What Is an Orrery?

The London instrument maker John Rowley built that first orrery around 1713, possibly after an earlier model by the clockmaker George Graham. It showed only the Sun, Earth and Moon, and it is now in the Science Museum Group collection. The name later spread to models of the whole solar system.

Every model of the solar system has to leave something out. Each worked example below starts from what a simple model would teach you, then opens a Moment in our Live Orrery that steps through the real sky, with its dates worked out from the day you open it.

Worked example: the seasons

A simple geared orrery carries Earth on a circle, so its distance from the Sun never changes. Earth's real orbit is a slight ellipse. Earth passes closest to the Sun in early January, about 147.1 million km away, in the middle of northern winter, and farthest in early July, about 152.1 million km away. That change in distance alters the sunlight reaching Earth by about 7 percent over the year, yet the north is coldest when Earth is closest. The seasons come from the 23.4-degree lean of Earth's axis, which tips the north toward the Sun in June and away from it in December.

Open the Seasons Moment

Worked example: retrograde motion

In a Sun-centered gear model no planet ever moves backward: each one turns steadily around the Sun. Switch to Earth's point of view and Mars loops. About every 26 months Earth overtakes Mars on the inside track, and for two to three months Mars seems to drift west against the stars before it turns east again. Nothing reverses; the backward stretch is our own motion, seen against the distant stars.

Open the Retrograde Moment

Worked example: the Moon's phases

A simple tellurion, the Sun, Earth and Moon kind of orrery, often keeps the Moon in the plane of Earth's orbit and draws that orbit many times too large, yet it still shows the phases correctly. A phase depends only on the angle between the Sun, Earth and Moon: the Sun always lights half of the Moon, and over 29.5 days we see a changing share of that lit half. What a model on a table cannot show is the side that is lit for your half of the world. A first-quarter Moon is lit on the right as seen from northern mid-latitudes and on the left as seen from southern ones, where observers face north to see it.

Open the Phases Moment

Worked example: eclipses

A flat model with the Moon in the plane of Earth's orbit predicts a solar eclipse at every new Moon and a lunar eclipse at every full Moon. The real orbit tilts about 5 degrees, so in most months the Moon's shadow passes above or below Earth, and Earth's shadow passes above or below the Moon. An eclipse needs a new or full Moon close to one of the two nodes, where the Moon's orbit crosses the plane of Earth's orbit. The Sun lines up with the nodes about every 173 days, and those are the eclipse seasons.

Open the Eclipse seasons Moment

Kinds of orreries

Geared desk models. Gears set each planet's period, so the order of the planets and their relative speeds come out right: Mercury laps the Sun a little more than four times for every Earth year, and Jupiter once in about 12. Most of them move each planet at one steady rate on a circle in one flat plane, so they show no ellipse and no tilt, and over the years their planets drift from the real ones.

Tellurions. The Sun, Earth and Moon only, like the first orrery. An Earth whose axis keeps pointing the same way in space shows day and night, the seasons and the Moon's phases. The Moon's orbit is drawn far too large, and a simple tellurion often keeps it in the plane of Earth's orbit, so it cannot show why eclipses are rare.

Computed orreries. A computed orrery works out each position from an ephemeris, the theory astronomers use to predict where each body will be at any instant. It can put every planet where it really is, on its real ellipse and tilt, and switch the point of view between the Sun and Earth. It still has to choose a scale, and every kind of orrery distorts scale more than anything else.

Why no orrery is to scale

Shrink Earth to a marble 1 centimeter across and the Sun becomes a ball about 1.1 meters wide, 117 meters away. Mercury orbits at an average of 0.39 AU (1 AU is Earth's average distance from the Sun) and Pluto reaches 49.3 AU, about 126 times farther out, which on the marble scale puts Pluto almost 6 kilometers from the Sun. No room holds that, and no screen can show it with the planets larger than specks.

Our Live Orrery offers two answers. The diagram's All bodies view and the 3D view's compact scale take the square root of each planet's distance from the center of the view, which shrinks Pluto's 126 times to about 11 and fits the whole system on one screen, with the planets drawn larger than life. The True scale switch in the 3D view shows the distances as they are, with every planet drawn at least 5 pixels wide so you can still find it. The directions are real in both, so each body sits where you would see it from the Sun or from Earth.

How ours is computed

Our Live Orrery is a computed orrery. It works out every position in your browser with Astronomy Engine, the open-source library behind the rest of the site, timed on Earth's measured rotation. Positions as seen from Earth: planets and Pluto within 1 arcminute of NASA JPL Horizons, 1700 to 2200 (the accuracy report). The Sun-centered positions come from the same engine, and that study did not measure them separately.

Frequently asked questions

What is an orrery?

An orrery is a working model of the solar system that shows where the planets are and how they move around the Sun. Classic orreries are driven by gears; a computed orrery, like our Live Orrery, works out each position from an ephemeris.

Why is it called an orrery?

It is named for Charles Boyle, 4th Earl of Orrery. Around 1713 the London instrument maker John Rowley made him a model of the Sun, Earth and Moon, the model took its owner's title, and the name later spread to models of the whole solar system.

Is an orrery to scale?

Almost never. If Earth were a marble 1 centimeter across, the Sun would be a ball about 1.1 meters wide and 117 meters away, and Pluto would reach almost 6 kilometers out, so orreries squeeze the distances and enlarge the planets. The 3D view of our Live Orrery has a True scale switch that shows the real distances.

What is the difference between an orrery and a planetarium?

An orrery shows the solar system from outside, with the planets going around the Sun. A planetarium today usually means a domed theater that projects the night sky as it looks from the ground. The two words once overlapped, and the Whipple Museum in Cambridge still calls its grand orrery of about 1750 a planetarium.

Where are the planets right now?

Open the Live Orrery: it opens on the current moment and keeps time with the clock, so it shows where every planet is right now, seen from the Sun or from Earth.

Sources & further reading

• Science Museum Group: Orrery made by John Rowley for the Earl of Orrery (object 1952-73): the orrery Rowley made in 1712 or 1713 for Charles Boyle, its Sun, Earth and Moon, the name, and George Graham's earlier model.
• Whipple Museum of the History of Science, University of Cambridge: The Grand Orrery: Rowley's orrery of around 1713, the Sun-centered models George Graham built with Thomas Tompion before it, and a grand orrery of about 1750 that the museum calls a planetarium.
• Astronomy Engine by Don Cross: the open-source library our Live Orrery computes with.
• NASA JPL Horizons: the reference ephemeris our accuracy study measured against.

See how these figures are computed on the methodology and sources page.

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