Celestial pole
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The north and south celestial poles are the two points in the sky where Earth's axis of rotation, indefinitely extended, intersects the celestial sphere. The north and south celestial poles appear permanently directly overhead to observers at Earth's North Pole and South Pole, respectively. As Earth spins on its axis, the two celestial poles remain fixed in the sky, and all other celestial points appear to rotate around them, completing one circuit per day (strictly, per sidereal day).
The celestial poles are also the poles of the celestial
An analogous concept applies to other planets: 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 (the apparent positions of the stars also change slightly because of parallax effects).[1]
Finding the north celestial pole
The north celestial pole currently is within one degree of the bright star Polaris (named from the Latin stella polaris, meaning "pole star"). This makes Polaris, colloquially known as the "North Star", 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 (though it can, of course, only be seen from locations in the Northern Hemisphere).
Polaris is near the north celestial pole for only a small fraction of the 25,700-year precession cycle. It will remain a good approximation for about 1,000 years, by which time the pole will have moved closer to Alrai (
To find Polaris, from a point in the Northern Hemisphere, face north and locate the
Finding the south celestial pole
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, more than one degree away from the pole, but with a magnitude of 5.5 it is barely visible on a clear night.
Method one: The Southern Cross
The south celestial pole can be located from the
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. But where should the third corner go? It could be on either side of the line connecting Achernar and Canopus, and the wrong side will not lead to the pole. To find the correct side, imagine that Archernar and Canopus are both points on the circumference of a circle. The third corner of the equilateral triangle will also be on this circle. The corner should be placed clockwise from Achernar and anticlockwise from Canopus. The third imaginary corner will be the south celestial pole. If the opposite is done, the point will land in the middle of Eridanus, which isn't at the pole. If Canopus has not yet risen, the second-magnitude Alpha Pavonis can also be used to form the triangle with Achernar and the pole. In this case, go anticlockwise from Achernar instead of clockwise, form the triangle with Canopus, and the third point, the pole, will reveal itself. The wrong way will lead to Aquarius, which is very far away from the celestial pole.
Method three: The Magellanic Clouds
The third method is best for moonless and clear nights, as it uses
Method four: Sirius and Canopus
A line from Sirius, the brightest star in the sky, through Canopus, the second-brightest, continued for the same distance lands within a couple of degrees of the pole. In other words, Canopus is halfway between Sirius and the pole.
See also
- Celestial sphere
- Celestial equator
- Circumpolar star
- Orbital pole
- Polaris
- Pole star
- Poles of astronomical bodies
References
- ^ Jim Kaler Professor Emeritus of Astronomy, University of Illinois. "Measuring the sky A quick guide to the Celestial Sphere". Retrieved 10 March 2014.
- ^ Loyola University Chicago. "Earth-Sky Relationships and the Celestial Sphere" (PDF). Archived (PDF) from the original on 2014-01-10. Retrieved 10 March 2014.
- ^ "Swirling Star Trails Over Yepun". Picture of the Week. ESO. Retrieved 11 January 2013.