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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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