A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Francois Tremblay
Date: 2022 Jun 27, 18:07 -0700
Just a little observation following my last post. Page 257 of the 2022 Almanach (Commercial Edition) states "at sunrise (...) 16' is allowed for semi-diameter and 34' for horizontal refraction...
I had assumed that the problem came from the semi-diameter. However it seems that this value is indeed taken into account. However, the 34' for horizontal refraction would perfectly explain the error between 52 and 53.4.
My understanting is that the values for sunrise and sunset in the Almanach are provided not to indicate the moment the center pount of the sun is at 0 deg but the moment the observer will see the sun pointing above the horizon, taking into account refraction.