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    Re: Apollo spacecraft sextant
    From: Frank Reed CT
    Date: 2004 May 2, 20:14 EDT
    Jim T wrote:
    "Now THAT's astronavigation.  This is a highly significant milestone in the evolution of the sextant, and in the history of human navigation.  It means that humans first navigated to the moon in a similar way to how Cook opened up the Pacific Ocean centuries ago -- by doing celestial navigation with an optical instrument. "

    It really wasn't like that. Celestial navigation with a sextant was a minor footnote in the Apollo missions.

    You have to be careful when you read the operations manual's account of the Apollo sextant. This is a lot like reading a copy of Bowditch from the 19th century and assuming that you couldn't cross an ocean unless you did lunars every other day.

    The sextant on Apollo was frequently used as a convenient telescope when the lone, slightly bored astronaut in the command module was trying to spot the lunar module when the two spacecraft were (up to) 500km apart. But this was just a convenient accident having little connection with the instrument's navigational role.

    Navigation during the Apollo missions was conducted almost entirely by radar from the ground accompanied by calculations from the ground computers with lesser help from the onboard computer. The sextant and the techniques for using it were seen as an almost useless backup system by the time the missions to the Moon actually flew.

    During the early Gemini missions real experiments were conducted in celestial navigation in space. These experiments showed numerous problems, but also indicated that there was at least a chance that standard celestial could be adapted for flights to the Moon. By the time of the later Gemini missions, the astronauts all knew that they would not be using sextants for navigation on the Moon missions (they were already training for the new computer-based system), and some of the later Gemini astronauts apparently resented the time wasted on these continuing experiments.

    That is not to say that the sextant on Apollo was never used for navigation. 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 know if those are among the stars Lovell actually used in 1968, but the series was a labor of love from actor Tom Hanks, a big fan of manned space flight, and he was known for his attention to detail (Hanks, of course, played Lovell in the movie "Apollo 13").

    The series "From the Earth to the Moon" was largely based on Andrew Chaikin's excellent book "A Man on the Moon". Chaikin wrote in his chapter on Apollo 8:
      "When Apollo was first conceived it was thought that the astronauts would act not only as pilots but as onboard navigators. But the task proved so time consuming and ate up so much space in the memory of the command module's computer that planners decided to let this work be done by computers in mission control. Still, there had to be a backup, in case Apollo 8 lost communications with earth. In that contingency, Lovell would use the stars to help himself and his crewmates get home."

    And when trouble did occur sixteen months later on Apollo 13, Lovell, now mission commander, found that the sextant was nearly useless because it had been designed for primary navigation rather than backup navigation. Rather than try using the balky, precise sextant, Lovell was instructed to align the spacecraft by rough "eyeball" alignments of the Sun and Moon out the lunar module's windows. [another problem with a spaecraft sextant in an emergency: debris doesn't "blow away" in space (even Jules Verne understood this in principle). Apollo 13 was accompanied by a blizzard of false stars which made sextant sights completely impossible except when they were in the Moon's shadow which was considered too short a time to be useful].

    Sextants, of a sort, have indeed been installed on many manned spacecraft in the past forty years, but they have not proven to be an effective means of navigation. It would be interesting to know if any are carried today "just in case", and if not, when the last one was launched.

    Frank E. Reed
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
       
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