Part of a portable sundial-calendar, Byzantine, c 520 AD.
© Science Museum / Science & Society Picture Library
Detail of the moon wheel. This fragment is from the remains of a portable sundial with calendrical gearing. The gear wheels are the second oldest surviving examples. They provide important evidence linking the earliest known geared mechanism, a Hellenistic calendrical instrument (dated to the 1st century BC), with the earliest gearing from the Islamic world (dated to c 1000 AD), thereby indicating the path of transmision of such technology.