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Fred,

The self-made program contains also a goographical database with the
coordinates of about half million locations and mountains and from the
mountains also their altitude. In addition I incorporated recently a
atmospheric model for approximate temperature and pressure values for a
given location at a given day and time... Ask my wife how many years I
already spent in this private project ;-)

Marcel

----- Original Message -----
From: "Fred Hebard" 
To: 
Sent: Thursday, August 04, 2005 7:35 PM
Subject: Re: Refraction


> Marcel,
>
> I am still wondering under what conditions you would get a negative
> elevation.
>
> If it's for locating yourself on land, it seems hopeless: If you're at
> an unknown location in a valley, and trying to use surrounding
> mountains as an indication of horizon, you wouldn't know your height
> above sea level to begin with, so wouldn't know how much higher the
> mountains were than you, not to mention not knowing how far away they
> were.  Also, the height of mountains varies, so you would have to know
> the precise spot on the mountain below the celestial object, to
> determine its height above sea level at that point.  It seems hopeless
> to me.  Better to use an artificial horizon or a plumb bob.
>
> Fred
>
> On Aug 4, 2005, at 12:20 PM, Marcel E. Tschudin wrote:
>
>> Fred,
>>
>> Yes, how do I get in this? Just trying to cover in a self-made program
>> the
>> situation from an object at the horizon (over sea level) as seen from a
>> mountain or air craft.
>>
>> The "real" calculation is done via integration. But since this is not
>> very
>> practical one uses approximative formulae like e.g. the one from
>> Bennett
>> which Meeus mentions in his book Astronomical Algorithms. All tables on
>> refraction I found so far do end at 0? elevation and for none of the
>> approximative formulae I could find an indication that they also would
>> be
>> valid for negative elevations.
>>
>> I also was wandering whether the approximate formulae could be used by
>> calculating the Refraction R for e.g. -2? the follwing way:
>>
>> R(-2?)  =  R(0?)  +  (  R(0?) - R(+2?)  )
>>
>> If this would be correct then one would not need separate formula for
>> negative elevations.
>>
>> Greetings from Marcel
>>
>

   

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