An orrery is a mechanical model that demonstrates the movements of the planets. A system of revolving wheels allows it to show astronomical events across days and years: the movement of the Earth, Sun and planets, the revolutions of our Moon, or of the moons of Jupiter and Saturn. Named after the fourth Earl of Orrery, these models were key to popularising the latest astronomical discoveries. This orrery was found in the observatory at the School of Military Engineering at Chatham and donated to the Science Museum. It is signed 'Ramsden London', but the signature is atypical of the workshop of Jesse Ramsden and he was not known to make orreries. A very similar but more complete unsigned orrery is in the collection of the Adler Planetarium, Chicago.