An astrarium, also called a planetarium, is the mechanical representation of the cyclic nature of astronomical objects in one timepiece. It is an astronomical clock.

The first astraria were mechanical devices. Archimedes is said to have used a primitive version that could predict the positions of the sun, the moon, and the planets. On May 17, 1902, an archeologist named Valerios Stais discovered a lump of oxidated material with in it a mechanism with cogwheels that had been recovered from a shipwreck near Antikythera. This mechanism, known as the Antikythera mechanism, was recently redated between [150 to 100 BC][1]. Extensive study of the fragments, using X-rays, have revealed enough details (gears, pinions, crank) to enable researchers to build partial replicas [2], [3][4]. Engraved on the major gears are the names of the planets (such as Venus), which leaves little doubt as to the intended use of the mechanism.

However, before the end of the Roman Empire, the know-how and science behind this piece of clockwork would be lost.