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    Re: Vernier sextant
    From: Alexandre Eremenko
    Date: 2007 Mar 6, 13:51 -0500

    
    On Tue, 6 Mar 2007, Bill wrote:
    
    > What is the purpose of the averager,
    > and how (mechanically) does it achieve
    > that function?
    
    The purpose of the averager is the same as the purpose of
    averaging sights with the usual sextant: to decrease the
    influence of small random errors. When instead of one sight
    you reduce the average of n sights, the theory says that
    the small random errors will be reduced by a factor of
    sqrt(n). So the average of 9 sights is likely to be more
    precise than one sight by a factor of 3.
    
    Now taking, recording and averaging 9 sights by hand
    is time consuming, even with a calculator.
    That's why the air sextants have a mechanical averaging
    device, which can average about 60 sights taken in 2 minutes.
    The small random error (of the sort the averaging kills)
    are expecially important with this sextant because
    a) airplane shakes and
    b) your hand shakes,
    c) the bubble shakes
    d) you cannot match the Sun and the bubble as well as
    you match two sun's images in the ordinary sextant.
    
    All this shaking produces small errors which are equally
    likely to be positive and negative, so they are supposed to cancel
    in the average.
    
    The mechanical averaging device is a funny clockwork mechanism.
    You have to wind it with the key, then align the Sun and the bubble.
    Then you trigger the averaging mechanism and try to keep the
    Sun and the Bubble alinged as well as you can for two minutes.
    A lever connected with the index glass protrudes from the sextant
    to the averager, and it is the position of this lever that
    the averager records every two seconds through a sophisticated
    cogwheel mechanism. The mechanism adds all these positions
    and divides by 60; all this is performed by a system
    of cogwheels.
    The principle of adding mechanism is simple, it is roughly the
    same as the bicycle device that records the distance traveled
    by counting the rotations of the wheel. Dividing is also easy
    with the cogwheels.
    
    In the end you see the result on a digital indicator
    which consists of two wheels: for 10ths of minutes and single
    minutes. Then you record the time by the usual watch,
    and subtract 1 minute (so that the time is the average time,
    corresponding to the middle of your
    observation).
    
    Now, I am not observing from a moving and shaking airplane,
    and I can align the bubble with the sun once pretty well.
    That is why, when observing from a balcony, sitting in a comfortable
    position, i usually do better without the averaging.
    Why exactly better? because in 2 minutes of keeping the Sun and
    the bubble aligned, my hand gets tired and starts to shake more:-)
    
    Anyway, my experience shows that the averager is useless for
    backyard navigation.
    However, on a small boat, which sometimes swings like crazy
    on the waves, I do not exclude that the averager may be of some use.
    One just has to experiment.
    
    The bubble sextant, with averager or without, certainly has some
    advantages on the small craft. Because the horizon is frequently
    obscured by waves.
    I know that C. Plath was making art horizon sextants (with averager!)
    for the use on submarines (the line of sight is very low),
    and that the Soviets cloned it after the war.
    They had an art horizon marine sextant.
    I almost bought one once, but then decided not to when I learned
    that its weight is 12 kg (!!). (Well, including the box,
    but anyway:-)
    
    Alex.
    
    
    
    
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