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    Re: Bubble sextant trials
    From: Bill B
    Date: 2007 Feb 15, 21:12 -0500

    Alex wrote:
    > Today I made my first observations with my new MkIXa
    > bubble sextant (purchased on E-bay for $41).
    With a lighted bubble sextant and a stable platform, twilight is no longer a
    limitation for star sights.
    Most texts use upper transits of the sun to derive latitude directly, or
    Polaris.  Despite lower availability, why not other stars etc.?  No pole
    star in the southern hemisphere to rely on. And why no mention of lower
    transits of circumpolar bodies for direct latitude?
    If trying to pull longitude from a sun transit, the exact time of transit is
    problematic by altitude, so we take sights before the expected transit and
    after and plot. Or take one perhaps 15 minutes before expected transit and
    then match that Ho after transit and split the time to determine transit
    time.  That requires a perfect observation on the front and backside, as
    well the problem of declination changing over a period of 30-40 minutes time
    (albeit about noise given the system resolution).  Stars have a relatively
    stable declination, but the viewing period with a conventional sextant would
    be a problem.
    Why not combine the best of all worlds?  The problem with a time site that
    gives you longitude directly is knowing your latitude.  It has been said, "A
    good sailor always knows his latitude."  Yet the basis for the discovery of
    Sumner's celestial LOP was the fact he was not sure about his latitude. Pull
    latitude off Polaris or another transit, and do a time site from a body to
    the east or west almost simultaneously.  Almost nstant lat and lon.
    I may have reinvented the wheel here, but it was fun to think about.
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