Question about Short Period Comets

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Offline Skyguy

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Question about Short Period Comets
« on: 11/09/2008 03:41:44 »
To get away for all the LHC talk for a moment...

Short Period Comets are defined as comets with orbital periods 200 years or less.  I've always wondered what is so special about the number 200?  When was it chosen?  Was it picked for any specific observationally based reasons or simply out of convenience (i.e.  200 years doesn't sound too short or too long) to prevent useless bickering?  Bickering like the raucous "What defines a planet?" debate that still continues.


Offline Democritus

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Question about Short Period Comets
« Reply #1 on: 11/09/2008 08:56:37 »
I'm guessing here, but it may have to do with short period comets being interplanetary.
Which is to say that they tend to orbit within the furtherest planetary orbits.

In earth years Pluto takes 248 years, and Neptune 165 years, to orbit the sun.
OK, so Pluto aint a planet these days (don't tell Clyde) but you get the gist.

So 200 years is a pretty good approximation to help define those comets which are short, and those which are long period.
That's my best shot.


Offline daveshorts

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Question about Short Period Comets
« Reply #2 on: 11/09/2008 09:01:12 »
Looking at a list of periodic comets
My feeling is that all of the periodic comets we know about (bar possibly 1) have a period less than 200 years and there are a load which just seem to drop in once and are then not predictable.

The figure of 200 years is probably a consequence of how long people had been looking for comets to come back, but there does appear to be two groups of comets, one with periods of years and tens of years, and another lot.