NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Navigation in Fog
From: Frank Reed CT
Date: 2007 Feb 19, 17:01 -0800
From: Frank Reed CT
Date: 2007 Feb 19, 17:01 -0800
Nicolas, you wrote: "I assume you will need to calibrate the whole setup so that when the polarising filter 'disappears' against the background the orientation is truly towards the sun. Is there an easy way to do this calibration? Would you be able to do this experiment while still observing the sun directly, or should the sun already be below the horizon?" Yes, the polarization is also visible when the sky is clear and the Sun is in the sky. You can observe this with polarizing filters, but you can also observe it with an ordinary piece of glass, painted black on the back, functioning as a front-surface mirror. Light reflected from glass is polarized, most strongly when the angle of incidence is around 60 degrees, if I remember correctly (just experiment to find it). The light of the sky is maximally polarized in a wide band centered along a great circle 90 degrees from the Sun, no matter what time of day it is. This polarization is strongest when the sky is very clear. A smooth pool of water will also serve as a polarizing filter so if you walk around a bucket of water looking at the reflecting light from the sky at the right angle, you will see a darker blue reflection from the sky in certain directions. One final example: car windows near sunset (or a similar event, which I understand is called "sunrise", for you morning people). I find that this is especially obvious with large, nearly horizontal windows like on car "hatchbacks". Near sunset, these windows seem to display a mottled pattern of indistinct blue and purple patches, each patch about an inch across. These are apparently polarizing effects in the glass or maybe a coating on the glass. They show best when the sky overhead is strongly polarized. -FER www.HistoricalAtlas.com/lunars --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---