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Yes, it does.  One gathers elevation information with radar ranging;
it's the same problem, you're just at a different elevation, so
there's a larger (!) dip correction.  It was the method proposed by
Weems, et al, in the delightful book, "Space Navigation Handbook,"
Navpers92988,  US Govt Printing Office: 1962 0-628762.


On Apr 15, 2008, at 7:27 PM, glapook@pacbell.net wrote:
>
> Gary LaPook writes:
>
> But that doesn't solve the problem. The only reason that CN works on
> the earth is that the direction of "up" varies with your position on
> the earth. The altitudes measured on earth (and in aircraft) rely on
> the direction of "up" for the measurement. The sea horizon used with a
> marine sextant is where it is due to the local gravitational field
> which causes water to assume a shape at right angles to "up" and
> gravitational "down." A bubble sextant uses a bubble to sense "up."
> Because local "up" changes at a constant rate of one nautical mile per
> minute of altitude we can find our place on or above the surface of
> the earth. This relationship does not hold on the way to the moon.
>
> gl
>
> On Apr 15, 2:12 pm, Fred Hebard  wrote:
>> 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,
>>>>>>>> frankr...@HistoricalAtlas.net wrote:
>>
>>>>>>>>> The fascinating article which Fred Hebard linked: http://
>>>>>>>>> www.physicstoday.org/vol-59/iss-3/p10.htmlincludes 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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