NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Refraction
From: Frank Reed
Date: 2008 Jul 04, 04:57 -0400
From: Frank Reed
Date: 2008 Jul 04, 04:57 -0400
Dave Walden, you wrote: "They are functions of pressure, temperature and wavelength. Two sets of conditions can be calculated for easy comparisons. Those included are for the Nautical Almanac conditions and a more conventional set to conditions: 760mb, 0 deg C, lamda .54 microns. The K =60.4 sec, should look familiar. The Naut Alm STP yields 58.2, also familiar." Aha. Very nice! So that does appear to explain it. Kent, did you believe that your refraction number was for 0 deg C or 10 deg C? And you wrote: "The formulas can be found in Spherical Astronomy by Green. This is the "successor" to Smart's work. Green was coauthor with Smart on later editions and finally wrote his own. (To my way of thinking, it's better.)" Damn. Another book to buy. :-) What's the full title of Green's book? And: "Also attached is a plot of errors of Bennett and simpler formulas (minutes refrac vs degrees alt). There is indeed, nothing magic about Bennett. It get 34 min at 0 deg in case you've forgotten, but the added complication to do so only makes things worse at higher altitudes." A simple way to keep the best of Bennett (it is indeed more convenient than typing in the whole table) is to retain it for altitudes below 15 degrees (or some other cut-off) and use some simple function of tan(z.d.) for higher altitudes. Anyone have any thoughts about the best color of light to use for the refraction formulae/tables? At very low altitudes, the star images really are smeared out substantially. What's the standard for astronomy? Is it appropriate for sextant users? -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---