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    Re: Gyro sextant
    From: David Pike
    Date: 2019 Jan 25, 15:47 -0800


    I now believe that a bottom heavy gyroscope will be affected by acceleration errors.  In the bubble sextant, the liquid lags on the acceleration, and the bubble moves in the direction of the acceleration.  If you like, the liquid acts like a pendulum.  Because it’s bottom heavy, the Plath gyro would also like to act as a pendulum, but it can’t act in the same way as the liquid in a bubble chamber, because of its angular momentum.  Instead, the torque due to the acceleration/inertia couple between the pivot cup and the CG causes the spinning gyro to start to precess at 90 degrees to the acceleration direction.  This isn’t quite the same as a bubble sextant.  In a bubble sextant a fore and aft acceleration affects a fore and aft observation.  With a gyro sextant a fore and aft acceleration looks like affecting a beam observation.  However, this initial tilt soon turns into a rotating tilt of the artificial horizon, because the spin axis starts to follow the surface of a cone.  Fortunately, when the accelaration stops, this wobble soon dampens out for reasons even the Admiralty Research Laboratory were struggling to explain in 1945, and we are trying hard to explain in simple terms today.  DaveP      

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