Frequently Asked Questions

What is an Orrery?

A planetary model of our Earth/Moon system around the Sun. This usually includes some movements in a third dimension, such as the inclination of the Moon's orbit or the Earth's axis fixed in space.
A model that shows the planets revolving round the Sun, normally without any planetry rotation or axial tilt, is called a Planetarium.

Why are they called Orreries?

In 1712 an instrument maker called John Rowley built an Earth/Moon/Sun model for Charles Boyle, who was the fourth earl of Orrery. John Rowley, most probably in an attempt to flatter his patron, named the device "an Orrery" and the name stuck!

How do they work?

Mainly they are driven round by a series of cogs and gears, and as you turn a small handle the various parts go round. Some are driven by a cord and pulley system, but these are not as accurate as the geared ones.

What is an Orrery used for?

They are a teaching aid for showing students how the various planets revolve and rotate. With an Earth/Moon/Sun system model you can answer all these questions by just turning the handle..
  • Why we have day and night
  • What a year is
  • What a lunar month is
  • Why there is summer and winter
  • Why there is 'six months day' at the North pole
  • How and why Solar and Lunar eclipses happen
  • Why eclipses don't happen very often
  • Why there are phases of the Moon
  • Why we always see the same face of the Moon
  • Why the tropics and the Arctic circles are where they are
  • Why we have to have a leap year every fourth year.

Are they to scale?

No - if the Sun and Earth were to scale and the Sun was 100mm in diameter, then the Earth would be 1mm across and 10.8 meters away.

Are they accurate?

They can be surprisingly accurate... using a carefully chosen set of gears a model Earth can rotate around the sun to an accuracy of 5 minutes a year. It's all down to the designer, and how much time is spent in looking at different gear combinations.

Where can I find out more about Orreries?

  • Ask Peter Grimwood: peter@orreries.co.uk
  • Visit a science museum and speak to the curator, there are orreries in the Science Museum (London, UK), The Museum of the History of Science (Oxford UK), Conservatoir National des Arts et Metiers (Paris, France), Museum of History and Technology (Washington DC), Adler Planetarium and Astronomical Museum (Chicago, USA)
  • Ask a library to access a book on the subject, good books to start with are:
    • Geared to the Stars - H C King and J R Millburn - ISBN 0 8020 2312 6
    • Wheelwright of the Heavens - J R Millburn - ISBN 0 946836 45 0
    • The Planetarium of Giovanni de Dondi - AHS - ISBN 0 901180 10 6
    • Antide Janvier - M Hayard - ISBN 2 911063 00 7
    • Jens Olsen's Clock - O Mortensen - Published by the Copenhagen technical Institute
  • Visit specialist auction sales, read the catalogues, ask for (half-price) back number catalogues, talk to the expert. Christie's hold three or four specialist sales each year at their South Kensington sale rooms.
  • Try designing and building your own ... you will learn a lot by doing this!