appeared around that time in the Greco-Roman world ( we speak of Ptolemy as “ Greek ,” but that ’ s only his origin , as he lived in Egypt which was then a part of the Roman Empire ). The name of the instrument comes from Greek ἄστρον ( ástron , “ star ”) and λαμβάνω ( lambánō , “ I take ”), meaning “ I take stars ,” as it is an early form of planispheric star map . The instrument gradually evolved into its “ modern ” form thanks to the Arabs and Muslims , who came upon it first through commercial exchanges with the Roman Empire , but later through their conquest of not only Egypt , but all of North Africa and even a good part of modern Spain after crossing the Gibraltar Strait . While the Western Roman Empire collapsed in 476 , it had been split in two almost 200 years prior to that , and the Eastern Roman Empire lived on until 1453 ; it was a major gateway for Greco- Roman science into the Islamic world . But back to the instrument . It is composed of a main plate , or mater ( Latin for “ mother ”; also called umm in Arabic , of same meaning ), which is usually recessed and surrounded by the limb . In the recess are housed interchangeable plates called tympans or climates , which are engraved with a set of lines corresponding to a given latitude — more on that soon . Most astrolabes are fitted with four tympans , covering eight latitudes on their front and back sides , or seven sides for the climates plus one side inscribed with other bits of information . On the front of European astrolabes , but rarely of Islamic ones , we find a movable ruler , which may or may not be inscribed with graduations , that can pivot around a central pin . The back of the astrolabe is inscribed with an ecliptic circle as well as a calendar circle , and most often also contains a shadow square and sometimes other information . On that side , the central pin holds a movable alidade ( from Arabic terms meaning “ arm ”) bearing pinnules , vanes through which one may aim at the Sun or a star . ( WARNING : Never look at the Sun directly , either through an astrolabe or a telescope ! It may permanently damage your eyes and turn you blind . To point the Sun with an astrolabe , use the alidade instead to project a shadow and align it with the bottom pinnule .) The set of lines engraved on the tympans are the almucantar ( from Arabic terms for “ arch ”), which are parallel to the horizon and are thus altitude lines , and the azimuth lines which cross each other at the local zenith , the point above the observer ’ s head . The final piece of the instrument , placed on the front of the instrument , between the ruler and the tympans , is the rete ( Latin for “ network ”; Arabic ‘ ankabūt , “ spider ”, hence the common French word “ araignée ” of same meaning ).
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