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The mariner's astrolabe, also called sea astrolabe, was an inclinometer used to determine the latitude of a ship at sea by measuring the sun's noon altitude (declination) or the meridian altitude of a star of known declination. Not an astrolabe proper, the mariner's astrolabe was rather a graduated circle with an alidade used to measure vertical angles. They were designed to allow for their use on boats in rough water and/or in heavy winds, which astrolabes are ill equipped to handle. In the sixteenth century, the instrument was also called a ring.〔 The Corpus Christi Museum of Science and History in Texas, United States, contains the world's oldest mariner's astrolabe with a confirmed date of 1554, salvaged from the wreck of the ''San Esteban''.〔(【引用サイトリンク】title=Astrolabe )〕 ==History== Many dates can be found for the appearance of the first mariner's astrolabes. The earliest date, 1295, is offered by the Majorcan astronomer Ramon Llull.〔''The Oxford Companion to Ships and the Sea'', Peter Kemp ed., 1976 ISBN 0-586-08308-1〕 Later dates centre around the late 15th century, with Samuel Purchas claiming that it was adapted for marine navigation by Martin Behaim,〔 a mariner not considered a trustworthy source by some historians. In any event, the instrument was certainly known by the end of the 15th century. Nevertheless, the creation and perfectioning of the mariner's astrolabe is attributed to Portuguese navigators during the beginning of Portuguese discoveries.〔http://www.ancruzeiros.pt/anci-astrolabio.html〕 The earliest known description of how to make and use a sea astrolabe comes from Martín Cortés de Albacar’s〔See :es:Martín Cortés de Albacar for the Spanish Wikipedia biography〕 ''Arte de Navegar'' published in 1551,〔Swanick, Lois Ann. ''An Analysis Of Navigational Instruments In The Age Of Exploration: 15th Century To Mid-17th century'', MA Thesis, Texas A&M University, December 2005〕 although the basic principle is the same as that of the archipendulum used in constructing the Egyptian pyramids. There is strong evidence that the mariner's astrolabe was derived directly from the planispheric astrolabe, as the earliest examples retain some of the markings (e.g. ''umbra recta'' and ''umbra versa'') of the prior device without having the same components.〔Daumas, Maurice, ''Scientific Instruments of the Seventeenth and Eighteenth Centuries and Their Makers'', Portman Books, London 1989 ISBN 978-0-7134-0727-3〕 The mariner's astrolabe would have replaced or complemented instruments such as the cross staff or quadrant as a navigator's instrument. The mariner's astrolabe was used until the middle or, at the latest, the end of the 17th century.〔 It was replaced by more accurate and easier-to-use instruments such as the Davis quadrant. Although their heavy brass construction permits their longevity in marine environments,〔 mariner's astrolabes are very rare today. In 1979, only 35 were known to exist.〔Turner, Gerard L'E. ''Antique Scientific Instruments'', Blandford Press Ltd. 1980 ISBN 0-7137-1068-3〕 while 65 were known by 1988.〔A. Stimson, ''The Mariner's Astrolabe. A survey of known, surviving sea astrolabes.'' Hes & De Graff Pub B V (March 1988) ISBN 978-90-6194-017-3〕 The biggest collection remains in museums in Portugal.〔 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「mariner's astrolabe」の詳細全文を読む スポンサード リンク
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