Astrolabes

The astrolabe is a measuring instrument from ancient Greece. It was used to tell time, locate oneself, measure heights and distances, predict the positions of the sun and stars at any given date and perform more than 500 different arithmetic operations. The ones shown here were calibrated for the latitude and longitude of Ormône in Savièse (Switzerland), and have an accuracy of about 5 minutes on real time.

By night: point the alidade at a low star on the eastern or western horizon and note the angle. Transfer this star from the spider to the angle of the corresponding almucantarat. Position the ruler on the present day and read the time on the outer face of the mother. Add one hour in winter and two in summer.
By day: point the alidade at the sun and set it by its shadow behind. Raise the degrees, turn the astrolabe over to the mother’s side and position the ruler on the zodiac of the day. Then place the zodiac/ruler intersection on the almucantarat corresponding to the angle of the sun’s height. The tip of the ruler indicates the time. Also add one hour in winter and two in summer.