Solar system model. Engraving of an orrery, a mechanical model of the planetary motions in the solar system. The planets rotated around the Sun (at centre); the Earth and Moon are just to the right of the Sun. Only the six planets nearest to the Sun are featured; Uranus, Neptune and Pluto were discovered in 1781, 1846 and 1930 respectively. The frame around the device represents the celestial sphere". This imaginary structure was used to calculate the movement of stars in the night sky due to the Earth's daily rotation. The orrery was a popular instrument of astronomical instruction during the 18th and 19th centuries."