thumb|upright=1.2|A small orrery showing Earth and the inner planets
An orrery () is a mechanical model of the Solar System that illustrates or predicts the relative positions and motions of the planets and moons, usually according to the heliocentric model. It may also represent the relative sizes of these bodies; however, since accurate scaling is often not practical due to the actual large ratio differences, it may use a scaled-down approximation. Mechanical planetary models are known to have existed since the Ancient Greeks, and are known by various names, but the term orrery is derived from a device produced by John Rowley and named for his patron Charles Boyle, 4th Earl of Orrery.
Orreries are typically driven by a clockwork mechanism with a globe representing the Sun at the centre, and with a planet at the end of each of a series of arms.
History
Ancient
thumb|[[Antikythera mechanism, main fragment, 205 to 87 BC]]
thumb|upright|Carlo G Croce, reconstruction of [[Astrarium of Giovanni Dondi dell'Orologio|Dondi's Astrarium, originally built between 1348 and 1364 in Padua]]
Discovered in 1901 in a shipwreck off the Greek island of Antikythera in the Mediterranean Sea, the Antikythera mechanism has been reconstructed as exhibiting the diurnal motions of the Sun, Moon, and the five planets known to the ancient Greeks. The geocentric mechanism has been dated between 205 and 87 BC and is considered one of the first orreries, likely used as a mechanical calculator to calculate astronomical positions.
Cicero, the Roman philosopher and politician writing in the first century BC, made reference to planetary mechanical models in his writings. According to Cicero, the Greek polymaths Thales and Posidonius each constructed a device modelling the motion of celestial bodies.
Medieval
thumb|upright|Astronomical clock (Venus-Mercury side) by Eberhard Baldewein, 1563–1568. Exhibit in the [[Mathematisch-Physikalischer Salon of Dresden, Germany.]]
thumb|The Orrery inside the Sphaera Copernicana, designed by Joseph of Gottorp and built by Andreas Bösch, 1653
In 1348, Giovanni Dondi built the first known clock driven mechanism of the system. It displays the ecliptic position of the Moon, Sun, Mercury, Venus, Mars, Jupiter and Saturn according to the complicated geocentric Ptolemaic planetary theories. The clock itself is lost, but Dondi left a complete description of its astronomic gear trains.
Early Modern
At the court of William IV, Landgrave of Hesse-Kassel two complicated astronomic clocks were built in 1561 and 1563–1568. These use four sides to show the ecliptical positions of the Sun, Mercury, Venus, Mars, Jupiter, Saturn, the Moon, Sun and Dragon (Nodes of the Moon) according to Ptolemy, a calendar, the sunrise and sunset, and an automated celestial sphere with an animated Sun symbol which, for the first time on a celestial globe, shows the real position of the Sun, including the equation of time. The clocks are now on display in Kassel at the Astronomisch-Physikalisches Kabinett and in Dresden at the Mathematisch-Physikalischer Salon.
In De revolutionibus orbium coelestium, published in Nuremberg in 1543, Nicolaus Copernicus challenged the Western teaching of a geocentric universe in which the Sun revolved daily around the Earth. He observed that some Greek philosophers such as Aristarchus of Samos had proposed a heliocentric universe. This simplified the apparent epicyclic motions of the planets, making it feasible to represent the planets' paths as simple circles. This could be modeled by the use of gears. Tycho Brahe's improved instruments made precise observations of the skies (1576–1601), and from these Johannes Kepler (1621) deduced that planets orbited the Sun in ellipses. In 1687 Isaac Newton explained the cause of elliptic motion in his theory of gravitation.
As late as 1650, P. Schirleus built a geocentric planetarium with the Sun as a planet, and with Mercury and Venus revolving around the Sun as its moons.
The eponymous orrery
There is an orrery built by clock makers George Graham and Thomas Tompion dated in the History of Science Museum, Oxford. 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 (in County Cork, Ireland), from which the device took its name in English. The name orrery may be a pun on the term , It was produced by John Rowley. The plaque on it reads:
:Orrery invented by Graham 1700
: Improved by Rowley, and presented by him to John Earl of Orrery, after whom it was named at the suggestion of Richard Steele.
The attribution of the naming to Richard Steele is spurious, as Steele himself attributes the naming to Rowley in his 1713 article popularizing the orrery.