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    Re: Basics of computing sunrise/sunset
    From: Frank Reed
    Date: 2009 Jun 19, 20:53 -0700

    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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