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Shadow square
The shadow square, also known as an altitude scale,〔(【引用サイトリンク】url=http://brunelleschi.imss.fi.it/museum/esim.asp?c=100314 )〕 was an instrument used to determine the linear height of an object, in conjunction with the alidade, for angular observations. It was invented by Muhammad ibn Mūsā al-Khwārizmī in 9th-century Baghdad. Shadow squares are often found on the backs of astrolabes.
==Uses==
The main use of a shadow square is to measure the linear height of an object using its shadow. It does so by simulating the ratio between an object, generally a gnomon, and its shadow.If the sun's ray is between 0 degrees and 45 degrees the umbra versa (Vertical axis) is used, between 45 degrees and 90 degrees the umbra recta (Horizontal axis) is used and when the sun's ray is at 45 degrees its shadow falls exactly on the umbra media (y=x) 〔(【引用サイトリンク】url=http://catalogue.museogalileo.it/indepth/ShadowSquare.html )〕 It was used during the time of medieval astronomy to determine the height of, and to track the movement of celestial bodies such as the sun when more advanced measurement methods were not available. These methods can still be used today to determine the altitude, with reference to the horizon, of any visible celestial body.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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