http://en.wikipedia.org/wiki/Pole_star wrote:
<<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.
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.
Polaris is about 430 light-years from Earth. It is a multiple star. α UMi A is a six solar mass F7 bright giant (II) or supergiant (Ib). The two smaller companions are: α UMi B, a 1.5 solar mass F3V main sequence star orbiting at a distance of 2400 AU, and α UMi Ab, a very close dwarf with an 18.5 AU radius orbit. There are also two distant components UMi C and UMi D.
Polaris B can be seen with even a modest telescope and was first noticed by William Herschel in 1780. In 1929, it was discovered by examining the spectrum of Polaris A that it had another very close dwarf companion (variously α UMi P, α UMi a or α UMi Ab), which had been theorized in earlier observations (Moore, J.H and Kholodovsky, E. A.). In January 2006, NASA released images from the Hubble telescope, directly showing all three members of the Polaris ternary system. The nearer dwarf star is in an orbit of only 18.5 AU (2.8 billion km; about the distance from our Sun to Uranus) from Polaris A, explaining why its light is swamped by its close and much brighter companion.
Polaris is a classic Population I Cepheid variable (although it was once thought to be Population II due to its high galactic latitude). Since Cepheids are an important standard candle for determining distance, Polaris (as the closest such star) is heavily studied. Around 1900, the star luminosity varied ±8% from its average (0.15 magnitudes in total) with a 3.97 day period; however, the amplitude of its variation has been quickly declining since the middle of the 20th century. The variation reached a minimum of 1% in the mid 1990s and has remained at a low level. Over the same period, the star has brightened by 15% (on average), and the period has lengthened by about 8 seconds each year.
Recent research reported in Science suggests that Polaris is 2.5 times brighter today than when Ptolemy observed it (now 2mag, antiquity 3mag). Astronomer Edward Guinan considers this to be a remarkable rate of change and is on record as saying that "If they are real, these changes are 100 times larger than [those] predicted by current theories of stellar evolution."
South pole
Sigma Octantis (σ Oct / σ Octantis) is a magnitude 5.6 star in the constellation Octans most notable for being the current South Star.
Sigma Octantis is the dimmest star to be represented on a national flag.
It appears on
http://en.wikipedia.org/wiki/File:Flag_of_Brazil.svg the flag of Brazil, symbolising the capital city Brasilia.
Sigma Octantis is approximately 270 light years from Earth, and is classified as a Giant, with a spectral type of F0 III. It is a Delta Scuti variable, with magnitude varying by about 0.03 magnitudes over 2.3 hours. A Delta Scuti variable (also known as a Dwarf Cepheid, AI Velae Star or AI Velorum Star) is a variable star which exhibits variations in its luminosity due to both radial and non-radial pulsations of the star's surface. Typical brightness fluctuations are from 0.003 to 0.9 magnitudes in V over a period of a few hours, although the amplitude and period of the fluctuations can vary greatly. The stars are usually A0 to F5 type giant or main sequence stars. The prototype of these sorts of variable stars is Delta Scuti, which exhibits brightness fluctuations from +4.60 to +4.79 in apparent magnitude with a period of 4.65 hours. Other well known Delta Scuti variables include Denebola (β Leonis) and β Cassiopeiae. Vega (α Lyrae) is a suspected Delta Scuti variable, but this remains unconfirmed.
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. Other planets
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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.
*
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.
*
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
The location of the celestial poles of the planets is shown in the following table:
Code: Select all
Planet__ North Celestial __ South Celestial
________ 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
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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.
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.
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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.>>
