"...when I seek out the massed wheeling circle of the stars, my feet no longer touch the Earth..."
Claudius Ptolemy, matematician and astronomer
An astrolabe is a device with which astronomers solved problems relating to time and the position of the sun and stars in the sky. Its main element is a two-dimensional circular stereographic projection of the hemispherical sky. The projection was most probably formalized by the Greek astronomer Hipparchus (190-120 B.C.E.), who worked on the island of Rhodes.
The astrolabe was suspended vertically and a cross-arm was used to measure the altitude above the horizon of the sun (in the day) and bright stars (at night). The rim of the astrolabe is marked off in months, days, and hours, and most astrolabes have a series of longitude-specific circular main plates each marked off with lines of constant altitudes, azimuths, declinations, and right ascensions. Fitting over the plate is a cutaway fretwork (a "rete") that delineates that portion of the celestial sphere that can be seen above the horizon at any specific time at a specific latitude. The rete contains pointers that mark the positions of about twelve of the brightest stars.
By noting the elevation of the sun, or these bright stars, the traveler can tell the time of day or night. By noting how well the observed star positions correspond to a specific plate, the travelers can estimate their latitude. The stellar positions also enable the accurate establishment of north on the horizon.
Claudius Ptolemy (c. 85-165 C.E.) wrote about the stereographic projection and probably owned an astrolabe. The astrolabe was popular in the Islamic world because it enabled Muslims to ascertain prayer times and the direction of Mecca. The oldest existing instruments date from the tenth century C.E.
