What is an orrery? Meaning, history and how it works
An orrery is a moving model of the solar system: the Sun in the middle and the planets, often with their moons, carried around it at their relative speeds. The classic orrery is a brass machine driven by a hand crank or clockwork; turn the handle and the planets wheel round in step, Mercury racing ahead, Saturn barely moving.
orrery (noun; plural orreries) — say it OR-er-ee: /ˈɒrəri/ (UK), /ˈɔːrəri/ (US). A mechanical model of the solar system that shows the planets moving around the Sun, usually driven by gears. Named after Charles Boyle, 4th Earl of Orrery.
Why it is called an orrery
The name comes from a person, not a thing. Around 1704 the London clockmakers George Graham and Thomas Tompion built a geared model showing the Earth and Moon moving around the Sun. The instrument maker John Rowley made copies of it, one of them for his patron Charles Boyle, 4th Earl of Orrery — an Irish title, taken from the barony of Orrery in County Cork — and the machine took the earl’s title as its name in English. It has been an orrery ever since.
How a mechanical orrery works
Each planet sits on an arm, and all the arms turn on one central post under the Sun. A train of gears turns each arm at its own rate, set so the ratios match the planets’ real orbital periods. For every full turn of Earth’s arm — one year — the others must turn:
| Planet | Orbit (Earth days) | Turns per Earth year |
|---|---|---|
| Mercury | 88 | 4.15 |
| Venus | 225 | 1.63 |
| Earth | 365 | 1 |
| Mars | 687 | 0.53 |
| Jupiter | 4,333 | 0.084 |
| Saturn | 10,759 | 0.034 |
Gear trains can only approximate ratios like these, so a mechanical orrery slowly drifts out of step with the sky. It also moves every planet in a perfect circle at a constant speed, where the real orbits are ellipses and each planet speeds up near the Sun, as Kepler showed.
Nor can an orrery be to scale. If Earth were a 1 cm bead, the Sun would be a ball 1.1 m across, 117 m away; Jupiter would be 11 cm across and 610 m out; and Neptune would be 3.5 km away. Even the Moon would sit 30 cm from the bead. Every orrery enlarges the planets and squeezes their orbits together so the whole thing fits on a table.
Kinds of orrery
- Grand orrery — all the known planets, with moons such as Jupiter’s and Saturn’s turning around them.
- Tellurion (or tellurium) — just the Sun, Earth and Moon, built to show day and night, the seasons, and why eclipses happen.
- Lunarium — the Earth and Moon alone, to show the Moon’s phases and its tilted orbit.
- Planetarium — in the 18th century, another word for an orrery. Today it usually means a domed theatre that projects the night sky.
- Armillary sphere — a different instrument: a framework of rings for the celestial equator, the ecliptic and other circles of the sky. It shows the sky around the Earth, not planets going round the Sun.
A short history
- About 2,100 years ago — the Antikythera mechanism, a Greek box of bronze gears recovered from a shipwreck in 1901, predicted the positions of the Sun and Moon and the dates of eclipses. Cicero describes an earlier sphere by Archimedes that showed the planets moving.
- 1364 — Giovanni Dondi completed his Astrarium in Padua, a clock showing the motions of the Sun, Moon and five planets.
- Around 1704 — George Graham and Thomas Tompion built the first modern orrery; John Rowley’s copy for the Earl of Orrery gave it its name.
- About 1766 — Joseph Wright of Derby painted A Philosopher Lecturing on the Orrery, the faces of his audience lit by the lamp standing in for the Sun.
- 1774–1781 — Eise Eisinga built a planetarium into the ceiling of his living room in Franeker, in the Netherlands. It still runs, the oldest working planetarium in the world, and is a UNESCO World Heritage Site.
- Today — orreries are made as teaching models and as works of art, and computers can do what gears never could: real elliptical orbits, any date you like, and true scale at the flick of a switch.
An orrery you can use online
Orrery.live is a free, interactive 3D orrery that runs in your browser. Instead of gears it computes every planet’s position from JPL orbital data, so the planets move on their real elliptical orbits and sit where they actually are on any date from 3000 BC to 3000 AD. Like a brass orrery it enlarges the planets so you can see them — and, unlike one, it has sliders that shrink them back to true size and spread the orbits out to true distance. It also carries the major moons, dwarf planets, famous comets such as Halley’s, and the spacecraft heading out of the solar system.
Two companion pages put the same model into words: where the planets are right now, and when the next planetary alignment is.
Questions
What does orrery mean?
A model of the solar system that shows the planets, and often their moons, moving around the Sun at their relative speeds. Traditional orreries are mechanical: turning a handle or a clockwork drive moves each planet on its arm through a train of gears. The word is also used for computer versions such as this one.
How do you pronounce orrery?
OR-er-ee, with the stress on the first syllable: /ˈɒrəri/ in British English and /ˈɔːrəri/ in American English. The plural is orreries.
Why is it called an orrery?
After Charles Boyle, 4th Earl of Orrery, for whom John Rowley built a copy of George Graham’s new solar-system machine in the early 1700s. The earl’s title, from the barony of Orrery in County Cork, became the device’s name.
Who invented the orrery?
The first modern orrery was built around 1704 by the London clockmakers George Graham and Thomas Tompion. Geared models of the heavens are far older: the Greek Antikythera mechanism, roughly 2,100 years old, predicted the positions of the Sun and Moon and the dates of eclipses.
What is the difference between an orrery and a planetarium?
Today a planetarium is usually a domed theatre that projects the night sky as seen from Earth; an orrery is a model of the solar system seen from outside. In the 18th century the words overlapped, and mechanical orreries were often called planetaria. A tellurion is a smaller orrery with just the Sun, Earth and Moon.
Are orreries accurate?
Mechanical ones get the order of the planets and their relative speeds right, but move each in a circle at a constant speed, and cannot be to scale: with Earth 1 cm across, the Sun would be 1.1 m across and 117 m away, and Neptune 3.5 km away. A computer orrery can use the real orbits; Orrery.live works from JPL orbital data and can switch sizes and distances to true scale.
Sources
- Orrery — Wikipedia (Graham, Tompion, Rowley and the Earl of Orrery; kinds of orrery).
- Antikythera mechanism, Astrarium, A Philosopher Lecturing on the Orrery and Eise Eisinga Planetarium — Wikipedia.
- Orbital periods and the scale figures are computed from standard values for the planets’ periods and sizes.