A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David C
Date: 2018 Mar 30, 15:13 -0700
You say you had a latitude error of a "few n.m." Did you update the Sun's declination correctly for UT?
Yes, I staffed up the UT calculation! We are currently +13 (changes to +12 tonight I think). When I looked at the Air Almanac I mentally subtracted 12. That gives an error of 0.9' in the dec.
The refraction tables in the official Nautical Almanac have changed very slightly since 1973, but it's not meaningful to navigation. You can use those old tables, no problem. One issue with the sun correction tables is that they jump from winter to summer values.
I am not trying to achieve the maximum possible accuracy. I want to know what is the practical accuracy I should aim for. With an altitude of 45° the jump in tabulated dec on 31st March (today) is 0.3'. That immediately suggests that 0.5' is a realistic precision.
When I factor in the poor quality optics and low magnification of my sextant, my aging eyesight and oval images in the AH maybe 1' is a more realistic figure. My vernier sextant can be read to 10 arc seconds but I suspect that I am wasting my time reading it to a greater precision than 1'.
Changing the subject slightly - what precision would Chichester have needed to make sure he did not miss Norfolk Island?