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The sine quadrant (Arabic: Rub‘ul mujayyab, الربع المجيب) was a type of quadrant used by medieval Arabic astronomers. It is also known as a "sinecal quadrant" in the English-speaking world. The instrument could be used to measure celestial angles, to tell time, to find directions, or to determine the apparent positions of any celestial object for any time. The name is derived from the Arabic "rub‘‘‘" meaning a quarter and "mujayyab" meaning marked with sine.〔http://cosmolabe.tripod.com/id1.html〕 It was described, according to King, by Muhammad ibn Mūsā al-Khwārizmī in 9th century Baghdad.〔David A. King, "Islamic Astronomy", in Christopher Walker (1999), ed., Astronomy before the telescope, p. 167-168. British Museum Press. ISBN 0-7141-2733-7.〕 ==Description== The instrument is a quarter of a circle made of wood or metal (usually brass) divided on its arc side into 90 equal parts or degrees. The 90 divisions are gathered in 18 groups of five degrees each and are generally numbered both ways from the ends of the arc. That is, one set of numbers begins at the left end of the arc and goes to 90 at the right end while the other set the zero is at the right and the 90 is at the left. This double numbering enables the instrument to measure either celestial altitude or zenith distance or both simultaneously. At the apex where the two graduated straight sides of the grid pattern meet in a right angle there is a pin hole with a cord in it and a small weight on the free end with a small bead that slides along the cord. The cord is called “Khait” and is used as a plumb line when measuring celestial altitudes. It is also used as the indicator of angles when doing calculations with the instrument. The sliding bead facilitates trigonometric calculations with the instrument. Traditionally the line from the beginning of the arc to the apex is called “Jaibs” and the line from the end of the arc to the apex is called “Jaib tamams”. Both jaibs and jaib tamams are divided into 60 equal units and the sixty parallel lines to the jaibs are called sitheeniys or” sixtys “ and the sixty parallel lines to the jaib tamams are “juyoobul mabsootah”. The reason for sixty divisions along the Jaibs and Jaib Tamams is that the instrument uses the Sexagesimal number system. That is it is graduated to the number base 60 and not to the base 10 or decimal system that we presently use. Time, angular measurement and geographical coordinate measurements are about the only hold overs from the Sumerian/Babylonian number system that are still in current use. Like the arc, the Jaibs and Jaib tamams have their sixty divisions gathered into groups of five that are numbered in both directions to and from the apex. The double numbering of the arc means that the “Jaibs” and “Jaib tamams” labels are relative to the measurement being taken or to the calculation being performed at the time and the terms are not attached to one or the other of the graduated scales on the instrument. 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「Sine quadrant」の詳細全文を読む スポンサード リンク
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