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    Re: Apollo spacecraft sextant
    From: Trevor Kenchington
    Date: 2004 May 2, 22:23 +0000

    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{at}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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