The oldest description we have of the design and use of an astrolabe was written in the VIth century by Johannes Philoponus of Alexandria (490-566). Thereafter the instrument was steadily perfected and became increasingly widespread, first in the Arab world and then in the Christian, being used extensively in astronomy and especially navigation, until it was replaced by the theodolite and sextant at the end of the XVIIIth century.
It was a portable instrument made of copper or bronze used for computing the elevation of the Sun or a star on the horizon and for quickly solving problems of spherical astronomy. It could also be used for working out the height of mountains and towers and for topographic measures in general. It was therefore an extremely useful instrument, more for sailors and astrologists than for astronomers for whom its inaccuracy was not much help for observations.
Its origin seems to lie in the armillary sphere described by Ptolemy in the IInd century AD but it was the Arabs who perfected it and helped make it more widespread. When, towards the end of the Xth century, arab astrology began to spread across Europe, instruments for observation and astronomical navigation became known, many of which were based on ancient Greek instruments like the astrolabe.
The oldest astrolabe in Europe dates from 912 and can be found in Paris. The spread of this kind of instrument led to the rise, in Europe too, of expert craftsmen, designers of original astrolabes - the Italian and French schools were famous - but also clever imitators of old Arab models.
The brass or copper plate has a graduated limb on which rotates an alidade fitted with a vane for sighting the star one wants to measure the elevation of.