A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bob Goethe
Date: 2017 Sep 16, 10:05 -0700
>>be suspicious with observations through double glazing units <<
I think as a general rule, this has got to be true. Your opportunity is there to verify a particular observing location by comparing a number of shots with your GPS position, and making some inferences about the quality of glass you are looking through.
I have taken some hundreds of sights through the double-glazed front windows of my living room. Whether it is a "personal error" related to how I hold the sextant, or a quirk of the windows I am taking my sight through, I have discovered that if I subtract 3 minutes from my Hs, I get a pretty accurate line of position.
Of course, while the existence of my error is unequivocal, the source of the error is a bit harder to identify. I can say that I have several windows in the frame of the living room bow window, and that 3 minute error is fairly consistent no matter which one I am looking out. I also apply that 3 minute correction to sights I take when I am bouncing around in a boat, and I have been pretty happy with the results. So I am slightly inclined to think that I "see" the sun touch the horizon 3 minutes of angle before it actually does, and that my windows are pretty good.