Cometarium

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Cometarium
Manufacturer	W. & S. Jones

Cometariums are mechanical devices which describe the orbit of a comet. They originated as a way of illustrating Kepler's second law of planetary motion. The first such device was constructed by J. T. Desaguliers in 1732 as a way to show the more rapid motion of the planet Mercury when near the sun (at perihelion) than when far from the sun (at aphelion).

This example was made by the instrument makers W. & S. Jones, London, England early in the nineteenth century.

[edit] How it works

The comet is represented by an ivory ball with a tail of bristles which moves around an elliptical groove, traversing equal areas in equal times. A brass ball representing the sun is at the focus of the ellipse. A brass pointer radiating from the sun shows how the comet appears to move against the background of the sky, which is represented by a large circle.

[edit] In the Science Museum

Inv. No: 1909-202