Thuban — りゅうの恒星

Thuban, Alpha Draconis, is a white giant star of spectral type A0 IV located about 303 light-years away, notable not for its present brightness but for having been Earth's North Star around 2700 BC. Thuban is a spectroscopic binary with a close companion in a 51.4-day orbit, and the primary is a mild chemical-peculiarity star. At magnitude 3.65 it is the third-brightest star in Draco, behind Gamma and Eta Draconis.

Despite its "alpha" designation, Thuban is visibly fainter than several of its constellation-mates. Find it by tracing the tail of Draco — it lies midway along the arc running between Mizar in Ursa Major and the bowl of the Little Dipper. Its position is steady and easily identified: just remember the line from Mizar to Polaris passes almost directly through Thuban. Circumpolar from northern temperate latitudes, so visible every clear night of the year.

Because of the precession of Earth's rotational axis, Thuban was the pole star from approximately 3942 BC to 1793 BC. The great Pyramid of Giza was oriented so that one of its internal shafts pointed directly at Thuban's meridian position around 2600 BC — likely by design. The name Thuban comes from the Arabic "ath-thu'bān," meaning "the serpent." Vega was the pole star around 12,000 BC and will become so again around 13,700 AD; Polaris holds the title from about 500 AD to 3000 AD.