Orrery

 

This article is about the mechanical device. For the British peerage, see Earl of Orrery.

A small orrery showing earth and the inner planets

An orrery is a mechanical device that illustrates the relative positions and motions of the planets and moons in the Solar System in a heliocentric model. Though the Greeks had working planetaria, the first orrery that was a planetarium of the modern era was produced in 1704, and one was presented to the Earl of Orrery — whence the name came. They 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 the arms.

History

Antikythera mechanism (main fragment), ca. 125 BC

A Philosopher Lecturing on the Orrery (ca. 1766) by Joseph Wright of Derby

According to Cicero, the Roman philosopher who was writing in the first century BC, Posidonius constructed a planetary model.

The Antikythera mechanism, discovered in 1900 in a wreck off the Greek island of Antikythera and extensively studied, exhibited the diurnal motions of the sun, moon, and the five known planets. The Antikythera hand driven mechanism is now considered one of the first orreries but for many decades was ignored as it was thought to be far too complex to be genuine and was not mentioned in the 1967 Science Museum booklet. It was heliocentric and used as a mechanical calculator designed to calculate astronomical positions.

Copernicus in De Revolutionibus Orbium Coelestium published in Nuremberg in 1543 challenged the Western religious teaching of a geocentric universe where the sun rotated around the earth. He found that some Greek philosophers 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 this motion in his theory of gravitation.

Christian Huygens published details of a heliocentric planetary machine in 1703, which he built while resident in Paris between 1665 and 1681. He calculated the gear trains that were needed to represent a year length of 365.242 days, and using that to produce the cycles of the principal planets. As late as 1650, P. Schirleus has produced a Geocentric planetarium showing the sun as a planet with Mercury and Venus moving around it as moons.

The first modern orrery was built circa 1704 by George Graham and Thomas Tompion. 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, from which the device took its name. This model was presented to Charles' son John, later the 5th Earl. Its importance was partially in that a mechanical model of the universe, correctly named a planetarium, gained the name Orrery,

Joseph Wright's picture "A Philosopher giving a Lecture on the Orrery in which a lamp is put in place of the Sun" (ca. 1766) which hangs in Derby Museum and Art Gallery, features a group (three men, three children, and a lone woman) listening to a lecture by a 'natural philosopher'—the only light in the otherwise darkened room is from the "sun" in the brass orrery, which, in this case, has rings that cause it to appear to be similar to an armillary sphere. The demonstration was thereby able to cover eclipses.

To put this in context, it was in 1762 that John Harrison's Chronometer allowed longitude to be accurately measured and also in 1766 that the astronomer Titius first demonstrated that the mean distance of the planets could be represented by the progression.

$\frac{4+0}{10},\frac{4+3}{10},\frac{4+6}{10},\frac{4+12}{10},\frac{4+24}{10},\frac{4+48}{10},.....$

That is, 0.4, 0.7, 1.0, 1.6, 2.8, 5.2 ... The numbers refer to astronomical units, that is 1.496 x 10⁸ km (93 x 10⁶ miles). The Derby Orrery does not show mean distance, but demonstrated the relative planetary movements: all part of a process understanding contemporary cutting edge scientific thinking.

Eisinga's "Planetarium" was built from 1774 to 1781 by Eise Eisinga in his home in Franeker, 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. This orrery, was a planetarium in both senses of the word- firstly a complex planetary machine, and secondly it was displayed in a special room, that was a sort of theatre for the observers. Eisinga house was bought by the Dutch Royal family who gave him a pension.

A 1766 Benjamin Martin Orrery, used at Harvard

In 1764, Benjamin Martin devised a new type of planetary model, where 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 be consist of three parts: The planetarium where the planets revolved around the sun, the tellurian 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.

Explanation

All orreries are planetariums or planetaria (alternative plural). The term orrery has only existed since 1714. A grand orrery is one that includes the outer planets known at the time of its construction. The word planetarium, has been captured and now usually refers to hemispherical theatres in which images of the night sky are projected onto an overhead surface. Planetariums (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 and transits is called an astrarium.

An orrery should properly include the sun, earth and the (earth's) moon (plus optionally other planets.) A model that only includes the earth, its moon and the sun is called a tellurion, and one which only includes the earth and moon a lunarium. A jovilabe is a model of Jupiter and its moons.

Planet	Mean Distance from the sun	Diameter	Mass	Density	No. of satellites	Orbital period	Inclination to ecliptic	Inclination of equator	Rotation rate
Mercury	0.39	0.4	0.05	5.1	0	0.24	7.0	?	59 days
Venus	0.72	1.0	0.9	5.3	0	0.61	3.4	23	243 days
Earth	1.00	1.0	1.0	5.52	1	1.00	0	23	1 day
Mars	1.52	0.5	0.11	3.94	2	1.88	1.9	24	24.5 hours
Jupiter	5.20	11	318	1.33	12	11.86	1.3	3	10 hours
Saturn	9.54	9.54	95	0.69	10	29.46	2.5	27	10 hours
Uranus	19.18	4	15	1.56	5	84.01	0.8	98	11 hours
Neptune	30.06	4	17	2.27	2	164.79	1.8	29	16 hours
Pluto	39.44	0.5?	?0.1	?	0	247.69	17.2	?	6 days

A planetarium will show the orbital period of each planet and the rotation rate, as shown in the table above. A tellurion will show the earth with the moon revolving around the sun. It will use the angle of inclination of the equator from the table above to show how it rotates around its own axis. It was show the earths moon, rotating around the earth. A lunarium is designed to show the complex motions of the moon as it revolves around the earth.

Orreries are usually not built to scale. Some fixed Solar System scale models have been built and are often many kilometres in size. 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.

Projection orreries

Many planetariums (buildings) have a projection 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 42 seconds to complete an orbit on an orrery, and Jupiter will take 11.86 minutes.

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, while Phobos and Deimos 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 tricks can be used to show Pluto and its three moons.

Notable orreries

Shoemaker John Fulton (instrument maker) of Fenwick, Ayrshire, built three between 1823 and 1833 - the last is in Glasgow's Kelvingrove Art Gallery and Museum.

The Franeker Planetarium built by a wool carder called Eise Eisinga in his own living room, in the small city of Franeker in Friesland, is in fact an orrery. It was constructed between 1774 to 1781. The "face" of the model looks down from the ceiling of a 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.

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.

Meccano Planetaria

The first Meccano Orrery was described in June 1918 Meccano Manual, though it is the last quarter of the 20th Century that Alan Partridge, John Nuttall, Pat Briggs and Michael Whiting have experimented with the limitations and possibilities of this medium. There are six methods of building orreries:

• Telescopic concentric tubing, though the tubing is not a Meccano part. For a full 9 planet system, 18 concentric tubes and 19 geared motions are needed
• Nested hollow turntables with internal gearing. A 1984 Jovilabe showing Jupiter and the 14 then known moons required 14 turntables and 324 gears; it is accurate to within 0.01%
• Fixed central rod, with top drive and sun and planet gearing. In this method, the sun drives the first planet and this then drives the second and so on. The errors are thus cumulative, but non-Meccano gearing can keep this within 0.02%
• Fixed central tube, with bottom drive and sun and planet gearing. This removes the need for an overhead gantry, improving the appearance
• Two component epicyclic gearing.
• Fixed Central Rod, Epicyclic gearing, top drive by 'by-pass gearing' from below.

In popular media

• An orrery appears in Aughra's observatory in The Dark Crystal.
• An orrery is used to determine when total eclipses occur in Pitch Black.
• The Orrery is an artificial construct consisting of life-sized planets and a sun in Green Lantern: New Guardians.

See also

• Apparent retrograde motion
• Astrarium
• Astrolabe
• Astronomical clock
• Ephemeris
• Eratosthenes
• Orbit of the Moon
• Stability of the Solar System
• Tellurion
• Torquetum
• Aughra from the Dark Crystal (film)

External links

• JPL Solar System Simulator
• Armagh Observatory Human Orrery
• Long Now Foundation Orrery
• University of Pennsylvania Orrery