Pole star

For other uses, see Polestar (disambiguation).

"North Star" redirects here. For other uses, see North Star (disambiguation).

A French "navisphere": a type of celestial globe formerly used for navigation at sea

The North 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.

1 Northern pole star (North Star)
- 1.1 Past and future
2 Southern pole star (South Star)
- 2.1 Past and future
3 Other planets
4 In world cultures
5 References
6 See also
7 External links

Northern pole star (North Star)

Time-lapse video of Polaris and neighboring stars.

Time-exposure photo of Polaris and neighbouring stars, taken from Cerro Noroeste. In 2006 Polaris was 42' from the north celestial pole, and so does not lie exactly at the center of the concentric circular star trails.

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.

Polaris is a moderately bright star with an apparent magnitude of 1.97 (variable) making it the brightest star in the Little Dipper. Its current declination is +89°15'50.8".

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.

Past and future

The path of the north celestial pole amongst the stars due to the effect of precession^{[clarification needed]}

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. Since the precession of the equinoxes is so slow, taking about 26,000 years to complete a cycle, a single star typically holds that title for many centuries.

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.^[1]

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

The brilliant Vega in the constellation Lyra is often touted as the best North Star (it fulfilled that role around 12000 BCE and will do so again around the year 14000 CE). However, it never comes closer than 5° to the pole.

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

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

Southern pole star (South Star)

Currently, there is no South Star as useful as Polaris. σ Octantis is the naked-eye star closest to the south celestial pole, but at apparent magnitude 5.45 it is too faint to serve as a useful pole star. Its angular separation from the pole is about 1° (as of 2000^[update]). The Southern Cross constellation functions as an approximate southern pole constellation, by pointing to where a southern pole star would be. At the equator it is possible to see both Polaris and the Southern Cross.

Past and future

The path of the south celestial pole amongst the stars due to the effect of precession^{[clarification needed]}

Although the south celestial pole currently lacks a bright star like Polaris to mark its position, slow changes over time (due to the effects of precession) mean that other stars will become southern pole stars.

The celestial south pole is moving toward the Southern Cross, which has nicely pointed to the south pole for the last 2,000 years or so. As a consequence, the constellation is no longer visible from subtropical northern latitudes, as it was in the time of the ancient Greeks.

In the next 7500 years, the south celestial pole will pass close to the stars Gamma Chamaeleontis (4200 CE), I Carinae, Omega Carinae (5800 CE), Upsilon Carinae, Iota Carinae (Aspidiske, 8100 CE) and Delta Velorum (9200 CE).^[2] From the eightieth to the ninetieth centuries, the south celestial pole travels through the False Cross.

Other planets

Pole stars of other planets are defined analogously: they are stars that most closely coincide with the projection of the planet's axis of rotation onto the celestial sphere. Different planets have different pole stars because their axes are oriented differently.

Alpha Pictoris is the south pole star of Mercury, while Omicron Draconis is the north star.^[3]
Phi Draconis is the closest star to the southern pole of Venus, however Eta¹ Doradus is brighter though not as close.
Delta Doradus is the south pole star of the Moon
Kappa Velorum is only a couple of degrees from the south celestial pole of Mars. The top two stars in the Northern Cross, Sadr and Deneb, point to the north celestial pole of Mars.^[4]
Delta Octantis is the south pole star of Saturn
Eta Ophiuchi is the north pole star of Uranus and 15 Orionis is its south pole star
The north pole of Neptune points to a spot midway between Gamma and Delta Cygni. Its south pole star is Gamma Velorum
Pluto's north pole points toward the constellation Delphinus

The location of the celestial poles of the planets is shown in the following table:

Planet	North Celestial Pole		South Celestial Pole
Planet	RA	Dec	RA	Dec
Mercury	281.01	+61.45	101.01	−61.45
Venus	272.76	+67.16	92.76	−67.16
Earth	—	+90.00	—	−90.00
Mars	317.68	+52.88	137.68	−52.88
Jupiter	268.05	+64.49	88.05	−64.49
Saturn	40.56	+83.54	220.56	−83.54
Uranus	257.43	−15.10	77.43	+15.10
Neptune	299.36	+43.46	119.36	−43.46
Pluto	313.02	+9.09	133.02	−9.09

The declination of Uranus' north celestial pole is negative as it spins 'backwards'. Pluto's north celestial pole would likewise be below the Earth's celestial equator were it not for the large angle of its orbit with respect to the ecliptic. Venus should have a negative declination also though the reference material omitted this.^[5]

Some asteroids have more than one axis of rotation, possible due to impacts with other bodies. These asteroids tumble in space and have no pole stars. It is possible that a planet could be similarly affected, though unlikely as the much greater mass of a planet and its (usually) spherical shape make it much more unlikely for an impact to create a second axis of rotation.

In world cultures

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In Japan the Pole Star was represented by Myouken Bosatsu, ja:妙見菩薩.

In Vedic Wisdom, the Pole star is called Dhruva, an ardent devotee of the god Vishnu, who was blessed to be in a high position in the sky.^{[citation needed]} According to Vishnu Puran; Dhruva [Pole Star] situated in the tail of a Porpoise shaped Galaxy and Center of our Universe. our Solar system is tied up with it by Aerial cords. vishnu puran

THE form of the mighty Hari which is present in heaven, consisting of the constellations, is that of a porpoise, with Dhruva situated in the tail. As Dhruva revolves, it causes the moon, sun, and stars to turn round also; and the lunar asterisms follow in its circular path; for all the celestial luminaries are in fact bound to the polar-star by aerial cords. The porpoise-like figure of the celestial sphere is upheld by Náráyańa, who himself, in planetary radiance, is seated in its heart; whilst the son of Uttanápáda, Dhruva, in consequence of his adoration of the lord of the world, shines in the tail of the stellar porpoise . The upholder of the porpoise-shaped sphere is the sovereign of all, Janárddana. This sphere is the supporter of Dhruva; and by Dhruva the sun is upstayed. Upon the sun depends this world, with its gods, demons, and men. In what manner the world depends upon the sun, be attentive, and you shall hear.