In 1900, some divers found the remains of a mechanism below the surface of the sea near the island of Antikythera, between Crete and mainland Greece. They were dated to 87 BCE.
The object was identified as a calendrical Sun and Moon mechanism. It incorporated a very sophisticated device of a differential gear assembly. The turntable was the largest gear element on the back of the mechanism.
One turn of the drive wheel introduced two separate rates of rotation at the same time. Nineteen revolutions of the drive wheel brought 19 and 235 rotations on the differential turntable. In 19 solar years, there are 235 lunar months.
