side of the body of a fish, the other side consisting of much fainter stars including 4 and 5 Ursae Minoris and 32 Camelopardalis.

Two stars in the bowl of the Little Dipper, Beta and Gamma Ursae Minoris, are sometimes referred to as the Guardians of the Pole. Their names are Kochab and Pherkad. Paul Kunitzsch has been unable to trace the origin of Kochab, but thinks that it may come from the Arabic word kaukab meaning ‘star’. Pherkad is from an Arabic word meaning ‘the two calves’, referring to both Beta and Gamma Ursae Minoris.

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