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    Re: Leap second today
    From: Geoffrey Kolbe
    Date: 2012 Jul 01, 09:39 +0100
    Frank wrote

    So here's a navigational question for you to ponder: how much should the GHA of objects listed in the Nautical Almanac be adjusted after midnight UT tonight? Is your Nautical Almanac now "wrong"? Or is it always "right"?

    It has been interesting to see the varied answers to this question. Also, very revealing too....

    The answer (or course) is that as far as the NA is concerned, it was no more right or wrong after the leap second adjustment at midnight UT last night than it was before.

    That is because the time system used in the Nautical Almanac is UT1, which is determined from the mean solar day, not the UTC or "broadcast" time as given out by NIST or WWF and which is derived from atomic clocks. The fact that we can use UTC or "broadcast" time in conjunction with the NA is because at present, leap seconds are inserted from time to time into UTC in order to keep it within 0.9 seconds of UT1. To be really precise about using "broadcast" time as given out by NIST or WWF in conjunction with the NA, a correction called DUT1 should be added to UTC to derive UT1. See http://maia.usno.navy.mil/search/search.html to get this correction with a precision of 100 nanoseconds. You will find that yesterday, broadcast time was about 0.6 of a second too fast. Today, it is about 0.4 seconds slow due to the insertion of the leap second.

    At the present time, the earth's rotation is considered to be the master clock - as it always has been - and so in this sense, UT1 is considered superior to UTC. If we ever do away with leap seconds, the NA will probably have to be formatted with reference to UTC rather than UT1, as it would appear that Andres Ruiz is already doing for his navigational software.

    Geoffrey Kolbe
       
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