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almucantar
An almucantar (also spelled almucantarat or almacantara) is a circle on the celestial sphere parallel to the horizon. Two stars that lie on the same almucantar have the same altitude.
The term was introduced into European astronomy by monastic astronomer Hermann Contractus of Reichenau, Latinized from the Arabic word ' ("the almucantar, sundial", plural: '), derived from ' ("arch, bridge")
==Almucantar staff==
An almucantar staff is an instrument chiefly used to determine the time of sunrise and sunset, in order to find the amplitude and consequently the variations of the compass. Usually made of pear tree or boxwood, with an arch of 15° to 30°,〔May, William Edward, ''A History of Marine Navigation'', G. T. Foulis & Co. Ltd., Henley-on-Thames, Oxfordshire, 1973, ISBN 0-85429-143-1〕 it is an example of a backstaff.
The sun casts that shadow of a vane (B in the image at the right) on a ''horizon vane'' (A). The horizon vane has a slit or hole to allow the observer to see the horizon in the distance. The observer aligns the horizon and shadow so they show at the same point on the horizon vane and sets the ''sighting vane'' (C) to align his line of sight with the horizon. The altitude of the sun is the angle between the shadow vane and the sighting vane.
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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