Celestial pole

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The north and south celestial poles and their relation to axis of rotation, plane of orbit and axial tilt. AxialTiltObliquity.png
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. North pole path.png
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 celestial sphere. The north and south celestial poles appear permanently directly overhead to observers at the Earth's North Pole and South Pole, respectively. As the 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).

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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). Despite their apparently fixed positions, the celestial poles in the long term do not actually 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). [1]

Finding the north celestial pole

Over the course of an evening in the Northern Hemisphere, circumpolar stars appear to circle around the north celestial pole. Polaris (within 1deg of the pole) is the nearly stationary bright star just to the right of center in this star trail photo. Star Trails Shoreline.jpg
Over the course of an evening in the Northern Hemisphere, circumpolar stars appear to circle around the north celestial pole. Polaris (within 1° of the pole) is the nearly stationary bright star just to the right of center in this star trail photo.

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 (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 the "North Star", though it will be about six degrees from the true north celestial pole.

To find Polaris, from a point in the Northern Hemisphere, 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. [2]

Finding the south celestial pole

A series of shots show the rotation of Earth's axis relative to the south celestial pole. The Magellanic Clouds and the Southern Cross are clearly visible. Near the end of the video, the Moon rises and illuminates the scene.
The south celestial pole over the Very Large Telescope Swirling Star Trails Over Yepun.jpg
The south celestial pole over the Very Large Telescope
Locating the south celestial pole Pole01-eng.jpg
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, 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 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 crossand 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. If Canopus has not yet risen, the second-magnitude Alpha Pavonis can also be used to form the triangle with Achernar and the pole.

Method three: The Magellanic Clouds

The third method is best for moonless and clear nights, as it uses two faint "clouds" in the Southern Sky. These are marked in astronomy books as the Large and Small Magellanic Clouds. These "clouds" are actually dwarf galaxies near the Milky Way. Make an equilateral triangle, the third point of which is the south celestial pole.

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

Related Research Articles

Crux Constellation in the southern celestial hemisphere

Crux is a constellation centred on four stars in the southern sky in a bright portion of the Milky Way. It is among the most easily distinguished constellations as its hallmark (asterism) stars each have an apparent visual magnitude brighter than +2.8, even though it is the smallest of all 88 modern constellations. Its name is Latin for cross, and it is dominated by a cross-shaped or kite-like asterism that is commonly known as the Southern Cross.

Carina (constellation) Constellation in the southern celestial hemisphere

Carina is a constellation in the southern sky. Its name is Latin for the hull or keel of a ship, and it was the southern foundation of the larger constellation of Argo Navis until it was divided into three pieces, the other two being Puppis, and Vela.

Octans

Octans is a faint constellation located in the deep southern sky. Its name is Latin for the eighth part of a circle, but it is named after the octant, a navigational instrument. The constellation was devised by French astronomer Nicolas Louis de Lacaille in 1752, and it remains one of the 88 modern constellations.

Right ascension Astronomical equivalent of longitude

Right ascension is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the point in question above the earth. When paired with declination, these astronomical coordinates specify the location of a point on the celestial sphere in the equatorial coordinate system.

Ecliptic coordinate system

The ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent positions and orbits of Solar System objects. Because most planets and many small Solar System bodies have orbits with only slight inclinations to the ecliptic, using it as the fundamental plane is convenient. The system's origin can be the center of either the Sun or Earth, its primary direction is towards the vernal (March) equinox, and it has a right-hand convention. It may be implemented in spherical or rectangular coordinates.

Pole may refer to:

Diurnal motion is an astronomical term referring to the apparent motion of celestial objects around Earth, or more precisely around the two celestial poles, over the course of one day. It is caused by Earth's rotation around its axis, so almost every star appears to follow a circular arc path called the diurnal circle.

Circumpolar star Star that never sets

A circumpolar star is a star, as viewed from a given latitude on Earth, that never sets below the horizon due to its apparent proximity to one of the celestial poles. Circumpolar stars are therefore visible from said location toward the nearest pole for the entire night on every night of the year.

Cardinal direction Directions of north, east, south and west

The four cardinal directions, or cardinal points, are the directions north, east, south, and west, commonly denoted by their initials N, E, S, and W. East and west are perpendicular to north and south, with east being in the clockwise direction of rotation from north and west being directly opposite east. Points between the cardinal directions form the points of the compass.

Achernar Star in the constellation Eridanus

Achernar is the primary component of the binary system designated Alpha Eridani, which is the brightest star in the constellation of Eridanus, and the ninth-brightest in the night sky. The two components are designated Alpha Eridani A and B. As determined by the Hipparcos astrometry satellite, it is approximately 139 light-years (43 pc) from the Sun.

Extraterrestrial sky Extraterrestrial view of outer space

In astronomy, an extraterrestrial sky is a view of outer space from the surface of an astronomical body other than Earth.

Pole star Visible star that is nearly aligned with Earths axis of rotation

A pole star or polar star is a star, preferably bright, nearly aligned with the axis of a rotating astronomical body.

Alpha Cephei

Alpha Cephei, officially named Alderamin, is a second magnitude star in the constellation of Cepheus near the northern pole. The star is relatively close to Earth at 49 light years (ly).

Beta Cephei

Beta Cephei is a triple star system of the third magnitude in the constellation of Cepheus. Based on parallax measurements obtained during the Hipparcos mission, it is approximately 690 light-years distant from the Sun. It is the prototype of the Beta Cephei variable stars.

Orbital pole

An orbital pole is either point at the ends of an imaginary line segment that runs through the center of an orbit and is perpendicular to the orbital plane. Projected onto the celestial sphere, orbital poles are similar in concept to celestial poles, but are based on the body's orbit instead of its equator.

True north is the direction along Earth's surface towards the geographic North Pole or True North Pole.

The poles of astronomical bodies are determined based on their axis of rotation in relation to the celestial poles of the celestial sphere. Astronomical bodies include stars, planets, dwarf planets and small Solar System bodies such as comets and minor planets, as well as natural satellites and minor-planet moons.

This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth. The field of astronomy features an extensive vocabulary and a significant amount of jargon.

Southern celestial hemisphere

The southern celestial hemisphere, also called the Southern Sky, is the southern half of the celestial sphere; that is, it lies south of the celestial equator. This arbitrary sphere, on which seemingly fixed stars form constellations, appears to rotate westward around a polar axis due to Earth's rotation.

References

  1. Jim Kaler Professor Emeritus of Astronomy, University of Illinois. "Measuring the sky A quick guide to the Celestial Sphere" . Retrieved 10 March 2014.
  2. Loyola University Chicago. "Earth-Sky Relationships and the Celestial Sphere" (PDF). Retrieved 10 March 2014.
  3. "Swirling Star Trails Over Yepun". Picture of the Week. ESO. Retrieved 11 January 2013.