# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Refraction
From: Peter Fogg
Date: 2005 Aug 7, 20:16 +1000

```I thought these mentions of his work on refraction might be of interest to
George Bennett, and so passed them along. His comment:

"The evaluation of Astronomical Refraction is an interesting subject. I see
that Marcel Tschudin has chosen one of the empirical formulas that I derived
when I was working at Greenwich. In my article I confined the apparent
altitudes to range of zero to ninety degrees. I would be interested to know
the result of any investigation into the validity of my formulas at small
negative altitudes. As a lead you might write to HM Nautical Almanac Office
who in turn could direct any enquiries to Dr A T Sinclair. He is an expert
on the subject and although retired now he may offer some helpful
information. Cheers, George."

________________________________________
On Behalf Of jcs
Sent: Friday, 5 August 2005 7:26 AM
Subject: Re: Refraction

Hi Marcel  Welcome to the list;

Be careful there are nearly as many different formula for refraction as
there are members of the list. There is not to my knowledge a definitive
formula. Almost all of them will work well at sextant altitudes above about
15 deg but  begin to fail at low angles, particularly negative angles, due
to the unpredictable behaviour of the atmosphere close to the sea or ground.

G.G.Bennet  wrote a short article in the (UK) Journal of the Institute of
Navigation.Vol 35 No 2. in which he compared several calculator programs
with a Fortran program written by G Garfinkel, described in The Astronomical
Journal,Vol 72,No 2  (1967), However it is not an easy method to use. I
believe his program was written for the benefit of Nasa space program but I

The following  is one I use,

Ref = Tan  (90-Altitude - 0.999139 * Altitude - (7.31 / (Altitude + 4.4)))

between -20 < Air Temp < +40, and  970 ```
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