gordon brown wrote:
> 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.
Just a couple of additional notes.
1) It also isn't true that "The orbital periods of planets is
obviously well established and constant." The orbital elements of the planets
are affected by the gravitational perturbations of other planets, just as are
those of comets, though the effect is are not as large.
2) The variability of comet orbits is most extreme in the case of one
like Shoemaker-Levy, which was destroyed by its encounter with Jupiter.
Shalom,
George
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