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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2025 Jan 20, 19:17 -0800
Alex E, you wrote:
"You are making a crude mistake when determining the index error from the sun. When the two suns touch, the reading (assuming zero error) is not zero but 2 times the Sun semi-diameter"
It's certainly possible that he's making this error, but from his description it appeared to me that he was using another method for checking index error, namely parking the Sun's image / disk directly on top of itself (reflected on direct). While that's no more accurate than the usual horizon on horizon method, some people use it. Similarly, some sextant users like to place a star's reflected image right next to the direct image to test the index error.
Peter, can you confirm what you did? Did you place the two circular images --the direct and reflected disks of the Sun-- seen through your sextant on top of each other (making one brighter disk), or did you place them edge to edge, just touching? If you did the latter, then the idea is to get the angle when they are ordered direct on top of reflected and then the angle when they're reversed to reflected on top of direct, subtract the two values and divide by two. But if, as I understood your post, you're dropping one Sun disk directly on top of the other and ending up with a noticeably different index correction than you find by the standard horizon test, that would indicate something unusual, like prismatic error in one of your shades.
Frank Reed