science

Starwatch: enter the dragon, home to former pole star Thuban


At this time of year in the northern hemisphere, the mighty constellation of Draco, the dragon, is perfectly placed in the evening sky. From mid-northern latitudes, the constellation is visible all year round, but in the summer months Draco is in prime position.

The chart shows the view looking high into the sky, facing north at midnight this week. It will take a dark sky and some patience to trace out the faint stars of Draco but once memorised it fills in between brighter, more obvious star patterns. Draco’s tail slithers between Ursa Major, the great bear, and Ursa Minor, the little bear, which contains the current pole star, Polaris; its forelegs appear to rest on Cepheus, the king, and its head rears up to menace Cygnus, the swan. Unfortunately, from Sydney, Australia, Draco merely pops his head above the northern horizon.

Over the course of 26,000 years, the Earth’s rotation axis slowly traces out a large circle on the sky. This process is known as precession, and leads to different pole stars throughout history. Five thousand years ago Draco’s star Thuban was the star nearest to the north pole. The gradual movement continues today. In AD4000, the star Errai in Cepheus will be the pole star.



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