A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Robin Stuart
Date: 2020 Jan 4, 08:49 -0800
As I was revisiting the definitions of UTC, UT1, ΔT, DUT1 the following (maybe obvious) thing occurred to me.
Times in the Nautical Almanac are given in Coordinated Universal Time (UTC) which is also shown on the chronometer. This is a uniform time scale apart from discrete jumps due to leap seconds that are added from time to time to keep it in synch with UT1 that is based on the Earth’s rotation angle. This means that, in principle, the positions that you come up with from celestial navigation are with respect to a fictitious uniformly rotating Earth and not the true surface. Just prior to the addition of a leap second you would expect the position you get through celnav to be biased to the west of your true position by up to 0.225' in longitude (0.9 seconds of time) but more typically around 1/8' of longitude. Agree?
This effect is obviously just on the edge of observability with a sextant but there are a few things (e.g. aberration, planetary parallax) for which I finding it surprising that they are detectible by a hand held instrument. If I'm right this effect should probably join the list.
Finally the reason my sights are never quite come out bang on!
Happy New Year!