Monday, February 22, 2021

Thuban

Thuban is a star visible in the constellation Draco, 300 light-years away from Earth. Although in literature it is known as Alpha Draconis (where the alpha indicates the brightest star in the constellation), Thuban with an apparent magnitude of +3.67 is only the eighth brightest star in Draco.

At first glance, Thuban may seem a fairly irrelevant star, similar to many others, as indeed its physical parameters demonstrate. Its mass is in fact just under three times that of the Sun while its radius is 3.4 times that of our star.

However, Thuban has a particular cultural significance: due to the precession of the Earth's axis, Thuban was in fact the closest star to the celestial North Pole between 4000 and 2000 BC.

Thuban reached a minimum distance from the North Pole around 2800 BC, when it was less than two and a half degrees away (for comparison, the Polar star is currently less than half a degree from the celestial North Pole). From that moment on, Thuban began to move further and further away from the pole and it will reach the maximum distance in about 8000 years. Also due to the precession of the Earth's axis it will get closer to the pole and can be considered again the north star in 18,000 years.

Credit: David Ritter.

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