astrolabe (1)

An ancient instrument for showing the positions of the Sun and bright stars at any time and date. Its invention is credited to Greek astronomers who worked in the second century BC. The basic astrolabe consists of a circular star map (the "tablet" or "tympan") with a graticule (the "rete") over the top, the two being joined at their common centre so that the rete can rotate over the tablet. Typically, it would be made of brass. Various engraved scales enable the positions of the stars and the Sun to be displayed for any time and date. There may be other scales giving further information. Astrolabes were often fitted with a sight on a movable arm so that they could be used to estimate the altitudes of stars, for navigational purposes, for example. However, the use of any particular astrolabe is restricted to places within the range of latitude for which it is constructed.