NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Refraction correction
From: Frank Reed
Date: 2009 Apr 25, 20:13 -0700
From: Frank Reed
Date: 2009 Apr 25, 20:13 -0700
Peter Hakel, you wrote: "I am wondering, if the sun or the moon are observed through a cloud layer that is thin enough so that the disk is still visible, what is the additional atmospheric refraction effect on the sextant altitude? Can the experts on this list point me to any discussions on the subject or is this a non-issue? If this does make a practical difference, one might have to consider the effective thickness of the cloud, its composition (liquid droplets vs. ice crystals), color dispersion, etc..." Most of those high thin clouds are composed of ice crystals. It's not a problem for refraction in navigation because any refraction that does occur as light passes through an ice crystal shifts the ray by a large angle. When you see a big ring around the Moon, you can think of that as a series of superimposed images of the Moon from those large angle refractions. There aren't any gradual refractions so the main image of the Moon that you see in the sky is not shifted. The same is largely true for water droplets though it might be a little harder to see. Like a Moon halo, a rainbow is nothing more than light that has been severely refracted away from the principal direction of the Sun. The principal image of the Sun in the sky is not affected. By the way, there is a relatively easy observational test that should convince you that the refraction is not significantly different. Look at the image of the Sun through your sextant, with appropriate shades of course, as it passes behind the edge of a layer of thin clouds. If there were a difference in refraction, you would see a distortion in the Sun's circular outline (similar to what you see when the Sun is very close to the horizon where the refraction is actually quite variable). -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---