The Cometarium is, by analogy with the planetarium,
a mechanical device to illustrate the path of a comet. This example, from
the Smithsonian Insititution, traces out the path of Halley's Comet of
1682, which was studied by Edmund Halley (1656-1742) who was a professor
at Oxford and later the Astronomer-Royal of England. The sun is shown at
one focal point of the earth's elliptical orbit, which seems to be far
too eccentric. Turning the crank makes the comet and the earth go around.
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