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I just wrote in to summarily thank everybody who made an effort to
contribute something to my question on the sunrise/set problem. I'm
privileged in having the means to communicate so casually with several
obviously distinguished professionals and other very knowledgeable people
through this online forum- I won't list any names lest I exclude someone I
shouldn't, though I ought to mention Frank Reed because he is its creator -
something unimaginable under different circumstances.

Christian Scheele

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From: 
To: 
Sent: Saturday, June 20, 2009 5:53 AM
Subject: [NavList 8740] Re: Basics of computing sunrise/sunset



Christian, you wrote:
"But in theory: Assume you know the athmospheric conditions to
"the  horizon" and beyond right up to GP of the sun, i.e. you are able to
monitor these conditions for this entire stretch; say you have an imaginary
string of weather balloons thousands of miles long. Would that solve
refraction?"

Oh yes, I agree with that. This is a simple enough problem in physics. It's
just a matter of applying the standard law of refraction to a medium whose
density varies continuously with position. If we know the function for the
density(x,y,z) exactly from observations, then we can solve the problem by
straight-forward numerical integrations, largely thanks to the extremely low
cost of modern computing power.

What's actually remarkable about astronomical refraction is that basically
none of the atmospheric detail matters for angular altitudes above three
degrees or so. If you know the local atmospheric density (which depends on
temperature and local pressure and to a very small extent on atmospheric
composition, humidity, etc.), then you can calculate the refractions for
altitudes above three degrees without access to the actual conditions at all
points along the line of sight.

-FER





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