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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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