Orrery
Mechanical model illustrating planetary motions, named after the Earl of Orrery.
An orrery is a clockwork model of the Solar System, usually built around the heliocentric idea, that shows or predicts where planets and moons are and how they move. It might also try to show their relative sizes, but because the real differences in size are so huge, accurate scaling is rarely practical. The model is typically driven by gears, with a globe for the Sun in the center and each planet mounted at the end of an arm.
The history of such mechanical models goes back to the Ancient Greeks. The Antikythera mechanism, found in a 1901 shipwreck off the Greek island of Antikythera, has been reconstructed to show the daily motions of the Sun, Moon, and the five planets known to the ancient Greeks. This geocentric device, dated between 205 and 87 BC, is considered an early orrery, likely used as a mechanical calculator for astronomy. The Roman writer Cicero, in the first century BC, mentioned planetary models built by the Greek scholars Thales and Posidonius.
In 1348, Giovanni Dondi created the first known clock-driven model of the system. It showed the ecliptic positions of the Moon, Sun, Mercury, Venus, Mars, Jupiter, and Saturn, based on the complicated geocentric Ptolemaic theories. The clock itself is lost, but Dondi left a full description of its gear trains.
At the court of William IV, Landgrave of Hesse-Kassel, two complex astronomical clocks were built in 1561 and 1563–1568. These clocks have four sides, displaying the ecliptic positions of the Sun, Moon, and planets according to Ptolemy, along with a calendar, sunrise and sunset times, and an automated celestial sphere. That sphere, for the first time, showed the Sun’s real position, including the equation of time. The clocks are now in Kassel and Dresden.
In 1543, Nicolaus Copernicus’s *De revolutionibus orbium coelestium* challenged the geocentric view, noting that some Greek philosophers, like Aristarchus of Samos, had proposed a heliocentric universe. This simplified the planets’ apparent epicyclic motions, making it possible to represent their paths as simple circles using gears. Tycho Brahe’s precise observations (1576–1601) and Johannes Kepler’s 1621 deduction that planets orbit in ellipses, along with Isaac Newton’s 1687 theory of gravitation, explained the cause of elliptical motion. As late as 1650, P. Schirleus built a geocentric planetarium with the Sun as a planet and Mercury and
- field
- Astronomy, horology
- known_for
- Mechanical model of the Solar System
- first_known_example
- Antikythera mechanism (c. 205–87 BC)
- term_coined
- c. 1712
- named_after
- Charles Boyle, 4th Earl of Orrery
Lore & Background
Mechanical planetary models have existed since the Ancient Greeks. The Antikythera mechanism, discovered in 1901 in a shipwreck off the Greek island of Antikythera, has been reconstructed as exhibiting the diurnal motions of the Sun, Moon, and the five planets known to the ancient Greeks. This geocentric mechanism, dated between 205 and 87 BC, is considered one of the first orreries, likely used as a mechanical calculator. Cicero, writing in the first century BC, referenced planetary models built by Thales and Posidonius. In 1348, Giovanni Dondi built the first known clock-driven mechanism displaying the ecliptic position of the Moon, Sun, and planets according to Ptolemaic theories. At the court of William IV, Landgrave of Hesse-Kassel, two complicated astronomic clocks were built in 1561 and 1563–1568, showing ecliptical positions and an automated celestial sphere.
Reader's Guide
The term 'orrery' derives from a device produced c. 1712 by John Rowley and named for his patron Charles Boyle, 4th Earl of Orrery. The device took its name in English, possibly as a pun on the term 'horary.' A plaque on the original orrery reads: 'Orrery invented by Graham 1700 / Improved by Rowley, and presented by him to John [sic] Earl of Orrery, after whom it was named at the suggestion of Richard Steele.' The attribution to Steele is spurious, as Steele himself attributes the naming to Rowley in a 1713 article. Orreries are planetariums in the sense of planetary mechanical models; the word 'planetarium' has since shifted meaning. A grand orrery includes the outer planets known at the time of its construction. Orreries range from hand-held to room-sized. A model including only the Earth, Moon, and Sun is called a tellurion; one with only Earth and Moon is a lunarium; a jovilabe models Jupiter and its moons. The Eisinga Planetarium, built from 1774 to 1781 by Eise Eisinga in Franeker, Netherlands, displays the planets across a room's ceiling and has operated almost continually since creation.
Did You Know?
- The Antikythera mechanism, discovered in 1901 in a shipwreck off the Greek island of Antikythera, is considered one of the first orreries.
- The term 'orrery' was derived from a device produced c. 1712 by John Rowley and named for his patron Charles Boyle, 4th Earl of Orrery.
- Joseph Wright's painting 'A Philosopher giving a Lecture on the Orrery' (c. 1766) hangs in the Derby Museum and Art Gallery.
- The Eisinga Planetarium, built from 1774 to 1781 in Franeker, Netherlands, has been in operation almost continually since it was created.
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