An ancient instrument for solving problems of location and distance, as well as time



Astrolábon órganon is the word used in Ptolemaeus Syntaxis 5.1 to describe a form of armillary sphere consisting of both fixed and rotating rings for determining the ecliptic co-ordinates of stars. Also called astrolabe were two-dimensional representations of the celestial sphere: over fixed disks with a representation of the horizon of the earth (in each case for a specific geographic latitude) and its parallels up to the zenith lies a rotating representation of the heavens (called ‘spider’ or ‘net’) with zodiac and indicators for some of the brightest stars. On the back is an indicator (‘alidade’) with visor plates for calculating the height of the stars. The astrolabe allows its user to determine the hours of day and night, the positions of stars, and the height and depth of objects on earth (mountains, buildings, wells). The oldest extant descriptions of the flat, planispheric astrolabe come from Iohannes Philoponus, 6th century AD, and (in Syrian) from Severus Sebokht, c. 660 AD. Indirect references may date back as early as Theon (4th cent.). Between the 8th and the 9th centuries the Arabs appropriated the knowledge of the astrolabe. At the end of the 10th century and again in the 12th century, they passed it on to Western Europe, where astrolabes were constructed up until the 17th cent. Today, one Byzantine astrolabe (from 1062) is extant, as are some 750 Arabic-Islamic astrolabes and approximately the same number of European astrolabes.

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