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From: jp@lanl.ARPA
Newsgroups: net.astro
Subject: Re: StarDate: June 11 Where the Stars Don't Twinkle
Message-ID: <27565@lanl.ARPA>
Date: Mon, 24-Jun-85 09:29:44 EDT
Article-I.D.: lanl.27565
Posted: Mon Jun 24 09:29:44 1985
Date-Received: Thu, 27-Jun-85 05:16:29 EDT
References: <224@utastro.UUCP> <71@denelvx.UUCP>
Organization: Los Alamos National Laboratory
Lines: 24

> ******************************************************************************
> > 				The twinkling we notice from the ground
> > comes from the effect of the atmosphere on incoming waves of
> 
> 	Given that the twinkling effect is caused by tthe atmosphere as
> stated above, why is it that the stars twinkle but the planets don't.
> 
> 	The explanation I've always heard is that it's because the planets are
> closer--but I don't see what that has to do with it; after all, they aren't
> closer than the atmosphere.  (Are they? :-)
> 
> 	In other words, why does the atmosphere have that effect on
> starlight but not on the light reflected from the planets?
> 
Your explanation is partly correct.  The stars are so far away that they may
be considered point sources of light.  The planets, on the other hand, are close
enough that they appear to be disks of light.  The effect of the atmosphere is the
same on all sources of light originating outside it, but the light reflected
from a planet can orginate at many points.  The effect of the atmosphere
causes each point to "twinkle" just as for the stars, but the average of many
twinkling points (not synchronized because each ray takes a slightly different
path) tends to be a steadier, non-twinkling, image.

Jim Potter  jp@lanl.arpa