This mechanical device illustrates the relative motions of the Earth and Moon about the Sun. It was invented (c1700) by George Graham, and a copy was made by the clockmaker John Rowley in the early 18th century. He presented it to Charles Boyle, the 4th Earl of Orrery and it was named an orrery in his honour. The orrery was controlled by a clockwork mechanism, but the movements were slow.

More elaborate orreries were made, including all the known planets and their moons. Many were large, almost a metre in diameter, and were ornately decorated. As Uranus was discovered in 1781, orreries can be dated by examining how many planets are shown. Orreries became important and popular teaching aids, although not everyone appreciated them. Some astronomers, notably William Hershel, rejected them as childish toys.

An interesting activity is to build a human orrery, based on the model from which the orrery got its name. Three people take the roles of the Sun, Moon and Earth. A piece of string is divided into 13 equal parts, and this string is then laid on the floor in a circle. The Sun stands at the centre. The Earth travels around the circle, stepping from one mark to another. Each mark represents a lunar month. For each of these steps the Moon walks about the Earth once. For each of the steps the Earth makes it should spin around 28 times, but this makes the Earth very dizzy. Representing the planets as spheres avoids this, as the spheres can be spun quickly with no adverse effects. The Sun can hold a torch directed at the Earth to show the variations of day and night.

For more about the planets try Planet 10 at www.scienceyear.com where you can design your own planet and launch it into space.

Sophie Duncan is a physicist and programme manager with Science Year