North Star

A pole star is a visible star, preferably a prominent one, that is approximately aligned with the Earth's axis of rotation; that is, a star whose apparent position is close to one of the celestial poles, and which lies approximately directly overhead when viewed from the Earth's North Pole or South Pole. A similar concept also applies to planets other than the Earth. In practice, the term pole star usually refers to Polaris, which is the current northern pole star, also known as the North Star.

The south celestial pole lacks a bright star like Polaris to mark its position. At present, the naked-eye star nearest to this imaginary point is the faint Sigma Octantis, which is sometimes known as the South Star.

While other stars' apparent positions in the sky change throughout the night, as they appear to rotate around the celestial poles, pole stars' apparent positions remain virtually fixed. This makes them especially useful in celestial navigation: they are a dependable indicator of the direction toward the respective geographic pole although not exact; they are virtually fixed, and their angle of elevation can also be used to determine latitude.

The identity of the pole stars gradually changes over time because the celestial poles exhibit a slow continuous drift through the star field. The primary reason for this is the precession of the Earth's rotational axis, which causes its orientation to change over time. If the stars were fixed in space, precession would cause the celestial poles to trace out imaginary circles on the celestial sphere approximately once every 26,000 years, passing close to different stars at different times. In fact, the stars themselves also exhibit proper motion, which causes a very small additional apparent drift of pole stars.

The closest bright star to the north celestial pole is Polaris. At magnitude 1.97 (variable), it is the brightest star in the constellation Ursa Minor (at the end of the "handle" of the "Little Dipper" asterism). As of October 2012 its declination is +89°19′8″ (at epoch J2000 it was +89°15′51.2″). Therefore, it always appears due north in the sky to a precision better than one degree, and the angle it makes with respect to the true horizon (after correcting for refraction and other factors) is equal to the latitude of the observer to better than one degree. It is consequently known as Polaris (from Latin stella polaris "pole star"). It was formerly sometimes known as Cynosura, from a time before it was the pole star, from its Greek name meaning "dog's tail" (as the constellation of Ursa Minor was interpreted as a dog, not a bear, in antiquity). The pole will be nearest Polaris in 2100 and will thereafter become more distant.

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Wikipedia