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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


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Trevor J. Kenchington PhD                         Gadus@iStar.ca
Gadus Associates,                                 Office(902) 889-9250
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