1. History
    Early versions  Modern orreries 

2. Explanation

3. Projection orreries

4. Notable orreries

5. In popular culture

6. See also

7. References

8. External links

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; but since accurate scaling is often not practical due to the actual large ratio differences, a subdued approximation may be used instead. 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 Charles Boyle, 4th Earl of Orrery – whence the name. 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

Early versions

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. It has been dated between 150 and 100 BC. 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.^[1] It was geocentric and used as a mechanical calculator designed to calculate astronomical positions.

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

In 1348, Giovanni Dondi built the first known clock driven mechanism which displays the ecliptical position of Moon, Sun, Mercury, Venus, Mars, Jupiter and Saturn according to the complicated Ptolemaic planetary theories.^[2][3] The clock itself is lost, but Dondi left a complete description of the astronomic gear trains of his clock.

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.^[4]

At the court of William IV, Landgrave of Hesse-Kassel two complicated astronomic clocks were built in 1561 and 1563-1568, which show on four sites the ecliptical position of Sun, Mercury, Venus, Mars, Jupiter and 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, show the real position of the Sun, including the equation of time.^[5][6] 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 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 modelled 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.^[7]

Modern orreries

Clock makers George Graham and Thomas Tompion built the first modern orrery around 1704 in England.^[8] 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 in English.^[9][10] This model was presented to Charles' son John, later the 5th Earl of Cork and 5th Earl of Orrery. Independently, Christiaan Huygens published in 1703 details of a heliocentric planetary machine which he had built while resident 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.^[4]

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

That is, 0.4, 0.7, 1.0, 1.6, 2.8, 5.2 ... The numbers refer to astronomical 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.

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.^[12] 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, the tellurion (also tellurian or tellurium) 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.^[1]