Pole Star

Polaris, Alpha Ursae Minoris, is the bright star closest to the north celestial pole. It is also known as the North Star, the Lode Star, or the Pole star.

Because it lies nearly in a direct line with the axis of the Earth's rotation "above" the North Pole -- the north celestial pole -- Polaris is apparently motionless from the Earth, and all the stars of the Northern sky appear to rotate around it. Therefore, it makes an excellent fixed point from which to draw measurements for celestial navigation. The antiquity of the use of this star is attested to by the fact that it is found represented on the earliest known Assyrian tablets. At present, Polaris is slightly over 1° away from the pole of rotation and hence revolves around the pole in a small circle about 2° in diameter. Only twice during every 24 hours does Polaris accurately define the true north azimuth; the rest of the time it is only an approximation and must be corrected using tables.

Although Shakespeare wrote "I am as constant as the northern star", due to precession of the equinox, other stars in the northern hemisphere have been and will likely again become the north star over thousands of years; Thuban was pole star in the past, and Vega will be in the future.

It is easy to find Polaris by following the line traced from Merak to Dubhe (β and α Ursae Majoris), the two stars at the end of the bowl of the Big Dipper. One can also follow the central point of the W shape of Cassiopeia.

Polaris is 431 light years (132 parsecs) from Earth, according to measurements made by the Hipparcos satellite. It is an F7 supergiant (Ib) or bright giant (II), with two smaller companions: an F3 V main sequence star about 2000 AU away and a close companion in an orbit with a 5 AU semi-major axis. The main star is a Population II cepheid variable, the pulsations of which cause it steadily cycle between 8% brighter than its average luminosity and 8% dimmer (0.15 magnitudes in total) with a 3.97 day period.

There is no real south star. The star, visible to the naked eye, that is closest to the south celestial pole is the dim Sigma Octantis. However, the bright Southern Cross (Crux) points towards the south celestial pole.

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