NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Refraction
From: Marcel Tschudin
Date: 2007 Dec 11, 11:24 +0200
From: Marcel Tschudin
Date: 2007 Dec 11, 11:24 +0200
Depending on what one intends to do, the different approximate functions which have been suggested for calculating refraction don't provide useful results near the horizon. The scatter of the observed refraction values is just too high. Victor Reijs has a nice plot on his Web-page http://homepages.iol.ie/~geniet/eng/tropospherelayers.htm showing how observed refractions do scatter around a calculated approximate value. The plot shows also nicely the (systematic) difference between sunrise and sunset. One set of data points shows also the result from Sinclair's approximate formula. This graph may help you to understand why one shouldn't use observations below about 3 to 5 degrees above the horizon for navigational purposes. The reason for the large scatter of observed refractions are the (diurnal) changes in the planetary boundary layer, i.e. the lowest part of the atmosphere where the heat exchange between the earth (or sea) and the atmosphere takes place. Those effects are high over land, less near the coast and minimal over the sea. Marcel --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---
