Astrolabium planisphericum

Description

The principle of this versatile instrument of medieval astronomy is the projection of the fixed stars on a flat surface, in this case a pendant disc. The oldest extant astrolabes come from mid C10th Persia; the first European ones date from the C13 or C14. The instrument consists of a round disc, the 'mater', with a raised rim, the 'limb', housing the plates, 'fabulae', which are set with their lines aligning with the latitudes and the horizon zenith co ordinates. Above lies the 'rete' with the zodiac and 23 star pointers (13 outer, 10 inner) in the shape of little flames, as well as the pointer, the 'index'. On the reverse is a rule with two upright sights, the 'alidade', with which to make astronomical estimations. The reading can be obtained on the basis of the month and the day and the numbers of days in the month, the zodiac position, division by 30°, and the shadow scale.