On Mon, 8 Jan 2001, Bill Payne wrote:
> The orbital periods of planets is
> obviously well established and constant. The same is true of comets
> bound gravitationally to the sun.
The above statement is not true. The period of Halley's comet, for
example, has been observed to vary from 73 to 77 years if I recall the
numbers correctly. The reason for this is simple. The sun is not the only
source of gravitational attraction that affects comets. When a comet comes
close to a planet, its orbit is altered. This can change a long-period
comet into a short-period comet, and it appears that this is the origin of
short-period comets. The planet can also alter an orbit to throw the comet
into a hyperbolic orbit, which would take it out of the solar system,
depending on which side of the planet the comet passes.
> Secondly, comets with parabolic orbits bound to the sun have observed
> periods up to I'm guessing a couple of thousand years.
If the orbit is truly parabolic (or hyperbolic), the comet is not periodic
and so has no observed period.
Gordon Brown
Department of Mathematics
University of Colorado
Boulder, CO 80309-0395
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