Pole Star - Northern Pole Star (North Star)

Northern Pole Star (North Star)

At the present time, the northern pole star, or North Star, is a moderately bright star with an apparent magnitude of 1.97 (variable), the brightest star in the Ursa Minor constellation (at the end of the or "handle" of the "Little Dipper" asterism). Its current (October 2012) declination is +89°19'8" (as per 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 horizon is equal to the latitude of the observer. It is consequently known as Polaris (from Latin stella polaris "pole star"). It also retains its older name, 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 North Star can even be seen slightly south of the Equator (because of atmospheric refraction); further south, it cannot be used for navigation. A common method of locating Polaris in the sky is to follow along the line of the so-called "pointer" stars in the bowl of the Big Dipper asterism, specifically, the two stars farthest from its "handle". The arc between the pointer stars & Polaris is nearly five times greater than the arc between the pointer stars.

The North Star has historically been used for navigation since Late Antiquity, both to find the direction of north and to determine latitude.

Due to the precession of the equinoxes (as well as the stars' proper motions), the role of North Star passes from one star to another. The name stella polaris has been given to α Ursae Minoris since at least the 16th century, even though at that time it was still several degrees away from the celestial pole. Gemma Frisius determined this distance as 3°7' in the year 1547. In the Roman era, the celestial pole was about equally distant from α Ursae Minoris (Cynosura) and β Ursae Minoris (Kochab). Before this, during the 1st millennium BC, β Ursae Minoris was the bright star closest to the celestial pole, but it was never close enough to be taken as marking the pole, and the Greek navigator Pytheas in ca. 320 BC described the celestial pole as devoid of stars. Polaris was described as αει φανης "always visible" by Stobaeus in the 5th century, when it was still removed from the celestial pole by about 8°. It was known as scip-steorra ("ship-star") in 10th-century Anglo-Saxon England, reflecting its use in navigation.

The precession of the equinoxes takes about 25,770 years to complete a cycle. Polaris' mean position (taking account of precession and proper motion) will reach a maximum declination of +89°32'23", so 1657" or 0.4603° from the celestial north pole, in February 2102. Its maximum apparent declination (taking account of nutation and aberration) will be +89°32'50.62", so 1629" or 0.4526° from the celestial north pole, on 24 March 2100.

Gamma Cephei (also known as Alrai, situated 45 light-years away) will become closer to the northern celestial pole than Polaris around AD 3000. Iota Cephei will become the pole star some time around AD 5200.

First-magnitude Deneb will be within 5° of the North Pole in AD 10000.

When Polaris becomes the North Star again around 27800 AD, due to its proper motion it then will be farther away from the pole than it is now, while in 23600 BC it was closer to the pole.

In 3000 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, and today it is invisible in light-polluted urban skies.

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