Celestial pole

The north and south celestial poles and their relation to axis of rotation, plane of orbit and axial tilt.
Diagram of the path of the celestial north pole around the ecliptic north pole. The beginning of the four "astrological ages" of the historical period are marked with their zodiac symbols: the Age of Taurus from the Chalcolithic to the Early Bronze Age, the Age of Aries from the Middle Bronze Age to Classical Antiquity, the Age of Pisces from Late Antiquity to the present, and the Age of Aquarius beginning in the mid 3rd millennium.

The north and south celestial poles are the two imaginary points in the sky where the Earth's axis of rotation, indefinitely extended, 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 respectively.

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 because of 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 nearly 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 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, with a period of about 25,700 years. The Earth's axis is also subject to other complex motions which cause the celestial poles to shift slightly over cycles of varying lengths; see nutation, polar motion and axial tilt. Finally, over very long periods the positions of the stars themselves change, because of the stars' proper motions.

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).


Finding the north celestial pole

See also Pole star.

The north celestial pole currently is within a degree of the bright star Polaris (named from the Latin stella polaris, meaning "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 can, of course, only be seen from locations in the northern hemisphere.

Polaris is near the celestial pole for only a small fraction of the 25,700-year precession cycle, and the fact that it is currently so is purely a coincidence. It will remain a good approximation for about 1,000 years, by which time the pole will have moved to be closer to Alrai (Gamma 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, but it will be about six degrees from the true north celestial pole.

To find Polaris, face north and locate the Big Dipper (Plough) and Little Dipper asterisms. 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.

Finding the south celestial pole

See also: Pole star.
Locating 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, over a 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 Southern Cross (Crux) and its two "pointer" stars α Centauri and β Centauri. Draw an imaginary line from γ Crucis to α Crucis—the two stars at the extreme ends of the long axis of the cross—and follow this line through the sky. Either go four and a half times the distance of the long axis in the direction the narrow end of the cross points, or 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 5 or 6 degrees from the south celestial pole. Very few bright stars of importance lie between Crux and the pole itself, although the constellation Musca is fairly easily recognised immediately beneath Crux.

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. These 'clouds' are actually galaxies close to our own Milky Way. Make an equilateral triangle, the third point of which is the south celestial pole.

See also


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Look at other dictionaries:

  • celestial pole — n. either of two points on the celestial sphere where the extensions of the earth s axis would intersect: see DECLINATION …   English World dictionary

  • celestial pole — noun one of two points of intersection of the Earth s axis and the celestial sphere • Syn: ↑pole • Hypernyms: ↑celestial point • Instance Hyponyms: ↑north celestial pole, ↑south celestial pole …   Useful english dictionary

  • celestial pole — dangaus polius statusas T sritis fizika atitikmenys: angl. celestial pole vok. Himmelspol, m rus. небесный полюс, m pranc. pôle céleste, m …   Fizikos terminų žodynas

  • celestial pole — Astron. each of the two points in which the extended axis of the earth cuts the celestial sphere and about which the stars seem to revolve. Also called pole. [1900 05] * * * …   Universalium

  • celestial pole — celes′tial pole′ n. astron. each of the two points in which the extended axis of the earth cuts the celestial sphere and about which the stars seem to revolve • Etymology: 1900–05 …   From formal English to slang

  • celestial pole — noun Date: 1848 either of the two points on the celestial sphere around which the diurnal rotation of the stars appears to take place …   New Collegiate Dictionary

  • celestial pole — noun An imaginary point situated where a planets axis intersects the celestial sphere …   Wiktionary

  • celestial pole — noun Astronomy the point on the celestial sphere directly above either of the earth s geographic poles, around which the stars appear to rotate …   English new terms dictionary

  • celestial pole — /səˌlɛstiəl ˈpoʊl/ (say suh.lesteeuhl pohl) noun → pole2 (def. 2) …   Australian English dictionary

  • north celestial pole — noun the celestial pole above the northern hemisphere; near Polaris • Instance Hypernyms: ↑pole, ↑celestial pole * * * noun : north pole 1 a * * * north celestial pole, the zenith of the northern end of the earth s axis from which every direction …   Useful english dictionary

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