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    Re: Dropping leap seconds and the impact on celestial navigation
    From: John Huth
    Date: 2011 Sep 9, 07:17 -0400
    I have a naive question, which may reflect my misunderstanding of UTC.    I always think of the time standard as 'mean solar time' - placing the sun, on average, over the prime meridian at noon.   Does this shift imply a slow drift away from this?    I think you say this in your message, but I wanted to double check.  

    That being the case, what becomes the standard?   A clock somewhere?   What happens to the definition of right ascension - does it remain the same?

    I guess I have a fairly deep philosophical issue - if the historical basis of 'time' was a synchronization to celestial events, then I have very serious reservations about severing this bond.   I suppose you can point out that the heavens are not immutable, but mean solar time seems to me to be a gold standard.  

    Of course, you can say that standardization only began for navigators around the period we set clocks to GMT and that standardization for normal folks occurred at least a century later, but the historical basis has always been the sun in one form or another.

    What's the motivation for the shift?
     

    On Thu, Sep 8, 2011 at 10:00 PM, Frank Reed <FrankReed{at}historicalatlas.com> wrote:

    I have committed to do a presentation at the colloquium next month in Pennsylvania (Oct. 5/6) regarding the proposal to drop leap seconds from the definition of UTC. One of the things that I want to do is get opinions from NavList members. Suppose leap seconds are dropped from time-keeping. Since the Earth's rotation does not quite match the definition of the second, the Earth's rotation will slowly fall out of line with the mean times kept on the vast majority of the world's clocks. If this proposal takes effect, then, in the year 2025 for example, if you calculate when the star Sirius crosses the meridian at your location naively using UTC, you might find that it is seven or eight seconds late. Similarly "local noon" would not occur at twelve o'clock, even on average, even at the center of a time zone. This is easily observable, and this sort of discrepancy could easily put a vessel's position out by a couple of miles. Of course this would only happen if we were to BLINDLY apply the rules without correction. The simple solution is to include a table in the annual Nautical Almanac and other "nautical almanac" equivalents giving the "watch error" of UTC relative to navigators' time (whatever that might be). There might be other ways to handle this, too. In principle, this should be no problem. In practice, there may be serious education issues. Since celestial navigation is primarily a backup method of navigation, and many people who would use it in an emergency could be fairly described as "rusty" on the details, would a navigator so be confused in an actual case where the methods were needed that navigation would be compromised?

    I would like to get as much input on this as possible. All opinions welcome! I need to finalize my presentation within the next couple of weeks, so start thinking and get back to me as soon as possible. Naturally, I will give individual credit where credit is due (for any non-obvious ideas!) and general credit to the NavList membership for your help.

    -FER

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