One of the finest instruments in the Physical Sciences collection, an equatorium was used to work out the position of the sun, moon and planets. It is thought to have been made about 1600 in Europe, possibly France, though it is not signed or dated. Surviving equatoria are extremely rare and this is one of only three examples left. It works by representing the orbits of sun, moon and planets as a series of circular discs, planets on one side, sun and moon on the other. The observed retrograde motions were calculated by smaller discs on the others. The five circles represent the five planets known at the time of making, Mercury, Venus, Mars, Jupiter and Saturn. The constellations of the zodiac are in the outermost circle giving their position in the sky. On the other side, the sun and moon circles allowed the prediction of lunar and solar eclipses.