
NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2025 Jan 20, 13:53 -0800
That sounds like a severe case of a "prismatic error" in a shade. Try the same test with the Moon or maybe the planet Venus. That is, place the target object right on top of its reflected image. Any index error by that test? Now try the same thing with the sea horizon or some linear feature more than a mile away? Any index error that way? You should see identical results. If you only get a bad index error by the Sun, then it's one of your shades.
So how can you test your shades, you might wonder... Test each shade individually on some suitably bright object, like the Moon. This should work with all but the darkest of sextant shades. You'll need to know exactly what altitude you should see at the instant you take your sight. An easy way to do this is with my app which you can read about here. Otherwise you can do it by longhand calculation (good practice?), or you can use another app if you understand what numbers it's producing.
By the way, to test each of the horizon shades independently, turn the sextant upside-down. :)
You may want to post a few photos of your sextant here. First, we like photos! Second, this is a good way to spot other potential "issues" with your sextant.
Frank Reed
My introductory workshop: Modern Celestial: Sextants & Sun Sights in two weeks online... or this coming weekend at Mystic Seaport Museum