The North Star is a title of the star best suited for navigation northwards. A candidate for navigation northward must be viewable from Earth and circumpolar to the north celestial pole. The current one is Polaris. It is the star at the end of the "handle" of the Little Dipper asterism in the constellation Ursa Minor. It is close to the celestial North Pole, but off by about two-thirds of 1°. The North Star has been historically used by explorers to determine their latitude. The angle the North Star makes with respect to the horizon is equal to the latitude of the observer. It is worth mentioning that the North Star is only visible in northern hemisphere skies and as such it is useless for determining latitude in the southern hemisphere. Polaris has a visual magnitude of 1.67 (second magnitude). Although some people mistakenly think this is the brightest star in the night sky (a title which truly belongs to Sirius), there are many others that are brighter.
Past and Future
Due to the precession of the equinoxes the direction of the Earth's axis is very slowly but constantly 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 3001 BC 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 Pole Star (situated at 430 light-years away) Gamma Cephei (also known as Alrai, situated at 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" 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. 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 south pole.
See also
| Nautical Portal |
