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    Re: Navigation without Leap Seconds
    From: Lu Abel
    Date: 2008 Apr 15, 09:33 -0700

    Fred:
    
    In theory, yes; in practice, no.
    
    To position oneself using star-star distances would require require
    measuring angles to billionths of an arc-second.   Maybe something an
    astronomer could do, but not something you or I are going to do with our
    sextants!
    
    BTW, I remember a conversation with a radio-astronomer about 20 years
    ago where he said that his team had measured the distance between two
    radiotelescopes on opposite sides of the US to within a cm or so using a
    technique called long-baseline interferometry.   But the whole
    experiment took them a year or so...
    
    Lu Abel
    
    Fred Hebard wrote:
    > Completely unrelated, but stemming from the same article.
    >
    > The author states that height can only be known to some few cm or
    > whatever because of variations in gravity, if I remember correctly.
    > It would seem that this is due to our tradition of assuming we are on
    > the surface of a spheroid or ellipsoid when doing navigation.
    > Confining ourselves to a surface makes the trig easier, but couldn't
    > one position oneself with greater accuracy (with feet firmly planted
    > on earth, not on a boat) using only stars or stars plus the sun,
    > ignoring the earth's horizon, by measuring star-star distances?  Make
    > it a true 3-D problem.  Or would uncertainties in the positions of
    > stars still hamper ones efforts, especially uncertainty in their
    > distance from us?
    >
    > Fred Hebard
    >
    > On Apr 14, 2008, at 9:50 PM, 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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