Tuesday, December 11, 2007


POLARIS - more commonly known as The Pole Star, The North Star is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole 42′ away, making it the current northern pole star.
Polaris stands almost motionless on the sky, and all the stars of the Northern sky appear to rotate around it. It makes an excellent fixed point from which to draw measurements for celestial navigation and for astrometry. At present, Polaris is 0.7° away from the pole of rotation (1.4 times the Moon disc) and revolves around the pole in a small circle 1½° in diameter. Only twice during every sidereal day does Polaris accurately define the true north azimuth; the rest of the time it is only an approximation and must be corrected using tables or a rough rule of thumb.
Due to the precession of the equinoxes, Polaris will not always be the pole star. Over tens of thousands of years, perturbations to the Earth's axis of rotation will cause it to point to other regions of the sky, tracing out a circle. Other stars along this circle were the pole star in the past and will be again in the future, including Vega. Polaris has been close to the actual position of the north pole for over 1000 years and during the course of the 21st century it will continue to approach the exact theoretical position, reaching its closest approach on 24 March 2100 (almost 0.45° away). Subsequently it will begin to pull away.

In the Northern Hemisphere, it is easy to find Polaris by following the line traced from Merak to Dubhe and α Ursae Majoris, also known as the Pointers), the two stars at the end of the bowl of the Big Dipper (or Plough). One can also follow the central point of the "W" shape of Cassiopeia. Polaris is not visible from the Southern Hemisphere except from an elevated position near the equator.

Polaris's fame as the North Star has given rise to a persistent misconception that it is the brightest star in the sky. Although Polaris is a relatively bright star and is conspicuous since no other stars of similar brightness are close to it, it is nowhere near the brightest; it is actually the 48th brightest star in the night sky. The brightest star in the night sky is Sirius.

There is no real southern pole star. The only star visible to the naked eye that is close to the south celestial pole is the dim Sigma Octantis, sometimes called Polaris Australis. The bright Southern Cross (Crux) points fairly accurately towards the south celestial pole.