Spherical astronomy

Japanese: 球面天文学 - きゅうめんてんもんがく（英語表記）spherical astronomy

A science that deals with the apparent position (direction) and apparent movement of celestial bodies on the celestial sphere. It is a branch of positional astronomy that deals with the relative positions of celestial bodies and the Earth. To indicate the apparent position of a celestial body on the celestial sphere, the spherical coordinates θ and are used (the distance r is not necessary). This is called celestial coordinates, and is determined based on a great circle (called the main sphere). The coordinates that use the astronomical horizon as the main sphere are called horizontal coordinates. In this case, θ is called the azimuth angle and is represented by A , and is called the altitude and is represented by h . The complementary angle ζ of h is called the zenith distance. The azimuth angle starts from the south, and is positive when going west and negative when going east. The altitude is positive when above the horizon and negative when below. Light from a celestial body is refracted by the atmosphere, so in order to know the exact position of a celestial body, it is necessary to correct for this "atmospheric refraction." Coordinates that take the main sphere as the equator and the vernal equinox as the starting point are called equatorial coordinates. θ is called right ascension and is measured eastward in units of time, with 15° being one hour. is called declination, with north being positive and south being negative. The declination of the North Celestial Pole is 90°. Ecliptic coordinates are those in which the main sphere is the ecliptic. Instead of the North Pole, the north pole of the ecliptic is taken, and the starting point is also the vernal equinox, and the ecliptic longitude and ecliptic latitude are determined in the same way. Galactic coordinates, which use the mean center line of the galaxy as the main sphere, have also been established and are used in research into the structure of the Milky Way and other large universes. Observation results using these coordinates require further corrections, such as geocentric parallax, which takes into account the size of the Earth (the distance between the observer and the center of the Earth) for close celestial bodies, annual parallax due to the annual motion of the Earth for close stars, and annual aberration, which is generally due to the Earth's orbital speed.

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

天球上における天体の視位置 (方向) および視運動を扱う学問。天体と地球との位置関係を扱う位置天文学の一分野。天球上の天体の視位置を示すには，球面座標のθとを用いる (距離 r は不要) 。これを天球座標といい，一つの大円 (主圏と呼ぶ) に準拠して定める。主圏として天文学的地平線をとったものを地平座標と呼ぶ。このときθを方位角といい A で表わし，を高度といい h で表わす。また h の余角ζを天頂距離という。方位角は南を起点とし，西回りを正，東回りを負にとる。高度は地平線より上を正，下を負とする。天体からの光は大気によって屈折するので，天体の位置を精密に知るにはその「大気差」の補正が必要である。主圏を赤道にとり，春分点を起点とする座標を赤道座標と呼ぶ。θを赤経といい，15°を1時間とする時間単位で東回りにはかる。を赤緯といい，北を正，南を負とする。天の北極の赤緯は 90°となる。主圏を黄道にとったものを黄道座標という。北極の代りに黄道の北極をとり，起点は同じく春分点として，同様に黄経と黄緯が定まる。また銀河の平均中心線を主圏とする銀河座標が定められ，銀河系その他大宇宙の構造の研究に用いられる。これらの座標による観測結果については，さらに近距離の天体に関しては地球の大きさ (観測者と地球の中心の距離) を考慮した地心視差，近距離の恒星に対しては地球の年周運動による年周視差，また一般に地球の公転速度による年周光行差などの補正が必要である。

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