A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2021 Oct 7, 16:39 -0700
Art Leung re. your 2021 Oct 7, 06:26 -0700
Re my ‘smart Alec’ comment about averaging over mer-pas time, I was just drawing attention to something I only realised myself about a year ago. At 53.10N 000.32W today 7th October, mer-pas time for the Sun was 11.50 UTC. According to ‘Navigator’, if I’d observed the Sun at 11.50, I should have measured H(a) of 31.10.3. If Instead I’d made observations at 11.45 and 11.55 I should have obtained H(a) s of 31.09.8 and 31.09.6 giving an average of 31.09.7, which is 0.6 minutes of arc less than my single observation at 11.50. Readings at 11.40 and 12.00 would have given an average of 1.3 minutes of arc below the 11.50 value.
In other words averaging around mer-pas times means all of my readings with have been taken below the top of the hill. I don't remember anyone ever pointing this out to me during training. This was probably because the difference from the mid-time value for a one or two minute shot using a mechanical averager is negligible.