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    Re: A Ball Bearing in a dish as a reference
    From: Paul Hirose
    Date: 2018 Jun 8, 22:18 -0700

    I have a photographic tripod with a metal ball bull's eye level. The
    thing is almost useless. It lacks the sensitivity and smoothness of a
    bubble.
    
    The Apollo 11 passive seismic experiment had a similar device, and Buzz
    Aldrin had trouble centering the ball. "I'm having a little difficulty
    getting the BB to go in the center. It wants to move around and around
    on the outside."
    
    Neil Armstrong came over to help and wasn't any more successful: "That
    little cup is convex instead of concave."
    
    "I think you're right... Houston, I don't think there's any hope of
    using this leveling device to come up with an accurate level." They told
    Aldrin to level the seismic experiment by eye and press on.
    
    A few minutes later Houston requested a picture of the ball. Armstrong
    volunteered to return to the experiment and take the shot. "Would you
    believe the ball is right in the middle now."
    
    The laser reflector assembly also had a level. Armstrong called it a
    "bubble" and had some difficulty, though he did finally get a good
    alignment. Near the end of the moonwalk the announcer in Mission Control
    said, "The Lick Observatory in California reports a return on the laser
    experiment."
    
    With the Apollo 11 laser reflector a new era began in astrometry. The
    modern JPL lunar ephemerides are based entirely on range measurements to
    reflectors left on the Moon by American astronauts and Russian landers.
    The former methods of the transit instrument and lunar "theory" have
    been replaced by the laser and numerical integration, at least at JPL. I
    think lunar theory may still enjoy some vogue in France.
    

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