The OrreryAn orrery is not strictly speaking a clock, however it might be considered a distant cousin!
What is an Orrery?An orrery is a mechanical device for tracking the relative positions of the sun and the planets. Some more advanced models also show the rotation of the various moons about their respective planets. Orreries are usually "heliocentric" - that is, the sun sits in the middle and the other planets rotate about it on the end of arms that extend outwards. The mechanism could either be run by clockwork or hand-turned.
Early orreries showed only the first six planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn.
The different celestial bodies are often represented by spheres of different sizes according to their actual size. However the size in the model - as well as the distances between the planets - is not to scale. In reality the variation between measurements is such that a true scale model would be impractical. Another simplification is that the planets in an orrery normally move in a circular orbit whereas the actual orbits are elliptical.
Who Invented the Orrery?The orrery is named after Charles Boyle who was the 4th Earl of Orrery. In about 1712 Boyle commissioned John Rowley to make a copy of the first such device which had been created some years earlier by George Graham.
Nowadays, of course, we have computer simulations to model the heavens far more accurately than the clockwork models ever did. However collectors still like to buy antique orreries and there are still a few mechanical orrery makers around.