A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David C
Date: 2017 Jan 1, 13:26 -0800
On page 254, paragraph 2 of the Nautical Almanac, it specifies that the volume is based on UT1. UT1 is based on the rotation of the earth...and hence has a second of variable length. UTC runs on a second of fixed length, which is slightly shorter than the second of UT1. At intervals, UTC gets ahead of earth time, and a leap second is inserted to bring it back in sync.
Yesterday, UTC was 0.5 seconds ahead of earth time - which introduced a 0.5 second error in your celestial calculations. Today, UTC is 0.5 seconds behind earth time (UT1). So you still have an error = 0.5 seconds in your fix...except the error goes in the opposite direction now. At some point in the future, since the UTC clock runs fast compared to the earth clock, they will cross over each other, and your fixes will gain in accuracy.
In the Explanations at the back end of the Nautical Almanac, they explain
The time argument on the daily pages is 12 hr + Greenwich Hour Angle of the mean sun a.k.a GMT, and here denoted UT. This differs from the broadcast time UTC by an amount which, if ignored, introduces an error of up to 0.2' in longitude.
They go on to say leap seconds are introduced so that UTC and "UT" are always within 0.9 seconds of each other. If the leap second was not introduced, you would have to adjust the time read from your watch (i.e. add 1 second) before using it as an entry in the NA.
I am struggling to get my head around this. From the quotes above I deduce:
UT1 variable length second
UTC fixed length second
UTC second slightly less than UT1 second
UTC is human time
Leap second inserted to sync UTC with UT1 (UT) (and thus syncs human time with the NA)
UT (as in Almanac) = UT1 = GMT
NA based on UT1
In addition I conclude:
UT1 vs UTC Variations in the rotation of the earth.
GMT vs GAT Variations in the orbit of the earth around the sun.
Does any of that make sense?