A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bob Goethe
Date: 2015 Dec 29, 11:53 -0800
>>Are any of our past (recent recorded history) major catastrophic geological or moon events significant enough to alter our almanac data on a percentage accuracy basis<<
Not a stupid question at all, Mark. Wobbles in the earth translate into changes in rate of what we think of as time. And of course, time is critical in celestial navigation. It is just this issue that is at the heart of the debate over whether or not UTC ought to have leap seconds or not.
UT1 is the time after you add in corrections for changes in the spin of the earth...and it is the time system the Nautical Almanac uses. Fundamentally, a "second" is not of an absolutely consistent length in UT1. It can vary over time...typically getting a little longer, but potentially getting a bit shorter, at unpredictable rates.
This unpredictability drives big-computer-network guys nuts. They like to have regular, predictable time...with a second that is of the same length now as it was in the past. UTC is the time scale that the world uses that has this second of a consistent length. Because the rate-of-spin of the earth is not entirely consistent, UTC and UT1 drift out of sync with one another. Before they are in danger of being out from each other by more than 1 second, the world agrees to throw in a leap second, giving one particular minute of a particular day 61 seconds instead of 60. In principle, they might also take away a second in a given minute to sync up UTC and UT1, but it hasn't happened yet.
The GPS guys can't even live with this amount of uncertainty. They don't want to goof around with leap seconds. So they synched their satellite clocks to Universal Time as it was at zero hours, January 6, 1980. So GPS time is built on seconds of a consistent length that never get corrected in any way for the spin of the earth. So GPS time is currently out of sync with UTC by 17 seconds. Those 17 seconds are the result of geological/orbital events that change the rate-of-spin of the earth since 1980.
The question, "What time is it?" is deceptively complicated. So I expect others to have more and better things to say than what you see here.