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    Re: It's Moon-landing Monday
    From: Frank Reed
    Date: 2009 Jul 23, 23:22 -0700

    Brad you wrote:
    "That would be an interesting exercise.  The sextant must therefore be aligned 
    to the thrust axis of the vehicle.  In other words, holding your sextant in 
    your hand and pointing it out the window (with the previously agreed exercise 
    on distance, RA and declination of the center of your vehicle) does not 
    really tell you how your vehicle is pointed!  Assuming that the roll of the 
    vehicle doesn't matter, then the vehicular pitch and yaw will not be known 
    UNLESS the sextant is referenced to the vehicle.  I am not sure how we do 
    that with our high end marine sextant!"
    How about this: take the telescope off the sextant and tape it to a window 
    (with duct tape, always assumed to be available) so that the end of the 
    telescope is flush against the glass. Now put the spacecraft in a slow roll 
    about the axis of thrust for the main engine. We can assume that this can be 
    done. Now you fire up your star chart software on your laptop and plot the 
    stars that you see while you look through the telescope during one roll. 
    Those stars will lie along a circle (probably not a great circle) on the 
    celestial sphere, and the center of that circle is the direction in which 
    you're oriented. When you fire your rocket, you will need to maintain that 
    orientation, so you have to fire the thrusters occasionally while you look 
    through the telescope maintaining whatever stars you see centered in the 
    field of view.
    On the Apollo missions, there was a sextant built into the spacecraft but it 
    was rarely used as a sextant is intended to be used --to measure angles 
    between objects. Instead it was used to get spacecraft orientation (since it 
    was built in, it was aligned to the spacecraft axes in a known way), and it 
    was used as a telescope to observe the lunar surface. 
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