Horizontal coordinates

A type of celestial coordinate system, which is a coordinate system that shows the position of a celestial body on the celestial sphere. The position of a celestial body is expressed by the angle a (altitude) measured perpendicularly from the horizon, and the azimuth angle Z (the azimuth angle is often measured eastward from the north point N). To explain in detail with a diagram , the reference point for horizontal coordinates is the vertical line ZO (the direction of the Earth's gravity), the plane that is perpendicular to it (intersects at a right angle) and passes through the observer O is the earth's horizon, and the great circle where this plane intersects with the celestial sphere is the horizon. The intersection point of the vertical line and the celestial sphere is the zenith Z, and the point P where the extension of the Earth's axis of rotation intersects with the celestial sphere is the celestial north pole. The great circle that passes through the zenith and the celestial north pole is the meridian, and the horizontal coordinate system is a coordinate system based on the horizon and meridian. The great circle that passes from Z through the celestial body S is called the azimuth circle. Let's call the intersection point of the azimuth circle and the horizon H. The altitude a of a celestial body is the angle ∠SOH measured along the azimuth circle, and the azimuth angle Z is usually measured as ∠NOH. Horizontal coordinates are used in surveying, celestial navigation, geodesy, etc.

[Naoaki Owaki]

"Calculating Sunrise and Sunset: How to Calculate the Times of Rising and Setting of Celestial Bodies" by Nagasawa Takumi (1999, Chijin Shokan)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

天球上の天体の位置を表す座標、すなわち天球座標の一種。天体の位置を、地平線から垂直に測った角a（高度）と、方位角Zで表す（方位角は北点Nを基点とし東回りに測ることが多い）。詳しく図で説明すると、地平座標で基準となるのは鉛直線ZO（地球重力の方向）で、これに垂直（直角に交わる）で観測者Oを通る平面が地平面、この面と天球と交わる大円が地平線である。また鉛直線と天球との交点が天頂Z、地球自転軸の延長が天球と交わる点Pが天の北極である。天頂と天の北極を通る大円が子午線で、地平座標系は、地平線と子午線とを基準とした座標系である。Zから天体Sを通る大円を方位圏という。方位圏と地平線との交点をHとしよう。天体の高度aとは方位圏に沿って測った角∠SOH、方位角Zは普通∠NOHで測る。地平座標は、測量、天文航法、測地学などで用いられる。

［大脇直明］

『長沢工著『日の出・日の入りの計算――天体の出没時刻の求め方』（1999・地人書館）』

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