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    Re: Celestial nav on land: info needed
    From: Geoffrey Kolbe
    Date: 2002 Feb 11, 08:48 +0000

    Hello Ron
    Celestial navigation on land is a different problem than celestial
    navigation at sea. Instruments and techniques that are efficient at sea
    become cumbersome on land - like a fish out of water! Different instruments
    and different techniques become more appropriate.
    Using a marine sextant with an artificial horizon on land can be difficult
    due to the resulting small field of view. Finding the sun in the reflected
    image of the sky during the day is not too bad. There is only one sun, it
    is pretty big, and pretty bright, and you can see where the artificial
    horizon is. But at night, when you are doing sightings of stars, you can
    get confused picking out the star of choice in the reflected image -
    assuming you don't loose the artificial horizon altogether in the gloom!
    Using many bubble sextants at night can be tricky too, due to the problems
    in finding the star of choice.
    For sightings at night, pre computation of the expected altitude and then
    setting the sextant to the appropriate angle helps to find the star of choice.
    One sextant which does not have this problem is the Link Aviation type A-12
    bubble sextant. With this sextant (more precisely an octant) the night sky
    can be viewed directly though the index mirror affording a good field of
    view so there is no problem in picking out the star of choice. The
    illuminated bubble is reflected off the back of the index mirror and the
    drum is adjusted so that the bubble is centered over the star. The
    illumination can be adjusted for good contrast.
    This sextant was designed before WWII for use in open cockpit aircraft. But
    for handiness and ease of use both by day and by night, I think that the
    Link A-12 sextant is about the best sextant for use on land that there is.
    Its only slight drawback is that the vernier is only calibrated in 2 minute
    intervals, but with care you can estimate the reading to one minute.
    I often use my A-12 to make sure my house has not drifted off down the
    valley since the last time I checked.  I am usually comforted to find that
    it is still within a mile of so of where is should be.
    I would recommend that you get your hands on a A-12 sextant if you can.
    Celestaire advertised them for about 15 years but have now stopped. Perhaps
    they ran out, but they may have a lead on one.
    Geoffrey Kolbe.
    At 21:29 10/02/02 -0800, you wrote:
    >Can anyone recommend a book or other source of info that details
    >techniques and tips for using celestial navigation on land? I'm new to
    >this, and although I've seen and tried an artificial horizon, I realize
    >there must be more to it. Any help would be appreciated.

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