Asked to name the earliest surviving set of gear-wheels, few people will think of an astrolabe, but this instrument contains the world's oldest geared mechanism which has survived in a complete state. One side of the instrument is an astrolabe, but the other is a calendar, showing the age of the moon, the relative positions of the sun and moon, and the position of the sun in the zodiac. The calendaric scales are linked by gear wheels inside the instrument.
Astrolabe with Geared Calendar, by Muhammad b. Abi Bakr, Isfahan, 1221/2 (Inv. 48213)