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    Re: sextants on aeroplanes
    From: Gary LaPook
    Date: 2008 Nov 29, 02:57 -0800

    A marine sextant has severe limitations on its use in flight due to
    indistinct horizon problems and large and inaccurate dip corrections.
    The higher you fly the further the horizon is away from your position.
    The distance in nautical miles is, for practical navigation purposes,
    the square root of the altitude as measured in feet. From one hundred
    feet the horizon is ten NM away but at 10,000 feet the horizon is 100
    NM away. At 10,000 feet, if the visibility is less than 100 NM (as it
    is all the time over the sea) then you can't see the actual horizon.
    When using the marine sextant you have to make a correction for "dip"
    which is caused by the fact that you are looking slightly downward to
    the visible horizon and since you are making your measurements in
    relation to this depressed horizon all of your measured altitudes are
    to great and you have to subtract the angle that the visible horizon
    is "dipped" below horizontal. This correction in minutes of arc or in
    nautical miles is also approximately equal the the square root of the
    altitude in feet. At 10,000 feet this correction is 100 minutes of arc
    (actually 97 minutes) or approximately 100 nm. Because you measure
    your altitude with a barometric altimeter any change in weather and
    atmospheric pressure will cause your altimeter to give you an
    incorrect altitude and you will apply an incorrect dip correction.
    After a long flight is is quite easy to have an altimeter that reads
    wrong by a thousand feet since this would be caused by a one inch of
    mercury change in barometric pressure.
    The following advice comes from someone who has some knowledge of
    using a marine sextant in flight.
    "If accurate results are required, first fly at sea level to correct
    the altimeter, then rise to the maximum height from which the horizon
    is clearly visible."
    (of course, you can't always do this.)
    This comes from Sir Francis Chichester in "The Observer's Book On
    Astro-navigation, part four."
    In fact, he did exactly this when making the first flight from New
    Zealand to Australia across the Tasman Sea in a Gypsy Moth light plane
    in 1931 using a marine sextant to find two small islands on the way
    where he could refuel. His book about this adventure, "Seaplane Solo,"
    is wonderful to read and I can email it to anybody who requests it off
    On Nov 19, 2:18�am, glap...@PACBELL.NET wrote:
    > Do you mean marine sextants?
    > gl
    > On Nov 18, 11:20�am, "Jackie Ferrari"  wrote:
    > > Dear List,
    > > I understand that sextants as opposed to bubble octants were used to 
    navigate aeroplanes. I was wondering therefore how prevalent this was and 
    what were the advantages and disadvantages.
    > > Regards,
    > > Jackie Ferrari.
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