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    Re: Bubble sextant trials
    From: Alexandre Eremenko
    Date: 2007 Feb 15, 21:53 -0500

    You are welcome to borrow my copy of Lecky.
    He is a big fan of stars (and opponent of the Lunars)
    and he advocates meridional altitudes of stars.
    Not only upper culminations but
    also lower culminations.
    (Of circumpolar stars)
    But he also writes that "soon the LOP navigation
    will be the only method used in practice",
    and was is right.
    The Russian manual of 1966 or so, has a brief discussion
    of "near meridian" Sun altitudes, but nothing else
    except LOP. The only reason why meridian and near meridian
    altitudes survived to 1966 was that the reduction
    is very simple. With the spread of calculators,
    this last reason disappeared.
    In asto navigation manuals for airplanes
    (I read the 2 volume set by Chichester
    he wrote this before his circumnavigation when he
    was working in the MkIX factory during WWII. Purdue
    aerospace library has it. I can check it for you
    if you wish)
    he also spends a lot of time on stars
    (rather than the Sun) emphasizing that you can have
    an immediate fix with two stars.
    And the stars are always visible from an airplane
    at night, and you don't care about the horizon etc.
    On Thu, 15 Feb 2007, Bill wrote:
    > 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.
    > Bill
    > >
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