Tuesday, May 26, 2009

Celestial Poles

The north and south celestial poles are the two imaginary points in the sky where the Earth's axis of rotation, intersects the imaginary rotating sphere of stars called the celestial sphere. The north and south celestial poles appear directly overhead to an observer at the Earth's North Pole and South Pole.

At night the stars appear to drift overhead from east to west, completing a full circuit around the sky in 24 (sidereal) hours. Of course, exactly the same motion occurs during the day, except that the stars are not visible due to the sun's glare. This apparent motion is due to the spinning of the Earth on its axis. As the Earth spins, the celestial poles 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. The celestial poles do not remain permanently fixed against the background of the stars. Because of a phenomenon known as the precession of the equinoxes, the poles trace out circles on the celestial sphere. The Earth's axis is also subject to other complex motions which cause the celestial poles to shift slightly over cycles of varying lengths. Over very long periods the positions of the stars themselves change, due to the stars proper motions.
A planet's celestial poles are the points in the sky where the projection of the planet's axis of rotation intersects the celestial sphere. These points vary because different planets axes are oriented differently and the apparent positions of the stars also change slightly due to parallax effects.
The north celestial pole has nearly the same coordinates as the star Polaris (also called the "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 nearly equal to the observer's geographic latitude. Polaris can only be seen from in the northern hemisphere.
To find Polaris, face north and locate the Big Dipper (Plough) and Little Dipper constellations. Looking at the "cup" part of the Big Dipper, imagine that the two stars at the outside edge of the cup form a line pointing upward out of the cup. This line points directly at the star at the tip of the Little Dipper's handle. That star is Polaris, the North Star.
Southern Cross
The south celestial pole is visible only from the southern hemisphere. It lies in the dim constellation Octans, the Octant. Sigma Octantis is identified as the south pole star, over a degree away from the pole, but with a magnitude of 5.5 it is barely visible on a clear night. The south celestial pole can be located from the Southern Cross (Crux) and its two "pointer" stars Centauri and Centauri. Very few bright stars of importance lie between Crux and the pole itself, although the constellation Musca is fairly easily recognised. 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.
For a moonless and cloudless night you can use two faint clouds in the southern sky. These are marked in astronomy books as Large and Small Magellanic Clouds. These clouds are actually galaxies close to our own Milky Way. They make an equilateral triangle, the third point of which is the south celestial pole.