Pole star

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Long exposure of stars appearing to circle the North Celestial pole. The daily rotation of Earth creates the appearance of rings around the pole.

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. There are potentially both northern and southern pole stars, but whether there is either depends on the current orientation of the Earth's axis, which moves over time (see precession of the equinoxes). The term the Pole Star usually refers to the star Polaris (colloquially referred to as the North Star), which is the current northern pole star.

Pole stars change over time because stars exhibit a slow continuous drift with respect to the Earth's axis. The primary reason for this is the precession of the Earth's rotational axis that 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 a pole star.

Pole stars are often used in celestial navigation. While other stars' positions change throughout the night, the pole stars' position in the sky essentially do not. Therefore, 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.

[edit] North pole

At the present time, Polaris is the pole star in the northern direction. Its 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 current declination is +89°15'50.8".) 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]

[edit] South pole

σ Octantis is the naked-eye star closest to the south celestial pole, but it is too faint to serve as a useful pole star. 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.

[edit] Precession

Precession of Earth's axis around the north ecliptical pole

Precession of Earth's axis around the south ecliptical pole

A consequence of the precession is a changing pole star. Currently Polaris is extremely well-suited to mark the position of the north celestial pole, as Polaris is a moderately bright star with a visual magnitude of 2.1 (variable), and it is located within a half degree of the pole.^[2] On the other hand, Thuban in the constellation Draco, which was the pole star in 3000 BC, is much less conspicuous at magnitude 3.67 (one-fifth as bright as Polaris); today it is invisible in light-polluted urban skies.

The brilliant Vega in the constellation Lyra is often touted as the best north star (it fulfilled that role around 12000 BC and will do so again around the year AD 14000), however it never comes closer than 5° to the pole.

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 came closer to the pole.

It is more difficult to find the south celestial pole in the sky at this moment, as that area is a particularly bland portion of the sky, and the nominal south pole star is Sigma Octantis, which with magnitude 5.5 is barely visible to the naked eye even under ideal conditions. That will change from the eightieth to the ninetieth centuries, however, when the south celestial pole travels through the False Cross.

This situation also is seen on a star map. The orientation of the south pole is moving toward the Southern Cross constellation. For the last 2,000 years or so, the Southern Cross has nicely pointed to the south pole. By consequence, the constellation is no longer visible from subtropical northern latitudes, as it was in the time of the ancient Greeks.

[edit] 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 sothern pole of Venus
• 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:

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.

[edit] In Srimad Bhagavatam

• 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.

[edit] References

1. ^ Jean Meeus, Mathematical Astronomy Morsels Ch.50; Willmann-Bell 1997
2. ^ "Precession of the Equinox". Astro 101. Western Washington University Planetarium. http://www.wwu.edu/depts/skywise/a101_precession.html. Retrieved on July 3, 2009. 
3. ^ 2004. Starry Night Pro, Version 5.8.4. Imaginova. ISBN 978-0-07-333666-4. www.starrynight.com
4. ^ http://www.eknent.com/etc/mars_np.png
5. ^ http://www.spaceflightweb.com/pfs/earth.html

[edit] Other resources

Look up Pole Star in Wiktionary, the free dictionary.

Look up pole star in Wiktionary, the free dictionary.

[edit] See also

• Celestial sphere
• Celestial pole
• Celestial equator
• North Star
• South Star
• Circumpolar star
• Precession of the equinoxes
• Astronomy
• Lode Star
• Celestial navigation
• Vega
• Gamma Cephei
• Iota Cephei
• Voyages of Christopher Columbus

[edit] External links

• Star trails around Polaris