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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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