NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Refraction
From: Frank Reed CT
Date: 2005 Aug 16, 14:24 EDT
From: Frank Reed CT
Date: 2005 Aug 16, 14:24 EDT
George H wrote: "Bennet has provided a formula which is an empirical attempt to fit that averaged data. At large angles of altitude, it becomes proportion to the tan of the zenith angle, as Snell's law requires. Near the horizon, where refraction rises sharply, the divergence from Snell's law shows up in correction terms which turn out to be remarkably simple." Of course, Snell's Law always applies. It's just that a simple "slab" model of the atmosphere doesn't work near the horizon. And: "In some publications, such as the Nautical Almanac, it appears that Bennett's formula itself is used as the basis for the refraction tables" I don't think there's any reason to suppose that Bennett's formula is used in the Nautical Almanac. The tables predate the formula. And: "(though the constants have recently been tinkered-with a bit to improve the fit to recent Pulkova data)" I know that someone (possibly you --?) previously speculated that the changes in the almanac refraction tables were based on "recent Pulkova data" but has that been confirmed? -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars