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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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