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    Re: Gyro sextant
    From: Bill Morris
    Date: 2019 Feb 12, 19:45 -0800

    Thank you, David, foryour comments.

    people might assume that the gyro artificial horizon is the panacea, and that it must be preferable to the bubble or the pendulous reference artificial horizons. What do aircraft pilots prefer? A piece of wool taped to the winscreen, a turn and slip indicator or an artificial horizon indicator?

    then there must be a side force applied to the pivot by the cup, and the gyro will begin to precess. I don't doubt this, but it must be for a very short time, and of very small magnitude, as the following suggests. I trained the sextant on the sun near noon, when its altitude was changing slowly. Rapidly rocking and moving back and forth, swaying and moving from side to side. and rotating resulted often in loss of the sun's image from between the graticule lines, but as soon as I got it back between the lines, the graticule embraced it perfectly steadily. I have asked the owner of a large launch to take me with him when he goes fishing so I can try the sextant's behaviour at sea, but I expect he will forget to do so...

    Might some of this be due to hang-off caused by Earth rotation? I am not certain that I have completely understood you, but I am inclined to doubt this, as I used a horizontal collimator set with a ten second level, which of course senses local vertical. Once the cross wire was centred in the graticule of the sextant, that is where it remained with no observable change. Try as I may, once the rotor has settled down for five minutes I cannot make an observation that suggests significant precession at an observable rate, though it must be there.

    Bill Morris

    Pukenui

    New Zealand

       
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