Spherical astrolabe.jpg
An astrolabe (Greek: ἀστρολάβος astrolabos; Arabic: ٱلأَسْطُرلابal-Asturlāb) is an elaborate inclinometer, historically used by astronomers and navigators to measure the inclined position in the sky of a celestial body, day or night. The word astrolabe means "the one that catches the heavenly bodies." It can thus be used to identify stars or planets, to determine local latitude given local time (and vice versa), to survey, or to triangulate. It was used in classical antiquity, the Islamic Golden Age, the European Middle Ages, and the Renaissance for all these purposes.

The astrolabe's importance not only comes from the early development of astronomy, but is also effective for determining latitude on land or calm seas. Although it is less reliable on the heaving deck of a ship in rough seas, the mariner's astrolabe was developed to solve that problem.

OED gives the translation "star-taker" for the English word "astrolabe" and traces it through medieval Latin to the Greek word astrolabos, from astron "star" and lambanein "to take". In the medieval Islamic world the Arabic word "al-Asturlāb" (i.e. astrolabe) was given various etymologies. In Arabic texts, the word is translated as "ākhdhu al-Nujuum" (Arabic: آخِذُ ٱلنُّجُومْ‎, lit. "star-taker"), a direct translation of the Greek word.

Al-Biruni quotes and criticizes medieval scientist Hamzah al-Isfahani who stated: "asturlab is an arabization of this Persian phrase" (sitara yab, meaning "taker of the stars"). In medieval Islamic sources, there is also a folk etymology of the word as "lines of lab", where "Lab" refers to a certain son of Idris (Enoch). This etymology is mentioned by a 10th-century scientist named al-Qummi but rejected by al-Khwarizmi.

An early astrolabe was invented in the Hellenistic civilization by Apollonius of Perga between 220 and 150 BC, often attributed to Hipparchus. The astrolabe was a marriage of the planisphere and dioptra, effectively an analog calculator capable of working out several different kinds of problems in spherical astronomy. Theon of Alexandria (c. 335 – c. 405) wrote a detailed treatise on the astrolabe, and Lewis argues that Ptolemy used an astrolabe to make the astronomical observations recorded in the Tetrabiblos. Some historians attribute the astrolabe's invention to Hypatia, the daughter of Theon of Alexandria, noting that her student Synesius credits her for the invention in his letters.

Astrolabes continued in use in the Greek-speaking world throughout the Byzantine period. About 550 AD, Christian philosopher John Philoponus wrote a treatise on the astrolabe in Greek, which is the earliest extant treatise on the instrument. Mesopotamian bishop Severus Sebokht also wrote a treatise on the astrolabe in the Syriac language in the mid-7th century. Sebokht refers to the astrolabe as being made of brass in the introduction of his treatise, indicating that metal astrolabes were known in the Christian East well before they were developed in the Islamic world or in the Latin West.

This page was last edited on 16 March 2018, at 19:16.
Reference: under CC BY-SA license.

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