NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Refraction
From: Nicol�s de Hilster
Date: 2007 Nov 09, 11:27 +0100
From: Nicol�s de Hilster
Date: 2007 Nov 09, 11:27 +0100
In NavList 3849 Gary LaPook wrote: > Back in March there was a discussion of various formulas to use to > predict refraction. I have found some other formulas that may be more > precise. The U.S. Naval Observatory used to publish its "Almanac For > computers" but stopped it after 1991. That publication has several > formulas for refraction: > > (1) R' = cot(a + (7.31/(a + 4.4))) > > (2) R = R' - 0.06sin(14.7R' + 13) > > > R is in minutes and a is the observed altitude. "At sea level, with > air temperature of 10�C and pressure of 1010 mb, R' of eq. (1) is > accurate to 0.07' and R of eq. (2) is accurate to 0.015' for an > altitude range of 0� - 90�." > > > "For nonstandard conditions the following formula is accurate to about > 0.2' for temperatures in the range of -20� to + 40� C and pressures of > 970 - 1050 mb. > > (3) R = R'((P - 80)/930)/(1 + 8x10 ^ -5(R' + 39) T - 10))) > > > I found another formula for temperature and pressure which I used for correcting the observations with the demi-cross and compared that to (2): (4) R = (0.267 x P / T)/(TAN(Altitude + ((4.848x10^-2)/(TAN(Altitude)+0.028)))) In our case (alt = 4m, P = 1018mbar, T = 12�) there was only 0.05 arc minutes difference with (2) --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---