Orrery

• 1. Introduction

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  The device pulled from a shipwreck near the Greek island of Antikythera in 1901 was an orrery. It had rested on the floor of the Mediterranean for roughly two thousand years. Scholars dated it between 205 and 87 BC. It tracked the Sun, Moon, and the five planets that ancient Greek astronomers could observe. These machines, built to mirror the motions of the heavens in clockwork, go by many names. The English term orrery was not coined until 1714, attached to a gift made for an Irish aristocrat. How did this tradition develop over two millennia? Who built these machines, and who paid for them? The Roman philosopher Cicero, writing in the first century BC, already knew of at least two such instruments built by Greek scholars.

• 2. Ancient Clockwork

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  The Antikythera mechanism is geocentric: it places the Earth at the centre of its calculations, as ancient Greek astronomy required. Scholars regard it as likely serving as a mechanical calculator for determining astronomical positions. The Roman writer Cicero named two builders who had tackled the same challenge. The polymaths Thales and Posidonius each constructed a device to model the motions of celestial bodies.

  In 1348, the Italian scholar Giovanni Dondi completed the first known clock-driven mechanism of the planetary system. His instrument showed the ecliptic positions of the Moon, Sun, Mercury, Venus, Mars, Jupiter, and Saturn, following the geocentric theories of Ptolemy. The clock itself has since been lost. But Dondi left behind a thorough written description of its astronomical gear trains.

  At the court of William IV, Landgrave of Hesse-Kassel, two elaborate astronomical clocks were built in 1561 and in the years 1563 to 1568. These displayed the positions of the Sun, Mercury, Venus, Mars, Jupiter, Saturn, and the Moon. They also showed calendar information, sunrise and sunset times, and an automated celestial sphere. The clocks survive today: one is now in Kassel at the Astronomisch-Physikalisches Kabinett, the other in Dresden at the Mathematisch-Physikalischer Salon.

  Nicolaus Copernicus had published his challenge to the geocentric model before the first of those Hesse-Kassel clocks was begun. His argument that planets moved in simple circles around the Sun would eventually transform how these machines were built.

• 3. The Copernican Shift

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  Published in Nuremberg in 1543, Copernicus's De revolutionibus orbium coelestium placed the Sun at the centre of the cosmos. This directly challenged centuries of geocentric teaching. Copernicus observed that the Greek philosopher Aristarchus of Samos had proposed the same arrangement long before. The heliocentric model had a practical consequence for instrument makers: it eliminated the need for complicated epicycles. Planets could now be assigned simple circular paths, and circular paths could be reproduced by gears.

  Tycho Brahe made precise observations of the sky between 1576 and 1601. Using that data, Johannes Kepler established in 1621 that planets orbit the Sun in ellipses rather than circles. Isaac Newton, in 1687, explained the underlying cause: his theory of gravitation accounted for the elliptic motion.

  As late as 1650, a builder identified only as P. Schirleus constructed a geocentric planetarium. He placed the Sun among the planets and treated Mercury and Venus as moons revolving around the Sun.

  George Graham and Thomas Tompion would soon build an instrument that gave the entire tradition its English name.

• 4. The Name

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  George Graham and Thomas Tompion built their orrery around 1710; it is now held at the History of Science Museum in Oxford. Graham passed the design to John Rowley, a celebrated instrument maker in London. Rowley first produced a copy for Prince Eugene of Savoy. He was then commissioned to make another copy for his patron, Charles Boyle. Boyle held the title 4th Earl of Orrery, with lands in County Cork, Ireland. From that commission, the device drew its English name.

  The plaque on Rowley's instrument credits Richard Steele with suggesting the name. Steele himself, in a 1713 article written to popularize the orrery, attributed the naming to Rowley. The plaque's attribution is an error. It traces back to a minor mistake in the writings of the lecturer John T. Desaguliers.

  The device Rowley produced for Boyle is dated around 1712. The name orrery may itself be a pun on a term related to timekeeping.

  Working independently in Paris between 1665 and 1681, the Dutch scientist Christiaan Huygens built his own heliocentric planetary machine.

• 5. Builders and Their Machines

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  In 1703, Christiaan Huygens published the technical details of the heliocentric planetary machine he had built. He had calculated the precise gear ratios needed to represent a year of 365.242 days. That allowed him to model the orbital cycles of the principal planets.

  Around 1766, the painter Joseph Wright depicted the cultural life of the orrery in a canvas now hanging in the Derby Museum and Art Gallery. His work, A Philosopher giving a Lecture on the Orrery, shows a natural philosopher demonstrating the device to a group of onlookers. The only light in the room comes from the Sun in a brass orrery. The depicted device has rings that give it the appearance of an armillary sphere; it was configured to demonstrate eclipses.

  A wool carder named Eise Eisinga chose a far larger canvas. Between 1774 and 1781, he built a planetary model inside his own home in Franeker, a small city in Friesland in the Netherlands. The planets move across the width of the living room ceiling. Most of the mechanical works are concealed in the space above the ceiling. A pendulum clock driven by nine weights powers the whole system. The planets move at the real, unaccelerated speed of their actual orbits.

  Eisinga's home was eventually purchased by the Dutch Royal family, who awarded him a pension. The planetarium has run almost without interruption ever since.

  A shoemaker named John Fulton of Fenwick in Ayrshire built three orreries between 1823 and 1833.

• 6. Varieties and Descendants

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  The final orrery Fulton built is now displayed at Kelvingrove Art Gallery and Museum in Glasgow.

  In 1764, Benjamin Martin proposed dividing the orrery into three separate instruments. He devised a design in which planets were carried on brass arms extending from a series of concentric tubes. That construction made the planets difficult to turn and the moons hard to animate. Martin proposed three distinct parts: a planetarium, where the planets revolved around the Sun; a tellurion, showing the Earth's tilted axis and its annual orbit; and a lunarium, tracking the Moon's complex motions around the Earth.

  A model showing only Jupiter and its moons has its own name: a jovilabe. The term grand orrery applies to any model that includes the outer planets known when it was built.

  Armagh Observatory in Northern Ireland has a permanent human orrery, in which people move as the planets. It represents the six ancient planets, Ceres, and comets Halley and Encke. The human orrery at Sky's the Limit Observatory in Twentynine Palms, California, operates at a scale of twenty billion to one. Its positions are accurate to within four days. The first four planets are grouped near the centre; visiting the outer four requires hiking.

  In a projection orrery, Earth completes its orbit in one minute. Venus, whose real year is 224.7 days, takes just 37 seconds. Jupiter, with a year of nearly twelve Earth years, takes eleven minutes and fifty-two seconds. These systems cast images of the planets onto a domed ceiling, typically ranging from Mercury to Saturn.

  The Meccano construction system has long been used to build precise working orreries. Its first dedicated design, Model 391, was described in the June 1918 edition of the Meccano Manual. In 2024, the LEGO Group released a commercial orrery of the Sun, Earth, and Moon. Built entirely from LEGO elements, it reproduces solar and lunar orbits and Earth's rotation on a tilted axis.

  A census of permanent human orreries worldwide has been initiated by the French group F-HOU. The aim is to study their value as teaching tools in schools.