Celestial pole
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The two celestial poles are the imaginary points where the Earth's spin axis intersects the imaginary rotating sphere of stars called the celestial sphere.
At night the sky appears to drift overhead from east to west, completing a full circuit around the sky in 24 (sidereal) hours. This phenomenon is due to the spinning of the Earth on its axis. The Earth's spin axis intersects the celestial sphere at two points. These points are the celestial poles. As the Earth spins, they remain fixed in the sky, and all other points seem to rotate around them. The celestial poles are also the poles of the celestial equatorial coordinate system, meaning they have declinations of +90 degrees and -90 degrees (for the north and south celestial poles, respectively).
The north celestial pole currently has nearly the same coordinates as the bright star Polaris (which is Latin for "Pole Star"). This makes Polaris useful for navigation in the northern hemisphere: not only is it always above the North point of the horizon, but its altitude angle is always (nearly) equal to the observer's geographic latitude. Polaris, however, can only be seen from locations in the northern hemisphere.
The fact that the star Polaris is near the pole is purely a coincidence. It will remain a good approximation for say 1000 years, but, because of precession of the equinoxes, Polaris is only near the pole for a small fraction of a 25,700 year period. Actually, Polaris will remain our north star for the next 1,000 years, by which time the pole will have moved to be closer to Alrai (Delta Cephei). In about 5,500 years, the pole will have moved near the position of the star Alderamin (Alpha Cephei), and in 12,000 years, Vega (Alpha Lyrae) will become our north star.
Finding the South Celestial Pole
Method one: The Southern Cross
The Southern Cross, together with the two pointers Alpha and Beta Centauri will help to find South if you draw an imaginary line from Gamma Crux through to Alpha Crux, and follow this line through the sky. Of course, you have to know when to stop. You join the two pointer stars with a line, divide this line in half, then at right angles draw another imaginary line through the sky until it meets the line from the Southern Cross. This point is the South Celestial Pole.Method two: Canopus and Achernar
The second method uses Canopus (the second brightest star in the sky) and Achernar. Make a large equilateral triangle using these stars for two of the corners. The third imaginary corner will be the South Celestial Pole.Method three: The Magellanic Clouds
The third method is best for a moonless and cloudless night as it uses two faint 'clouds' in the southern sky. These are marked in astronomy books as Large and Small Magellanic Clouds. They are described as white birds both by westerners as swans, and by Aboriginals as brolgas. These 'clouds' are actually galaxies close to our own. Make an equilateral triangle, the third point of which is the South Celestial Pole.Sources
- This article originates from Jason Harris' Astroinfo which comes along with KStars, a Desktop Planetarium for Linux/KDE. See http://edu.kde.org/kstars/index.phtml''
- http://library.thinkquest.org/C005462/astronomy.html
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