An ancient device for a range of purposes including timekeeping and measuring the altitudes of stars, like a simple sextant. A basic astrolabe consists of a flat base plate engraved with lines of altitude and azimuth, overlain by a cut-out mask called the rete which is rotated to replicate the nightly movement of the sky; pointers on the rete indicate the positions of prominent stars. Attached to the rear of the plate is a pivoting sight (the alidade) which can be turned to point at a chosen star. The local time can then be read off the face of the astrolabe. Different base plates are required for different latitudes.

The universal astrolabe, also known as the astrolabum catholicum, is a more sophisticated version developed in the fifteenth and sixteenth centuries that could be used at any latitude without need for different tablets. The mariner’s astrolabe was a simplified device consisting of a vertical ring that was used to measure the altitude of the Sun at noon, from which latitude could be deduced.

http://www.astrolabes.org/pages/saphea.htm Website about astrolabes.

http://www.mhs.ox.ac.uk/astrolabe/ Online guide to world’s largest collection of astrolabes.