Celestial pole,It refers to the two imaginary points where the earth's rotation axis (earth axis) extends to the celestial sphere and meets the celestial sphere at infinity.
The stars in the night sky seem to move from east to west from overhead, which makes people feel that the celestial sphere is also rotating from east to west. This is because people observe the sky with the Earth as the reference system;Since the earth is not an inertial system, but rotates continuously around the earth's axis, the illusion of the celestial sphere rotating around the earth's axis will occur relatively to observers.The "rotation" period of the celestial sphere is the same as the rotation period of the earth, which is 23 hours, 56 minutes and 04 seconds in stellar time.
The earth's axis extends to infinity and intersects the celestial sphere at two points, which is called the celestial pole.With the earth as the reference system, the observer will observe that these two points are a pair of fixed points on the celestial sphere, and the line between these two points (i.e., the earth axis) is the reference axis, the geocenter is the origin, and the equatorial plane is the reference planecelestial coordinate system Is the celestial equatorial coordinate system, and the third coordinate (declination) of the corresponding two celestial polar coordinates are respectively+90 °（North Celestial Pole）And - 90 °（Southern Celestial Pole）。
For tracking photography in astronomical photographyEquatorialYou must aim at the celestial pole before you can accurately track and photograph the celestial bodies.
There is aApparent magnitude2 classfixed star（Ursa MinorAlpha star, Chinese star namedGou Chen Yi, also known as Beichen, with a latitude of+89 ° 15 ′ 51 ″, which is close to 90 degrees, but still has a difference of 0.7 degrees), so it is calledPolaris(Polaris in English, derived from the Latin "Pole Star").
Compared with observers in the northern hemisphere, Polaris is always above the horizonHorizon angleObserverGeographic latitude, can almost be considered consistent.Therefore, Polaris has become a useful indicator for orientation when traveling in the northern hemisphere, but Polaris can only be seen in the northern hemisphere.
In fact, it is a coincidence that Polaris is near the North Celestial Pole.It is estimated that it will be called Polaris in 1000 years,Gou Chen YibecomePolarisOnly 25700 years oldprecession of the equinoxesA small part of the cycle.1000 years later, CepheusShaowei increases eight(Cepheus gamma) will become the star closest to the North Celestial Pole (Polaris).
In about 7500 AD, (Alpha Cepheus, Chinese starSky hook five）Will become Polaris;Around 10200 AD, Tianjin IV will become the Polaris;In 11600 AD, Polaris wasTianjin II；In 14000 AD,Vega(Alpha Lyra) will become Polaris, but then it will still be 6 ° away from the North Celestial Pole.
Southern Celestial Pole
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There is a dark star that can be used as the Antarctic star at 1 ° from the south celestial pole,AntarcticOfAntarctic σ。However, its apparent star magnitude is 5.5, and it is not easy to see even on a sunny night, so it is not enough to be used as the indicator of the southern celestial pole. Therefore, there is no Antarctic star, and it is also difficult to find the position of the southern celestial pole.
There are usually three ways to find the South Celestial Pole:
Southern cross
Southern crossIt is one of the constellations easy to see in the southern hemisphere,Southern crossAndCentaurAlpha (Centaur α) andAgena (Centaur β) can help us findSouthern Celestial Pole。First draw an imaginary line throughSouthern crossLong axisCross One(Southern Cross γ) andCross II(Southern cross α), and continue to extend southward.When the length is 4.5 times the distance between the two starsAlpha andAgena Draw a vertical line on the line until it intersects with the line from the Southern Cross. The intersection point isSouthern Celestial PoleLocation.
Elderly Star and Water Committee I
The second method is to use the second light in the whole dayElderly Star（Bottom seat α）AndWater Commission I（Poseidon alpha）。Use these two stars as the two vertices of an equilateral triangle, and the connection between the two stars as a giantEquilateral triangleIf the third angle is the south celestial pole, this method can only determine the approximate position.
Magellan Cloud
The third method can only be used at night when there is no moonlight and clouds, because the two methods that are not very bright in the southern hemisphereMagellan Cloud。They are respectively marked asLarge Magellanic CloudandWheat ZhelunyunThey are called "brolgas" among local aboriginesswan），Magellan CloudActually, it's close toGalaxyTwo irregularities ofCompanion galaxy, ofLocal galaxy cluster；Make an equilateral triangle according to the method of determining the celestial pole by the Elderly Star and the Water Commission, then the third angle of the hypothesis is the southern celestial pole.
Among the three methods mentioned above, the second and third method will actually determine two points on the celestial sphere (there are two ways to make an equilateral triangle on a plane according to two points). For astronomers who are not familiar with the stars, it is better to have a constellation map orConstellation diskDetermine.
