NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Refraction
From: Peter Fogg
Date: 2005 Aug 7, 20:16 +1000
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." ________________________________________ From: Navigation Mailing List [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM] On Behalf Of jcs Sent: Friday, 5 August 2005 7:26 AM To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM 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 may be wrong about this. . 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