A pole star is a visible star, especially 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 other planets.)
The term "the Pole Star" usually refers to Polaris, which is the current northern pole star, also known as the North Star. The south celestial pole currently 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 essentially fixed. This makes them especially useful in celestial navigation: they are a dependable indicator of the direction toward the respective geographic pole, 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. However, the stars themselves exhibit motion relative to each other, and this so-called proper motion is another cause of the apparent drift of pole stars.
Northern pole star (North Star)
At the present time, the northern pole star, or North Star, is Polaris, which lies about three-quarters of a degree from the north celestial pole, at the end of the "bob" of the Little Dipper asterism in the constellation Ursa Minor. A common method of locating Polaris in the sky is to follow along the line of the so-called "pointer" stars, the two stars farthest from the "handle" of the Big Dipper.
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