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    Re: Navigation without Leap Seconds
    From: Fred Hebard
    Date: 2008 Apr 15, 12:45 -0400

    Lu,
    
    Why billionths of an arcsecond?  One arcsecond gets one to 1/60th of
    100 feet in traditional surveying, or about 50 cm.  One-thousandth of
    an arcsecond would drop one to 5 mm.  I wonder if refraction is a
    problem here.
    
    Fred
    
    On Apr 15, 2008, at 12:33 PM, Lu Abel wrote:
    >
    > 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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