Astronomy

Pole Star



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The Pole Star


Ursa Minor, the Little Bear or the Little Dipper, would be a relatively insignificant constellation, but for its brightest star, Alpha Ursae Minoris, or Polaris. Apart from the Sun, this star is the most important star in the heavens for those of us who live north of the equator. Its location places it a little less than 1 degree from the celestial north pole, so that as the Earth revolves beneath the heavenly dome, the relative position of this star barely changes, so that for any observer north of the equator, its angle above the horizon is equivalent to the latitude of the observer, and with this unchanging celestial guidepost, the observer knows the direction of his travels.

Aside from its value to the traveler or mariner, Polaris is an interesting star by itself. It belongs to spectral class F8, meaning it is a yellow star of medium temperature. It is actually a triple star, with a small spectral class F3 companion of the ninth magnitude, lying at 2000 AU. The capacity to distinguish the two stars is a good test of a small telescope. There is another companion at 5 AU, whose light cannot be detected but whose presence is postulated because of the variation of radial velocity of the visible pair. Polaris is also a Cepheid variable with a period of 3.9696 days and a fluctuation of 0.1 magnitude. Recent observations suggest that this period is moving toward a period of 5.7 days.

Polaris is estimated to lie about 430 light-years from the Sun, and its actual luminosity is 2200 times greater than our own sun.

The Pole Star has had a variety of names. Legend states that it was recognized as a navigational aid by the Phoenicians, and thus named Phoenice by the Greeks. The Anglo-Saxons called it Scip-Steorra, the Ship-Star, and Arab astronomers named the star Al Kaukab al Shamaliyy, Star of the North, or Al Kutb al Shamaliyy, the Northern Axis or Northern Spindle. Muslims called it Al Kiblah, in recognition of its value in the determination of true compass points, so the observer could know the direction of Mecca. Througout Asia the pole star appears in legends. In Hindu lore it is regarded as the pinnacle of the "Cosmic Mountain", the dwelling of the deities, reflected in the amazing spires found on the temples throughout the region. In China it was variously called the "Axis of the Universe", "The Center", "The Wheel of the Universe", and "The Wheel of Life", as well as the pole star.

Polaris has not always been the Pole Star. Because of the precession of the Earth's axis, the position of celestial north slowly moves in a circle with a 47-degree diameter. Around 2102 A.D., Polaris will be closest to true celestial north. After that, it will recede, until around 4000 A. D., Er Rai (Gamma Cephei) will be closest to true north. By 7500 A. D., Alderamin (Alpha Cephei) will be be in that position, and in 12000 A. D. the bright star Vega, in the constellation Lyra, will be a brilliant guidepost for future navigators. Around the year 25,000 A. D., that position will move to Thuban (Alpha Draconis), which was the pole star at the time of the building of the Great Pyramid in Egypt. Before returning to Polaris, the circle will also pass Kochab (Beta Ursae Minoris), one of the two bright stars on the opposite end of the Little Dipper. Beta Ursae Minoris and nearby Gamma Ursae Minoris are often called the Guardians of the Pole, and some astronomers believe that around the time of Homer Beta-Ursae Minoris may have been regarded as the actual Pole Star.

As the direction of the Earth's axis sweeps in this 25,800 year cycle, the "precessional circle" or "Platonic year", its path will pass a number of other stars which have not been officially considered for the honor of being future pole stars. As the pole recedes from Gamma Cephei, it will pass Pi Cephei, Xi Cephei, and Nu Cephei, finally reaching bright Alpha Cephei. Before reaching Vega, the pole will pass Alpha Cygni and Delta Cygni. Receding from Vega, it will pass dim stars such as Iota Herculis and Tau Herculis, Theta Draconis, and Iota Draconis on the way to Alpha-Draconis. So as the pole recedes from Vega, navigators of the distant future may find themselves in the same situation as travelers in the southern hemisphere in the present.

Navigators and travelers in the southern hemisphere do not have a bright star close to the celestial south pole. The closest is Sigma Octantis,which is a dim 5.46 magnitude star (the higher the magnitude, the dimmer the star). Sigma Octantis was closest to the pole in 1900 A. D., and is now over a degree away. Southern travelers thus use a different set of celestial guides. A line through the two bright stars Gamma Crucis and Acrux (Alpha Crucis) to the Small Magellanic Cloud, an irregular galaxy which appears as a misty patch in the southern sky, passes right through the celestial south pole. A second line from the bright star Canopus (Alpha Puppis) meeting the first perpendicularly marks the actual position of the pole. Millenia from now, will travelers of the northern lands and oceans have to devise a similar system, should the compass be lost and the modern navigational aides fail?

References

Allen, Richard Hinkley, Star Names: Their Lore and Meaning, Dover Publications Inc., New York, 1963

Berman, Bob, Secrets of the Night Sky, William Morrow and Co. Inc., New York, 1995

Robert Burnham, Jr., Burnham's Celestial Atlas, Dover Publications, Inc., New York, 1978

Dickenson, Terrence, Night Watch: A Practical Guide to Viewing the Universe, Firefly Books, 2006



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