An orrery [awr'ur E] is essentially a simple planetary machine. It is a partial model of the solar system as seen from the "outside," with the Sun at the center. Early orreries (such as the one shown here) typically showed no planets other than the Earth revolving about the Sun plus the Moon revolving around the Earth, but later, more complex, models included all known planets and, in very complex orreries, the moons revolving around those planets. The first known planetary machines were made by London clockmaker George Graham, between 1704 and 1709.

The first planetary machine to be called an orrery was made by another London clockmaker, John Rowley, who made the one shown here for Charles Boyle, 4th Earl of Orrery (for whom he named the device). Under the central glass dome, the brass ball of the Sun rotates once every twenty-seven and a half Earth days. One turn on the hand crank on the rim of the orrery rotates the ivory sphere of the Earth (under the other glass dome) exactly once -- equivalent to one full twenty-four-hour day. The Moon in turn revolves about the Earth in a lunar month of twenty-nine and a half Earth days, while also rotating on its own axis so as to keep the same face directed towards the Earth. The flat brass ring around the top represents the Earth's path around the Sun. The whole mechanism was controlled by a series of gear-trains, each hand-crafted with all the precision found in very expensive (and, hence, very accurate) clocks and watches.

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Solar System

THE ROBINSON LIBRARY
Science.--Astronomy.--Instruments.

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