A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Tony Oz
Date: 2019 Oct 14, 01:53 -0700
Dear David, you asked:
Just how much change in atmospheric refraction would you expect in half a degree change in altitude at medium altitudes?
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.
Yes, the check against the horizon is also good - the parallax is small enough to tell if the sextant is in working conditions - was not dropped/banged/etc. I still prefer to do the I.E. measurement across the Sun - with the ability to do the sanity check by obtaining the S.D. from the same data.