Home      Discussion      Topics      Dictionary      Almanac
Signup       Login
Pole star

Pole star

Overview
"North Star" redirects here. For other uses, see North Star (disambiguation)
North Star (disambiguation)
North Star is the prominent star that lies closest in the sky to the north celestial pole, and which appears directly overhead to an observer at the Earth's North Pole.North Star may also refer to:-Places:In Australia:...

.


The term "Pole Star" usually refers to Polaris
Polaris
Polaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star....

, which is the current northern pole star, also known as the North Star.

In general, however, a pole star is a visible star, especially a prominent one, that is approximately aligned with the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's axis of rotation; that is, a star whose apparent position is close to one of the celestial pole
Celestial pole
The north and south celestial poles are the two imaginary points in the sky where the Earth's axis of rotation, indefinitely extended, intersects the imaginary rotating sphere of stars called the celestial sphere...

s, and which lies approximately directly overhead when viewed from the Earth's North Pole
North Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets its surface...

 or South Pole
South Pole
The South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is one of the two points where the Earth's axis of rotation intersects its surface. It is the southernmost point on the surface of the Earth and lies on the opposite side of the Earth from the North Pole...

.
Discussion
Ask a question about 'Pole star'
Start a new discussion about 'Pole star'
Answer questions from other users
Full Discussion Forum
 
Unanswered Questions
Encyclopedia
"North Star" redirects here. For other uses, see North Star (disambiguation)
North Star (disambiguation)
North Star is the prominent star that lies closest in the sky to the north celestial pole, and which appears directly overhead to an observer at the Earth's North Pole.North Star may also refer to:-Places:In Australia:...

.


The term "Pole Star" usually refers to Polaris
Polaris
Polaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star....

, which is the current northern pole star, also known as the North Star.

In general, however, a pole star is a visible star, especially a prominent one, that is approximately aligned with the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's axis of rotation; that is, a star whose apparent position is close to one of the celestial pole
Celestial pole
The north and south celestial poles are the two imaginary points in the sky where the Earth's axis of rotation, indefinitely extended, intersects the imaginary rotating sphere of stars called the celestial sphere...

s, and which lies approximately directly overhead when viewed from the Earth's North Pole
North Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets its surface...

 or South Pole
South Pole
The South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is one of the two points where the Earth's axis of rotation intersects its surface. It is the southernmost point on the surface of the Earth and lies on the opposite side of the Earth from the North Pole...

. A similar concept also applies to other planets.

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
Sigma Octantis
Sigma Octantis is a magnitude 5.6 star in the constellation Octans most notable for being the current South Star. Sigma Octantis is approximately 270 light years from Earth, and is classified as a giant, with a spectral type of F0 III...

, 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
Celestial navigation
Celestial navigation, also known as astronavigation, is a position fixing technique that has evolved over several thousand years to help sailors cross oceans without having to rely on estimated calculations, or dead reckoning, to know their position...

: 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
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

.

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
Celestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...

 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
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...

 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
Polaris
Polaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star....

, which lies about one degree from the north celestial pole, at the end of the "bob" or "handle" of the Little Dipper asterism
Asterism (astronomy)
In astronomy, an asterism is a pattern of stars recognized on Earth's night sky. It may form part of an official constellation, or be composed of stars from more than one. Like constellations, asterisms are in most cases composed of stars which, while they are visible in the same general direction,...

 in the constellation Ursa Minor
Ursa Minor
Ursa Minor , also known as the Little Bear, is a constellation in the northern sky. Like the Great Bear, the tail of the Little Bear may also be seen as the handle of a ladle, whence the name Little Dipper...

. 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
Big Dipper
The Plough, also known as the Big Dipper or the Saptarishi , is an asterism of seven stars that has been recognized as a distinct grouping in many cultures from time immemorial...

 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.

Polaris is a moderately bright star with an apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

 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
Navigation
Navigation is the process of monitoring and controlling the movement of a craft or vehicle from one place to another. It is also the term of art used for the specialized knowledge used by navigators to perform navigation tasks...

, both to find the direction of north and to determine latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

. 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 can be seen slightly south of the Equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

 (because of atmospheric refraction
Atmospheric refraction
Atmospheric refraction is the deviation of light or other things like humanelectromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of altitude...

); further south, it cannot be used for navigation.

Past and future



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 25,770 years to complete a cycle, a single star typically holds that title for many centuries.

Polaris' mean position (taking account of precession
Precession
Precession is a change in the orientation of the rotation axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle is constant...

 and proper motion
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...

) will reach a maximum declination
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...

 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
Nutation
Nutation is a rocking, swaying, or nodding motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight, or as an intended behavior of a mechanism...

 and aberration
Aberration of light
The aberration of light is an astronomical phenomenon which produces an apparent motion of celestial objects about their real locations...

) will be +89°32'50.62", so 1629" or 0.4526° from the celestial north pole, on 24 March 2100.

Gamma Cephei
Gamma Cephei
Gamma Cephei , traditionally named Errai, Er Rai, and or Alrai, is a binary star system approximately 45 light-years away in the constellation of Cepheus. Gamma Cephei contains an apparent magnitude of 3.22. The visible part of the system is a stellar class K1III-IV orange subgiant star on its...

 (also known as Alrai, situated 45 light-years away) will become closer to the northern celestial pole than Polaris around AD 3000. Iota Cephei
Iota Cephei
Iota Cephei is a Class K0, third-magnitude star in the constellation Cepheus. Iota Cephei is an orange giant star located about 115 light-years from Earth. Due to precession of the equinoxes, it will become the pole star some time around 5200 CE.-External links:...

 will become the pole star some time around AD 5200.

First-magnitude Deneb
Deneb
Deneb is the brightest star in the constellation Cygnus and one of the vertices of the Summer Triangle. It is the 19th brightest star in the night sky, with an apparent magnitude of 1.25. A blue-white supergiant, Deneb is also one of the most luminous nearby stars...

 will be within 5° of the North Pole in AD 10000.

The brilliant Vega
Vega
Vega is the brightest star in the constellation Lyra, the fifth brightest star in the night sky and the second brightest star in the northern celestial hemisphere, after Arcturus...

 in the constellation Lyra
Lyra
Lyra is a small constellation. It is one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations recognized by the International Astronomical Union. Its principal star, Vega — a corner of the Summer Triangle — is one of the brightest...

 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
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...

 it then will be farther away from the pole than it is now, while in 23600 BC it came closer to the pole.

In 3000 BC the faint star Thuban
Thuban
Thuban also known by its Bayer designation Alpha Draconis is a star in the constellation of Draco. A relatively inconspicuous star in the night sky of the Northern Hemisphere, it is historically significant as having been the north pole star in ancient times...

 in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....

 Draco
Draco (constellation)
Draco is a constellation in the far northern sky. Its name is Latin for dragon. Draco is circumpolar for many observers in the northern hemisphere...

 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
Polaris
Polaris |Alpha]] Ursae Minoris, commonly North Star or Pole Star, also Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star....

. Sigma Octantis
Sigma Octantis
Sigma Octantis is a magnitude 5.6 star in the constellation Octans most notable for being the current South Star. Sigma Octantis is approximately 270 light years from Earth, and is classified as a giant, with a spectral type of F0 III...

 is the naked-eye star closest to the south Celestial pole, but at apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

 5.45 it is too faint to serve as a useful pole star. Its angular separation from the pole is about 1° . The Southern Cross
Crux
Crux is the smallest of the 88 modern constellations, but is one of the most distinctive. Its name is Latin for cross, and it is dominated by a cross-shaped asterism that is commonly known as the Southern Cross.-Visibility:...

 constellation functions as an approximate southern pole constellation, by pointing to where a southern pole star would be. At the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

 it is possible to see both Polaris and the Southern Cross.

Past and future


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
Crux
Crux is the smallest of the 88 modern constellations, but is one of the most distinctive. Its name is Latin for cross, and it is dominated by a cross-shaped asterism that is commonly known as the Southern Cross.-Visibility:...

, which has 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.

There have been many pole stars throughout the millennia. Around 2000 BC, the star Eta Hydri
Hydrus
Hydrus is a small constellation in the southern sky, created in the sixteenth century. Its name means "male water snake", and it should not be confused with Hydra, a much larger constellation which represents a female water snake.-History:...

 was the nearest bright star to the Celestial south pole.

In the next 7500 years, the south Celestial pole will pass close to the stars Gamma Chamaeleon
Chamaeleon
Chamaeleon is a small constellation in the southern sky. It is named after the chameleon, a form of lizard. It was first defined in the sixteenth century.-History:...

tis (4200 AD), I Carinae
I Carinae
I Carinae is a star in the constellation Carina. It is a yellow-white F-type subgiant with an apparent magnitude of +3.99 and is approximately 52.9 light years from Earth....

, Omega Carinae
Omega Carinae
Omega Carinae is a star in the constellation Carina. With a declination greater than 70 degrees south of the celestial equator, it is the most southerly of the bright stars of Carina , and it is part of a popular southern asterism known as the Diamond Cross. ω Carinae is a blue-white B-type giant...

 (5800 AD), Upsilon Carinae
Upsilon Carinae
Upsilon Carinae is a binary star in the constellation Carina. It is part of the Diamond Cross asterism in southern Carina....

, Iota Carinae
Iota Carinae
Iota Carinae is a star in the constellation Carina. It is one of the brighter stars in the night sky.It has the traditional names Aspidiske, Scutulum and Turais...

 (Aspidiske, 8100 AD) and Delta Velorum
Delta Velorum
Delta Velorum is a star system in the constellation Vela. It is approximately 79.7 light years from Earth. It is sometimes given the name Koo She, Chinese for 'Bow and Arrows', along with ω Velorum and stars in Canis Major....

 (9200 AD). From the eightieth to the ninetieth centuries, the south Celestial pole will travel 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. (See Poles of astronomical bodies
Poles of astronomical bodies
The poles of astronomical bodies are determined based on their axis of rotation in relation to the celestial poles of the celestial sphere.-Geographic poles:...

.)
  • Alpha Pictoris
    Alpha Pictoris
    Alpha Pictoris is the brightest star in the constellation Pictor with an apparent magnitude of 3.30. It is located about 99 light years from the Sun...

     is the south pole star of Mercury
    Mercury (planet)
    Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

    , while Omicron Draconis
    Omicron Draconis
    Omicron Draconis is a supergiant star in the constellation Draco located 322.93 light years from the Earth. It has a radius of 30 solar radii, and a luminosity of 269 suns...

     is the north star.
  • Phi Draconis
    Phi Draconis
    Phi Draconis is a fourth-magnitude star in the constellation Draco. It has the Flamsteed designation 43 Draconis....

     is the closest star to the southern pole of Venus
    Venus
    Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

    , however Eta¹ Doradus is brighter though not as close.
  • Delta Doradus
    Delta Doradus
    δ Doradus is a faint star in the Dorado constellation that has the distinction of being the Moon's south pole star. It is better aligned than Earth's Polaris , but much fainter.-External links:...

     is the south pole star of the Moon
    Moon
    The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

    .
  • Kappa Velorum
    Kappa Velorum
    Kappa Velorum is a binary star in the constellation Vela. It also has the traditional name Markab, often spelled Markeb to distinguish it from similarly named stars such as Alpha Pegasi....

     is only a couple of degrees from the south Celestial pole of Mars
    Mars
    Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

    . The top two stars in the Northern Cross
    Cygnus (constellation)
    Cygnus is a northern constellation lying on the plane of the Milky Way. Its name is the Latinized Hellenic word for swan. One of the most recognizable constellations of the northern summer and autumn, it features a prominent asterism known as the Northern Cross...

    , Sadr
    Gamma Cygni
    Gamma Cygni is a star in the constellation Cygnus. It has the traditional name Sadr ....

     and Deneb
    Deneb
    Deneb is the brightest star in the constellation Cygnus and one of the vertices of the Summer Triangle. It is the 19th brightest star in the night sky, with an apparent magnitude of 1.25. A blue-white supergiant, Deneb is also one of the most luminous nearby stars...

    , point to the north Celestial pole of Mars.
  • Delta Octantis
    Delta Octantis
    Delta Octantis has the distinction of being Saturn's southern pole star....

     is the south pole star of Saturn
    Saturn
    Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

    .
  • Eta Ophiuchi
    Eta Ophiuchi
    Eta Ophiuchi, also known as η Oph or η Ophiuchi, is a star in the constellation Ophiuchus. It also has the traditional name Sabik....

     is the north pole star of Uranus
    Uranus
    Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

     and 15 Orionis
    15 Orionis
    15 Orionis has the distinction of being the southern pole star of Uranus.It is an early F-type subgiant located 318 ly from the solar system....

     is its south pole star.
  • The north pole of Neptune
    Neptune
    Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

     points to a spot midway between Gamma and Delta Cygni. Its south pole star is Gamma Velorum
    Gamma Velorum
    Gamma Velorum is a star system in the constellation Vela. At magnitude +1.7, it is one of the brightest stars in the night sky. It has the traditional names Suhail and Suhail al Muhlif, which confusingly also apply to Lambda Velorum...

    .


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



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

In the Greek Magical Papyri
Greek magical papyri
The Greek Magical Papyri is the name given by scholars to a body of papyri from Graeco-Roman Egypt, which each contain a number of magical spells, formulae, hymns and rituals. The materials in the papyri date from the 2nd century BC to the 5th century AD...

 the Pole star was identified with Set-Typhon, and given authority over the Gods.

See also


  • Celestial equator
    Celestial equator
    The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator. In other words, it is a projection of the terrestrial equator out into space...

  • Circumpolar star
    Circumpolar star
    A circumpolar star is a star that, as viewed from a given latitude on Earth, never sets , due to its proximity to one of the celestial poles...

  • Celestial navigation
    Celestial navigation
    Celestial navigation, also known as astronavigation, is a position fixing technique that has evolved over several thousand years to help sailors cross oceans without having to rely on estimated calculations, or dead reckoning, to know their position...

  • Voyages of Christopher Columbus
    Voyages of Christopher Columbus
    In the early modern period, the voyages of Columbus initiated European exploration and colonization of the American continents, and are thus of great significance in world history. Christopher Columbus was a navigator and an admiral for Castile, a country that later founded modern Spain...


External links