From TheBestLinks.com
An orrery is a mechanical device that illustrates the relative positions and motions of the planets and moons in the solar system in heliocentric model. They are typically driven by a large clockwork mechanism with a globe representing the Sun at the center, and with a planet at the end of each of the arms.
A normal mechanical clock could be used to produce an extremely simple orrery with the Sun in the center, Earth on the minute hand and Jupiter on the hour hand; Earth would make 12 revolutions around the Sun for every 1 revolution of Jupiter. Note however that Jupiter's actual year is 11.86 Earth years long, so this particular example would lose accuracy rapidly. A real orrery would be more accurate and include more planets, and would perhaps make the planets rotate as well.
Orrerys are sometimes referred to as planetariums, although generally, planetariums are hemispherical theaters in which images of the night sky are projected onto on overhead surface. Orreries can range widely in size from hand-held to room-sized.
The first orrery was built in 1713 by George Graham. He was supported (as a patron) by Charles Boyle, 4th Earl of Orrery, and thus the name.
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