A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2019 Oct 14, 12:31 -0700
Tony Oz you wrote: Indeed, at medium altitudes the effect is negligible. But who said it is at medium altitudes one needs to do the index check? What if I'm preparing for an evening sight? The Sun is low and the refraction-caused effects are greatest on it.
Maybe at 60 North such a sight might be necessary (7.5 degrees at lunchtime on 22nd December), but anything below ten degrees altitude is not to be advised. If I had to take such a sight out of force of circumstance, I think I’d skip the index error check for once and trust to my existing index error figure, which hasn’t changed since I bought the Hughes Mate’s sextant 20 years ago, and there’s always the horizon for a check. DaveP