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    Re: The moons of Jupiter for Time or Position?
    From: Don Seltzer
    Date: 2016 May 6, 09:20 -0400


    On Fri, May 6, 2016 at 1:48 AM, Francis Upchurch <NoReply_Upchurch@fer3.com> wrote:
    >
    > Seriously though, does anyone know about gyro telescope mounts? Has anyone tried this?

    Yes, the Trident guidance system people.  Part of the navigational hardware is a gyro-stabilized telescope that is 'pointed' at a chosen reference star prior to launch. During the boost phase the gyro platform attempts to keep the telescope pointed in the same direction.  After the booster rocket stages have burned out, a cover retracts and the telescope looks for the star at the center of its field of view.  Any angular errors are used to correct the inertial guidance system estimate of position and orientation.
     
    >
    > One of my fictional naval heroes, Jack Aubrey, in O’Brian’s books was a keen amateur astronomer/cel  nav. Enthusiast. He experimented with his version of the Irwin Chair, but I do not think they had gyros in those days.


    His telescope was a disappointment. It was not that he could not see Jupiter: the planet gleamed in his eyepiece like a banded gold pea. But because of the ship’s motion he could not keep it there long enough or steadily enough to fix the local time of its moons’ eclipses and thus find his longitude. Neither the theory (which was by no means new) nor the telescope was at fault: it was the cleverly weighted cradle slung from the maintopgallantmast stay that he had designed to compensate for the pitch and roll that did not answer, in spite of all his alterations; and night after night he swung there cursing and swearing, surrounded by midshipmen armed with clean swabs, whose duty it was to enhance the compensation by thrusting him gently at the word of command.

     - Mauritius Command

    Don Seltzer
       
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