astrolabe
An instrument that measures the vertical angle between a celestial body and the horizontal plane at an observer's position.
According to Synesius of Ptolemais (4th century) he made the first astrolabion: this may have been an armillary sphere (which Ptolemy however says he constructed, in Almagest V.1); or the predecessor of the planar instrument called astrolabe (also ...
On how to determine the longitude of places, he suggested that eclipses of the moon could be used to determine it, and described an instrument he called a méthéoroscope, an astrolabe modified by adding a compass.
Supported corrections of scale distortions in other maps, i.e. introduced map projections. Mentions use of gnomon and astrolabe for pole star & sun. Discounts travel stories. Included a gazeteer of know places with lat. & long.
 See also: Map, Navigation, Longitude, Latitude, Server
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