Re: [Phys-l] Comets

[John Mallinckrodt <ajm@csupomona.edu>]

On Dec 10, 2006, at 11:29 AM, JMGreen wrote:

> With what speed can a comet appear to move?

Anything, including comets, can appear to move at any speed.

> Can it move across the sky from East to West in several days?  Weeks?

Yes.

But in all seriousness (not that the above wasn't), isn't it clear  
that the time it takes comets to "move across the sky" depends almost  
entirely on how close they are to us.  Nearby comets are moving at  
speeds measured in the 10's of km per second relative to us or any  
other nearby things.  So one that is a typical 100 million km away  
from us will take a typical couple of months to move across the sky.   
One that passes 10 times closer would take on the order of a week.   
One that passes 100 times closer (on the order of the distance to the  
moon) would take on the order of a day.

And when you see one moving across the sky in an hour or less, count  
your blessings.

John Mallinckrodt

Professor of Physics, Cal Poly Pomona
<http://www.csupomona.edu/~ajm>

and

Lead Guitarist, Out-Laws of Physics
<http://outlawsofphysics.com>