Re: [Phys-l] Comets

["R. McDermott" <rmcder@gmail.com>]

----- Original Message ----- 
From: "John Mallinckrodt" <ajm@csupomona.edu>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Sunday, December 10, 2006 4:47 PM
Subject: Re: [Phys-l] Comets


> 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.

The other question is, do you mean from when one first sees the comet?  One 
coming very close to us would still be visible for some time (weeks at 
least) before getting close enough to exhibit significant lateral motion.

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

Indeed!