North Star
From Wikipedia, the free encyclopedia
The North(ern) Star is the prominent pole star that lies closest in the sky to the north celestial pole, and which appears (approximately) directly overhead to an observer at the Earth's North Pole. The current North Star is Polaris, which lies about two-thirds of a degree from the pole at the end of the "bob" of the Little Dipper asterism in the constellation Ursa Minor. Polaris has a visual magnitude of 1.97 (second magnitude). (Some people mistakenly think that Polaris is the brightest star in the night sky. This title belongs to Sirius, and there are many others stars also brighter than Polaris.)
The North Star has historically been used for navigation, both to find the direction of north and to determine latitude. It always appears due north in the sky, and the angle it makes with respect to the horizon is equal to the latitude of the observer. The North Star is visible only in northern hemisphere skies and so cannot be used for navigation south of the equator.
Currently, there is no South Star as useful as Polaris; the faint star σ Octantis is closest to the south celestial pole. However, the constellation Crux, the Southern Cross, points towards the pole.
A similar concept applies to other planets; see Pole Star for details.
[edit] Past and future
Due to the precession of the equinoxes the direction of the Earth's axis is very slowly but continuously changing, and as the projection of the Earth's axis moves around the celestial sphere over the millennia, the role of North Star passes from one star to another. Since the precession of the equinoxes is so slow, a single star typically holds that title for many centuries.
In 3000 BCE the faint star Thuban in the constellation Draco was the North Star. At magnitude 3.67 (fourth magnitude) it is only one-fifth as bright as Polaris, the current North Star (situated 430 light-years away).
Gamma Cephei (also known as Alrai, situated 45 light-years away) will become closer to the northern celestial pole than Polaris around 3000 AD, and be at its closest approach around 4000 AD. The title of North Star then will pass to Iota Cephei (ι Cephei, situated 115 light-years away) some time around 5200 AD. The first magnitude star Vega (26 light-years away) will then become the North Star by 14000 AD.
