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    Re: Leap second today
    From: Geoffrey Kolbe
    Date: 2012 Jul 02, 07:27 +0100
    At 23:13 01/07/2012, you wrote:

    Geoffrey explains it well. But in practice we all use UTC, and as you explain UT1 (what NA uses) drifts from UTC. So in that context (when users insist on using the wrong time standard) the NA is "more wrong" without a leap second.

    In practice, as the NA effectively states, it is assumed that we always correct UTC to UT1 using DUT1. The NA also says that if you are happy with an error of 0.2' in longitude then you may assume UTC is the same as UT1, but never-the-less the underlying assumption remains true that a conversion from UTC to UT1 is always being made.


    There is no leap second inserted into the NA, but it does need UT1 which does not change in a predictable way, and so the NA must use a "best guess" on how UT1 will change in the following year.

    Well Bill, you raise an interesting point. The USNO publish current (and past) values of DUT1 with which you can correct UTC to UT1. So what is UT1 in this context? In practice, there is a model for the positions of the celestial bodies using algorithms based on UT1. Those algorithms contain a  "best guess" on delta T, but never-the-less that model defines UT1 for the purposes of the model with absolute accuracy and with a precision that is only limited by the computer that is calculating it. The fact that it might be an approximation is neither here nor there. What the USNO publish is the difference in time between where the celestial bodies (in particular, the moon) are expected to be from the UT1 model and when they actually appear in those positions using a UTC clock. Given that you had access to the complete model for the motions of the celestial bodies, the limiting factor in the precision of your observations would be the accuracy of the USNA observatory observations on the motion of the moon and their resultant determination of DUT1.

    So, what it comes down to is that how delta T varies during the year is irrelevant as far as the NA is concerned. UTC is corrected by DUT1 so that the model for the motions of the celestial bodies - notably the moon - which are based on UT1, are correct when compared to observation. UTC itself is corrected to track the assumed model for UT1 by the insertion of leap seconds from time to time. It is the assumed model for UT1 that is the superior time standard, not what the mean solar day actually is at any given time.

    Geoffrey Kolbe
       
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