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What Dr. Kenchington says makes perfect sense (oh to have a sextant with
that kind of precision!). This brings up an interesting side discussion
that may or may not relate to celestial navigation.

Exactly how do space scientists (I use that term to describe the vast
collective of technicians and scientists required to keep a space program
going) propose that spacecraft will navigate in deep space; i.e. beyond
our solar system? I suppose there will come a time when humans have the
ability to travel beyond sight of the earth (the cosmic version of
offshore navigation) and will require some form of navigation independent
of our home planet.

Assuming that each star has a unique electromagnetic signature, perhaps
we will utilize a combination of this, and perhaps doppler shift, to
navigate in deep space? Clearly, this would require an instrument of
great compexity and sophistication; in which case there would be no
simplistic, non electronic backup navigation system.

Perhaps there is a die hard "trekkie" out there who can enlighten us with
a combination of science fiction an science fact.

Robert




>Frank Reed wrote:
>
>> On the Apollo 8 mission, for example, which orbited the Moon
>> in December of 1968, Jim Lovell entertained himself by shooting some
>> celestial sights. They confirmed the spacecraft's position to the level
>> of accuracy required, but everyone understood that Lovell was just
>> exercising his personal favorite astro-skill (Lovell was a Navy flyer,
>> more famous for commanding Apollo 13). And that's about all they did
>> with it. The astronauts found the sextant much useful for its telescope
>> than for its angle-measuring capabilities.
>>
>> By the way, if you would like to see a dramatization of the sextant
>> incident in Apollo 8, watch episode 4 of the HBO mini-series "From the
>> Earth to the Moon" (I highly recommend this series, by the way). The
>> actor playing Lovell mentions sighting Antares and Sirius.
>
>
>I don't understand what navigational information that would provide.
>
>Antares is 230 light years away from us, or some 70 parsec. (Sirius is
>about 8 light years or a bit over 2 parsec -- next door to us, for this
>purpose.) With the diameter of the Moon's orbit being about 0.25% of the
>Earth's, the effect of parallax on the measured angle between those
>stars would only be some 0.2 seconds as an Apollo capsule moved from
>Earth orbit to Lunar orbit. So, unless the sextant was incredibly
>accurate, Lovell would have got a much better idea of his position by
>looking out of the window to judge where he was between Earth and Moon
>than he could have obtained by measuring an angle between Sirius and
>Antares -- or any other pair of stars, come to that.
>
>Or was the intention to measure the angle of each star (separately)
>relative to the rim of Earth, Moon or Sun, then correct for
>semi-diameter? Three such angles would give enough data for a 3-D
>equivalent to an Earth-bound navigator's fix using horizontal sextant
>angles.
>
>Would sight-reduction of the latter type of observations have to follow
>notions analogous to the plotting of horizontal sextant angles, rather
>than the approaches that we use for celestial navigation on Earth, since
>the approximation of assuming that all heavenly bodies lie on a
>celestial sphere would become too imprecise? Or would that approximation
>remain sufficient for the stars? If so, there would still be the problem
>that the primary piece of information of interest would be the capsule's
>distance from Earth -- meaning that the navigator would be trying to
>estimate his "height of eye" from his star "altitudes", rather than
>treating it as a fixed input to the calculations.
>
>
>Maybe I am missing something here.
>
>
>Trevor Kenchington
>
>
>--
>Trevor J. Kenchington PhD                         Gadus@iStar.ca
>Gadus Associates,                                 Office(902) 889-9250
>R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
>Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
>
>                     Science Serving the Fisheries
>                      http://home.istar.ca/~gadus

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