Anorrery (/ˈɒr.ə.ri/ ^ⓘ) is a mechanicalmodel of the Solar System that illustrates or predicts the relative positions and motions of theplanets andmoons, usually according to theheliocentric 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 sincethe Ancient Greeks, and are known by various names, but the termorrery is derived from a device producedc. 1712 by John Rowley and named for his patronCharles Boyle, 4th Earl of Orrery.

Orreries are typically driven by aclockwork mechanism with a globe representing theSun at the centre, and with a planet at the end of each of a series of arms.

TheAntikythera mechanism, discovered in 1901 in a wreck off the Greek island ofAntikythera in the Mediterranean Sea, exhibited thediurnal motions of theSun,Moon, and the fiveplanets known to theancient Greeks. It has been dated between 205 and 87 BC.^[1][2][3] The mechanism is considered one of the first orreries.^[4] It wasgeocentric and used as a mechanical calculator to calculate astronomical positions.

Cicero, the Roman philosopher and politician writing in the first century BC, has references describing planetary mechanical models. According to him, the Greek polymathsThales^[5] andPosidonius^[6] each constructed a device modeling celestial motion.

In 1348,Giovanni Dondi built the first known clock driven mechanism of the system. It displays theecliptic position of the Moon, Sun,Mercury,Venus,Mars,Jupiter andSaturn according to the complicatedgeocentric Ptolemaic planetary theories.^[7][8] The clock itself is lost, but Dondi left a complete description of its astronomicgear trains.

As late as 1650, P. Schirleus built a geocentricplanetarium with the Sun as a planet, and with Mercury and Venus revolving around the Sun as itsmoons.^[9]

At the court ofWilliam 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 toPtolemy, a calendar, the sunrise and sunset, and an automatedcelestial sphere with an animated Sun symbol which, for the first time on a celestial globe, shows the real position of the Sun, including theequation of time.^[10][11] The clocks are now on display inKassel at the Astronomisch-Physikalisches Kabinett and inDresden at theMathematisch-Physikalischer Salon.

InDe 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 theEarth. He observed that some Greek philosophers such asAristarchus of Samos had proposed a heliocentric universe. This simplified the apparentepicyclic 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 theseJohannes Kepler (1621) deduced that planets orbited the Sun inellipses. In 1687Isaac Newton explained the cause of elliptic motion in histheory of gravitation.^[12]

There is an orrery built by clock makersGeorge Graham andThomas Tompion datedc. 1710 in theHistory of Science Museum, Oxford.^[13] Graham gave the first model, or its design, to the celebrated instrument maker John Rowley of London to make a copy forPrince Eugene of Savoy. Rowley was commissioned to make another copy for his patronCharles Boyle, 4th Earl of Orrery (inCounty Cork, Ireland), from which the device took its name in English.^[14][15] The nameorrery may be apun on the termhorary,^[16] a term related to timekeeping.^[17] It was producedc. 1712 by John Rowley.[16] The plaque on it reads:

Orrery invented by Graham 1700

Improved by Rowley, and presented by him to John [sic]^[a] Earl of Orrery, after whom it was named at the suggestion ofRichard Steele.^[16][18]

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.^[19] This oft-repeated attribution error ultimately derives from a minor error in the writings of the lecturerJohn T. Desaguliers.^[20]

The device is atellurion.

Independently,Christiaan Huygens published in 1703 details of a heliocentric planetary machine which he had built while living in Paris between 1665 and 1681. He calculated the gear trains needed to represent a year of 365.242 days, and used that to produce the cycles of the principal planets.^[9]

Joseph Wright's paintingA Philosopher giving a Lecture on the Orrery (c. 1766), which hangs in theDerby Museum and Art Gallery, depicts a group listening to a lecture by anatural philosopher. The Sun in a brass orrery provides the only light in the room. The orrery depicted in the painting has rings, which give it an appearance similar to that of anarmillary sphere. The demonstration was thereby able to depicteclipses.^[21]

To put this in chronological context, in 1762John Harrison'smarine chronometer first enabled accurate measurement oflongitude. In 1766, astronomerJohann Daniel Titius first demonstrated that the mean distance of each planet from the Sun could be represented by the following progression:

${\displaystyle \frac{4+0}{10},\frac{4+3}{10},\frac{4+6}{10},\frac{4+12}{10},\frac{4+24}{10},...}$

That is, 0.4, 0.7, 1.0, 1.6, 2.8, ... The numbers refer toastronomical units, the mean distance between Sun and Earth, which is 1.496 × 10⁸ km (93 × 10⁶ miles). The Derby Orrery does not show mean distance, but demonstrated the relative planetary movements.

TheEisinga Planetarium was built from 1774 to 1781 byEise Eisinga in his home inFraneker, in the Netherlands. It displays the planets across the width of a room's ceiling, and has been in operation almost continually since it was created.^[22] This orrery is a planetarium in both senses of the word: a complex machine showing planetary orbits, and a theatre for depicting the planets' movement. Eisinga house was bought by the Dutch Royal family who gave him a pension.

In 1764, Benjamin Martin devised a new type of planetary model, in which the planets were carried on brass arms leading from a series of concentric or coaxial tubes. With this construction it was difficult to make the planets revolve, and to get the moons to turn around the planets. Martin suggested that the conventional orrery should consist of three parts: the planetarium where the planets revolved around the Sun, thetellurion (alsotellurian ortellurium) which showed the inclined axis of the Earth and how it revolved around the Sun, and the lunarium which showed the eccentric rotations of the Moon around the Earth. In one orrery, these three motions could be mounted on a common table, separately using the central spindle as a prime mover.^[4]

All orreries areplanetariums, in the sense of planetary mechanical models. (That sense ofplanetarium was the common one historically. The word planetarium has shifted meaning, and now usually refers to hemispherical theatres in which images of the night sky are projected onto an overhead surface.) The termorrery has only existed since 1714. Agrand orrery is one that includes theouter planets known at the time of its construction. Orreries can range widely in size from hand-held to room-sized. An orrery is used to demonstrate the motion of the planets, while a mechanical device used to predict eclipses andtransits is called anastrarium.

An orrery should properly include the Sun, the Earth and the Moon (plus optionally other planets). A model that only includes the Earth, the Moon, and the Sun is called atellurion or tellurium, and one which only includes the Earth and the Moon is alunarium. Ajovilabe is a model of Jupiter and its moons.^[23]

A planetarium will show theorbital period of each planet and therotation rate, as shown in the table above. A tellurion will show theEarth with the Moon revolving around the Sun. It will use the angle ofinclination of the equator from the table above to show how it rotates around its own axis. It will show the Earth's Moon, rotating around the Earth.^[24] A lunarium is designed to show thecomplex motions of the Moon as it revolves around the Earth.

Orreries are usually not built toscale. Human orreries, where humans move about as the planets, have also been constructed, but most are temporary. There is a permanent human orrery atArmagh Observatory inNorthern Ireland, which has the six ancient planets,Ceres, and cometsHalley andEncke. Uranus and beyond are also shown, but in a fairly limited way.^[25] Another is at Sky's the Limit Observatory and Nature Center inTwentynine Palms, California; it is a true to scale (20 billion to one), true to position (accurate to within four days) human orrery. The first four planets are relatively close to one another, but the next four require a certain amount of hiking in order to visit them.^[26] A census of all permanent human orreries has been initiated by the French group F-HOU with a new effort to study their impact for education in schools.^[27] A map of known human orreries is available.^[28]

A normal mechanical clock could be used to produce an extremely simple orrery to demonstrate the principle, 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. As Jupiter's actual year is 11.86 Earth years long, the model would lose accuracy rapidly.

Manyplanetariums have aprojection orrery, which projects onto the dome of the planetarium a Sun with either dots or small images of the planets. These usually are limited to the planets from Mercury to Saturn, although some include Uranus. The light sources for the planets are projected onto mirrors which are geared to a motor which drives the images on the dome. Typically the Earth will circle the Sun in one minute, while the other planets will complete an orbit in time periods proportional to their actual motion. Thus Venus, which takes 224.7 days to orbit the Sun, will take 37 seconds to complete an orbit on an orrery, and Jupiter will take 11 minutes, 52 seconds.

Some planetariums have taken advantage of this to use orreries to simulate planets and their moons. Thus Mercury orbits the Sun in 0.24 of an Earth year, whilePhobos andDeimos orbit Mars in a similar 4:1 time ratio. Planetarium operators wishing to show this have placed a red cap on the Sun (to make it resemble Mars) and turned off all the planets but Mercury and Earth. Similar approximations can be used to showPluto and its five moons.

ShoemakerJohn Fulton ofFenwick, Ayrshire, built three between 1823 and 1833. The last is in Glasgow'sKelvingrove Art Gallery and Museum.

The Eisinga Planetarium built by awool carder named Eise Eisinga in his own living room, in the small city ofFraneker inFriesland, is in fact an orrery. It was constructed between 1774 and 1781. The base of the model faces down from the ceiling of the room, with most of the mechanical works in the space above the ceiling. It is driven by a pendulum clock, which has 9 weights or ponds. The planets move around the model in real time.^[29]

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 at the Armagh Observatory.^[25]

In 2024, the LEGO Group commercially produced an orrery of the Sun, Earth, and Moon. The model is assembled exclusively from LEGO elements and reproduces solar and lunar orbits, as well Earth's rotation about a tilted axis.^[30]

• The construction systemMeccano is a popular tool for constructing highly accurate orreries. Model 391, the first Meccano Orrery, was described in the June 1918 Meccano Manual.^[31][32]

• Buick, Tony (2020).Orreries, Clocks, and London Society: The Evolution of Astronomical Instruments and Their Makers, 2nd Ed. Cham, Switzerland: Springer.ISBN 978-3-030-61776-9.

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