This is a remarkable device which simulates automatically the complex movements of the sun, the moon and the planets known in antiquity, as they appear from any point on earth. It constitutes the most complex mechanism of antiquity. It was a remarkable invention as attested by the relevant reports of ancient sources. According to Cicero (1st c. A.D.), it was the only piece of the spoils that general Marcellus kept for himself after the fall of Syracuse.
With every manual turn of the sphere, the central inner disc rotates isochronously through suitable cogwheels, having a transmission ratio 1:365,25 (3651/4), which was the assumed year duration at the time. Going further, the differing periodicity of the above celestial bodies is achieved with astonishing accuracy through a complex system of cogwheels, as well as the acceleration, deceleration and reverse movement of the disc. The sun and its inner planets (Mercury and Aphrodite-Venus), according to the geocentric system, are equipped with a suitable arrangement of cogwheels and jointed bars with slots, in a way simulating their apparent yearly movements (angular velocities and bidirectional movements). The outer planets (Mars, Jupiter and Cronus-Saturn), on the contrary, carry a separate pair of eccentric cogwheels with a brilliant pin and slot system for their periodic and bidirectional movements. The moon carries the cog-wheel arrangement corresponding to the mechanism of Antikythera, in order to effect its periodicity and angular velocity. Its phases are shown through its simultaneous rotation around its axis, via a pair of vertical cogwheels and a leather string inside its supporting tubelet.
