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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Digital Camera Celestial Navigation
From: Paul Hirose
Date: 2008 Jul 06, 21:12 -0700
From: Paul Hirose
Date: 2008 Jul 06, 21:12 -0700
Marcel Tschudin wrote: > Why not measuring the sun's horizontal diameter? This measure is > (almost?) not affected by refraction. I discovered the "almost" a few days ago, when programming a function to compute refracted semidiameter. The values at the 12 and 6 o'clock position angles (in the horizontal frame) were exactly what I expected for a Sun-sized object, but at 3 and 9 o'clock they were about .2" too small. After some debugging, I realized there was nothing wrong with my code. Refraction reduces the semidiameter in every direction. When the left and right limbs refract up, they move on great circles which converge at the zenith. With a body of 16' semidiameter, and sea level refraction, this decreases the semidiameter about .27". I tested this at several altitudes from 10 degrees to 80, and it didn't vary more than .01". In this case, the algorithm was smarter than its designer. Later I remembered that Jean Meeus ("Astronomical Algorithms", chapter on refraction) says the Sun's horizontal diameter is reduced slightly by refraction. His figure (credited to Danjon) is .6", practically independent of altitude. -- I block messages that contain attachments or HTML. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---