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    Re: Keep checking your GPS
    From: Peter Monta
    Date: 2015 Feb 12, 02:13 -0800
    Hi Bill,



    How on Earth (an Earth that seems to be slowing down its rotation in relation
    to the Sun--for now and the foreseeable future in an unpredictable
    manner) do we "publish the leap-second schedule 10 or 20 or 50 years in
    advance?"
     
    We could only do so to some lesser accuracy than we do now.  Suppose we publish 50 years' worth of leap seconds right now, perhaps simply by using the average rate of leap seconds over the past few decades and distributing them uniformly.  After 50 years, our prediction might be in error by, let's say, 10 seconds, violating the current requirement that civil time be at most 1 second away from UT1.  But there's no need for such a tight tolerance---UT1 is widely and immediately available to much better accuracy.

    After 25 years we reassess and publish the next batch of 25 years' worth, so there's always at least 25 years of lead time (rather than a mere 6 months with the current scheme).  Errors won't accumulate because the leap-second rate would be "steered" to minimize the expected error.

    Opinions differ as to whether leap seconds are the right mechanism, assuming there's agreement that civil time should track UT1 within some broad window (but still much less than 1 day).  There are quite a few interesting variations that have been discussed (none of this is original with me by the way), such as the "leap hour" or, almost equivalently, ring-around-the-rosie with time zones.  If leap seconds were abandoned and civil time allowed to drift, it would still be several centuries before problems started, but it seems impolite to punt on the issue to that extent.

    I'm not sure where the 1-second tolerance originally came from.  It could have been celestial navigation, which would have been still relevant (barely) in 1970.

    Cheers,
    Peter

       
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