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    Re: Navigation without Leap Seconds
    From: Lu Abel
    Date: 2008 Apr 14, 20:47 -0700

    Frank:
    
    The passage of time is very real -- it's the thing that physicists worry
    about -- for example the slowing of time with velocity or gravitational
    dilation, or the fact that it's the one thing in physics that seems
    irreversible (although Richard Feynman claimed that a positron
    (positively charged electron) was in fact an electron moving backwards
    in time).
    
    But answering the question "what time is it?" is a man-made thing.  Over
    time (hah!) mankind has had lots of ways of answering the question.
    Days started at different points -- sundown, midnight (which in itself
    is really half a day from noon), sunrise, and so forth.  And how to
    describe the passage of time over that day?   We use hours, minutes and
    seconds.   But the French tried to introduce a time measurement of 10
    hours of 100 minutes of 100 seconds as part of the metric system  (what
    fascinates me is why meters and grams and such stuck, but metric time
    didn't stick)
    
    At some point we got so good at measuring time (or, accurately, the
    passage of time) that we could see that the rotational period of the
    earth varied and that it was slowing.   We could average the earth's
    rotation and thereby figure out very precisely how many vibrations of
    some atom would constitute 1/86400 of a day.   But the earth's slowing
    required the injection of "leap seconds" so midnight would still occur
    at midnight and not a second or two or seven earlier.   But again, this
    is a man-made artifact.   A second is still a second, a minute is still
    60 seconds, an hour is still 60 minutes -- but a day (the rotational
    period of the earth) is sometimes a bit more than 24 hours.
    
    All that GPS requires is that all satellites have the same notion of
    "what time is it"   Yes, GPS doesn't use leap-seconds, but as long as we
    all agree on what time it is, it doesn't matter if our "time" is off by
    a second or two from leap-second time, just as GPS doesn't care what
    time zone we're in or whether we're practicing Daylight time.
    
    And that last sentence is the key to your question of "how would I
    practice celestial navigation if we didn't have leap seconds"   As long
    as I could convert my local time into Almanac time, it wouldn't matter,
    just as I adjust from local time to GMT.  If the celestial data in the
    almanac were calibrated in leap-second-less time, no correction would be
    required.  But if my watch somehow showed leap-second-less time while
    the Almanac used them, then I'd have to know how many leap seconds had
    occurred since 1972 (the year leap-seconds were adopted).
    
    Meanwhile, GPS uses a "what time is it" based on UTC in 1980 but not
    adjusted for leap-seconds thereafter (which means GPS time is 14 seconds
    fast as compared to UTC).   GPS receivers correct for this to give a
    "true" (ie, leap-second-corrected time display), but buried somewhere in
    the setup menu for most GPS receivers is an option to display GPS time
    instead of UTC or local time.
    
    Hope this helps.
    
    Lu Abel
    
    
    frankreed@HistoricalAtlas.net wrote:
    > The fascinating article which Fred Hebard linked:
    >  http://www.physicstoday.org/vol-59/iss-3/p10.html
    > includes a detailed discussion about the problems of gravitational time
    > dilation and extremely accurate clocks. That's the main topic, and it's
    > great stuff.
    >
    > The article also mentions leap seconds and navigation:
    > "Celestial navigators --that vanishing breed-- also like leap seconds. The
    > Global Positioning System, however, cannot tolerate time jumps and employs a
    > time scale that avoids leap seconds."
    >
    > So here's my question: what's the best way of doing celestial navigation if
    > leap seconds are dropped from official time-keeping? I don't think it should
    > be all that difficult to work around, but I'm not sure what the best
    > approach would be. Assume we get to a point where the cumulative time
    > difference is, let's say, 60 seconds (that shouldn't happen for decades, so
    > this is just for the sake of argument). Should we treat the difference as a
    > 60 second clock correction before working the sights? Or should it be a 15
    > minute of arc longitude correction after working the sights? Or something
    > else entirely??
    >
    >  -FER
    > Celestial Navigation Weekend, June 6-8, 2008 at Mystic Seaport Museum:
    > www.fer3.com/Mystic2008
    >
    >
    >
    > >
    >
    
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