The astrolabe is a model of earth and heaven. It follows the Ptolemaic view of the world: The earth is fixed in the centre, and the heaven rotates around the earth.
The main problem of the "Astrolabium planisphaerium" is the representation of the celestial sphere on a plane. This is done with stereographic projection from the southern pole onto a perpendicular plane to the celestial axis, e.g. the plane of the equator. The classical astrolabe consists of a round disk with a rim (limbus) and a suspension device. Inside the rim (mater) are several plates with horizontal co-ordinates for different geogr. latitudes. Above these plates rotates a celestial map (rete), in stereographic projection too, with star-pointers and the eccentric ecliptic circle. The back shows circular scales of degrees, calendar and ecliptic, further a shadow square for terrestrial measurements and often the diagram of temporal hours. Above rotates a ruler (alhidade) with diopters for altitude-measurements. The main purpose of the astrolabe is the finding of the hour. The altitude of the sun or a star is measured. By revolving the rete, the sun-position or the star-pointer is set to the corresponding altitude-circle of the plate below. The rim of the instrument is divided in 2 x 12 hours (western astrolabes), and there we read true local time. instruments
History of the astrolabe
The astrolabe has been known since late antiquity. It is an instrument for astronomical and astrological calculations and demonstrations.
The principles of the astrolabe are known since Apollonius (ca. 200 BC) and Hipparchos (ca. 150 BC). In the 6th/7th century AD we find the first descriptions of construction and use of the instrument by Johannes Philoponos and Severus Sebokth. May be, the astrolabe was known by the most famous astronomer of antiquity, by Ptolemy (ca. 150 AD), but we do not know.
The oldest existing astrolabes date from the 10th century and are produced by Persian astronomers. In Middle Ages the instrument was much developed by Islamic scientists and reached Latin Europe in the 11th century through Spain. Splendid examples were constructed in Renaissance Europe (Arsenius, Habermel). Even after the Copernican revolution the astrolabe remained in use right up into the 18th century, in Islamic culture for another hundred years or even longer.