Hipparchus is credited with the invention of this instrument. Among other things, it was used to calculate the rising and setting of stars. There have been several different types. Portable types were usually 10-20 cm (4-8 in) in diameter. However, there have been both larger and smaller ones.
The early modern version consisted of two metal plates, one on top of the other. The outer of these was a star map in the form of a skeleton network, with dagger points showing the positions of prominent stars. Other lines on the plate were projected elements of an earth-centred coordinate system.
Most astrolabes required the user to position the moving part (rete) correctly in relation to the fixed plate by reference to the observed altitude of a star. The rete was rotated until the star’s marker fell against the correct altitude line (almucantar) on the plate below.