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 25, 14:21 EDT
From: Frank Reed CT
Date: 2005 Aug 25, 14:21 EDT
George H you wrote: "She tells me that the refraction data were tinkered-with to conform with modern refraction values used by astronomers, which are based on an integration made by Sinclair, of JPL data provided by Standish." I suspect that this is exactly the same integration procedure that we've been working with lately. Standish says he made it widely known in the early 70s (did you get a chance to read the article? it's here in gif format: http://www.HistoricalAtlas.com/lunars/ref.html ). There is no sense in which these modern refraction integrations are superior to earlier ones. They are somewhat more flexible and they converge faster, but they can cover the same atmospheric complexity as earlier integrations. The difference between the present Nautical Almanac refraction table and the earlier one apparently comes down to a rather slight difference in temperature structure in the atmosphere. The pre-2004 refraction table can be derived (using the Standish integration) from an atmosphere model with a temperature lapse rate of 7.25 degrees C per kilometer up to 11km and constant temperature above that. The post-2004 refraction table can be derived from an atmosphere model with a lapse rate of 9.0 deg C/km up to 2km and 6.5 deg C/km up to 13km and constant temp above that level. These are both reasonable temperature curves which occur somewhere on the Earth every day. They are both "right" in that sense. It seems from the integrations that any plausible temperature curve will yield essentially the same refraction table for altitudes above 3.0 degrees with very little sensitivity to atmospheric structure. The refraction values within a degree or two of the horizon, on the other hand, are very sensitive to the lapse rate structure in the lowest part of the troposphere. And also: "Although Frank's comment, that the changes are hardly relevant to navigators, is broadly correct, it seems to me that the NAO are being responsible in keeping their published refractions in line with modern information, and in accord with values currently in use by astronomers, rather than sticking to "traditional" values." Bear in mind that the "traditional" refraction table is every bit as good as the "new" table. The difference for celestial navigators from a practical standpoint is completely insignificant, and in addition both are valid refraction tables under the appropriate weather conditions. Even from a theoretical standpoint, astronomers and marine navigators probably *should* use different refraction tables. Astronomers tend to work on desert mountaintops while marine navigators... er... don't! If you had to pick just one lapse rate structure for the atmosphere in order to compile a refraction table for angular altitudes below three degrees, you probably wouldn't pick the same structure for the open ocean that you would choose for desert mountains. Finally, I think it's worth pointing out that a difference in refraction values of less than 1% is smaller than the refraction difference between the red and blue end of the spectrum. -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars