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    Re: The development of bubble sextants
    From: Gary LaPook
    Date: 2009 Aug 15, 02:27 +0200

    Well, of course, coriolis is a fictional acceleration. You calculate it 
    with the formula:
     Z (coriolis correction) = .0262 times the ground speed in knots times 
    the sine of the latitude.
    What you are actually doing is correcting for the change in the 
    orientation of the latitude- longitude grid as the plane moves over the 
    surface of the earth which makes it appear that the vertical was 
    deflected as computed by the formula. You make the adjustment by 
    shifting the LOP or the AP or the final fix to the right (in the 
    northern hemisphere) of the track over the ground the amount calculated 
    by the formula. See attached table.
    douglas.denny@btopenworld.com wrote:
    > Three points:
    > 1)The curvature of the upper bubble chamber lens is equal to that of the 
    focal length of the lens/mirror arrangement so the bubble is in focus at all 
    times in all positions.
    > 2) The bubble chamber is circular so the bubble chamber/sextant can have 
    lateral movement (roll) as well as the important movement in pitch, to allow 
    some degree of lateral movement not easily possible in a straight (standard) 
    bubble tube arrangement. This is probably why the straight tube bubble 
    chamber was abandoned on marine sextants, or at least, not developed further 
    in favour of other methods.
    > 3) The argument seems to be - if I have it right: to attempt to eliminate 
    side accelerations affecting the bubble.  You cannot escape side (horizontal) 
    accelerations affecting the apparent gravitational vertical whatever you do, 
    as the 'compensating mechanisms'  will be referenced to the local 
    gravitational field too. Even if some hysteriesis is present between bubble 
    and 'compensator' mechanism,  both will have to come to the same conclusion 
    as to where the gravitational field is pointing. In other words you are back 
    to square one. 
    > Eary type aircraft artificial horizons are such a mechanism that uses local 
    gravitational field to provide an horizon (or hence vertical whichever you 
    prefer) - they have a long time constant errecting mechanism using small 
    pivoted vanes on four points around the gyro. The vanes 'dangle' downwards 
    due to gravity and provide precessional forces to bring the gyro to an 
    horizontal plane.
    > Coriolis force is such an example of horizontal acceleration that cannot be 
    eliminated; it has to be allowed for - luckily because it is a constant 
    feature and thus a known quantity.
    > Douglas Denny.  Chichester. England.
    > >
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