Orrery

[[image:Glikerson_orrery.png|thumb|250px|Brass orrery by Gilkerson and Co., Tower Hill, London (c.1810), in the archives of the

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Armagh Observatory (image by Miruna Popescu).]]

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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 centre, and with a planet at the end of each of the arms.

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A normal mechanical clock could be used to produce an extremely simple orrery with the Sun in the centre, 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.

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Orreries are sometimes referred to as planetariums, although generally, planetariums are hemispherical theatres in which images of the night sky are projected onto an overhead surface. Orreries can range widely in size from hand-held to room-sized.

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The first orrery was built circa 1704 by George Graham. Graham gave the first model (or its design) to the celebrated instrument maker John Rowley of London to make a copy for Prince Eugene of Savoy. Rowley was commissioned to make another copy for his patron Charles Boyle, 4th Earl of Orrery, thus the name. It was presented to Charles' son John, later the 5th Earl.

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Joseph Wright's picture "The Orrery" (c. 1766) features a group (four men, three boys) listening to a lecture by a 'natural philosopher'—the only light in the otherwise darkened room is apparently from the centre of the brass orrery.

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Orreries are usually not built to scale. Some fixed Solar system scale models have been built and are often many kilometres in size. An innovative concept is to have people play the role of the moving planets and other Solar system objects. Such a model, called a Human Orrery, has been laid out with precision at the Armagh Observatory.

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Planet: A planet in common parlance is a large object in orbit around a star that is not a star itself. The name comes from the Greek term πλανήτης, planētēs, meaning "wanderer", as ancient astronomers noted how certain lights moved across the sky in relatio...

Moon: :For other moons in the solar system see natural satellite. For the astrological meaning of the Moon, see Solar system in astrology. For other uses see Moon (disambiguation)....

Solar system: The solar system is the retinue of objects gravitationally bound to our Sun. Traditionally, it is said to consist of nine planets and their 156 (at last count) moons; however a large number of other objects, including asteroids, meteoroids, planetoids and comets orbit the Sun along with them....