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The horizontal coordinate system is a celestial coordinate system that uses the observer's local horizon as the fundamental plane. It is expressed in terms of altitude (or elevation) angle and azimuth. ==Definition== This coordinate system divides the sky into the upper hemisphere where objects are visible, and the lower hemisphere where objects cannot be seen since the Earth obstructs vision. The great circle separating hemispheres is called celestial horizon or rational horizon. The pole of the upper hemisphere is called the zenith. The pole of the lower hemisphere is called the nadir. There are two independent horizontal angular coordinates: * Altitude (Alt), sometimes referred to as elevation, is the angle between the object and the observer's local horizon. For visible objects it is an angle between 0 degrees to 90 degrees. * * Alternatively, zenith distance, the distance from directly overhead (i.e. the zenith) may be used instead of altitude. The zenith distance is the complement of altitude (i.e. 90°-altitude). * Azimuth (Az), that is the angle of the object around the horizon, usually measured from the north increasing towards the east. Exceptions are, for example, ESO's FITS convention where it is measured from the south increasing towards the west, or the FITS convention of the SDSS where it is measured from the south increasing towards the east. The horizontal coordinate system is sometimes also called the az/el system,〔(hawaii.edu )〕 the Alt/Az system, or the altazimuth system〔(''Encyclopædia Britannica'' entry on “horizon system” )〕 (from the name of the altazimuth mount for telescopes, whose two axes follow altitude and azimuth). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Horizontal coordinate system」の詳細全文を読む スポンサード リンク
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