A working model of the solar system showing the planets, possibly with some of their moons, in their orbits around the Sun. The term orrery was first applied to such a model in 1713 when one was made for the Fourth Earl of Cork and Orrery. The gear systems in such models are usually made so that the orbital periods of the planets are in the correct ratios. However, it is impossible to construct a demonstration model of this kind in which the planet sizes and the distances between them are both to scale because of the great range of distance scales.