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    Re: Navigation without Leap Seconds
    From: Fred Hebard
    Date: 2008 Apr 15, 17:12 -0400

    I believe they measured altitudes from a limb of the Earth, more-or-
    less in the "normal" way.
    On Apr 15, 2008, at 4:00 PM, Gary J. LaPook wrote:
    > Gary LaPook wrote:
    >
    > If I remember correctly, the Apollo spacecraft had a sextant on
    > board used to mesure angles of celestial bodies in order to compute
    > their position in space on the way to the moon, (maybe only as a
    > backup.)
    >
    >
    > gl
    > Fred Hebard wrote:
    >>
    >> So it would have to be sun/moon/planet-star distances. I suppose
    >> those are limited by the low degree of parallax of the planets and
    >> sun, not to mention one has to know where one is on earth to
    >> determine the "position" of other bodies in the solar system,
    >> which I guess would be a circular argument. On Apr 15, 2008, at
    >> 12:54 PM, Lu Abel wrote:
    >>>
    >>> Fred: You're right about traditional surveying. But your proposal
    >>> is to use star-to-star distances to locate one (if I understand
    >>> correctly) in 3-D space relative to some very distant stars.
    >>> Imagine a couple of stars several hundreds of light-years away
    >>> (that's on the order of 10^20 cm). Suppose I move a few cm closer
    >>> to them. By how much would the angle between them change? Not by
    >>> much at all. Lu Fred Hebard wrote:
    >>>>
    >>>> 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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