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    Re: Difficult observations
    From: Bruce Hamilton
    Date: 2009 Nov 20, 10:23 -0800
    Urban and Bush Celestial Observation Problems:

    You sea dogs will laugh at me, but I did get my interest in land celestial navigation from Geoffrey Kolbe's wonderful web page on his desert travels. The only time I use a sextant on water is when i am on BC Ferry somewhere.

    In the city, if a body is close to the top of a building on a cold night, the refraction caused by the heat of the building can cause error.  I can usually get an accuracy of 3 to 5 miles with my A12, but one night I was observing Venus as it was just over the roof of a high rise about a mile from my position, and I could not get any sight to be closer than 10 miles. Once it was far away, my sites were in the 3 mile range.

    When I use my sextants in the bush, I find that shooting a body when close to the top of a hill on a hot day can cause very erratic sights. Taking a site at a body peeking through dense leaves has also given me wild readings. I was shooting the moon just as it rose above a close rock hill after a hot (35 DegC) day.  I could see the heat shimmering, and my reading were way off until it got way clear of the hill.

    Bruce

    On Fri, Nov 20, 2009 at 9:17 AM, Anabasis <jcaoy---.com> wrote:
           In the world of taking observations, as opposed to reducing them, I
    was thinking of the observations that are the most difficult to shoot
    at sea and thought that I’d make a short list.  I am wondering what
    others might think and how they rate different observations in order
    of observation difficulty.  I am not going to list such main stays as
    sun lines, azimuths, and upper transits.
           By difficulty I mean to not only observe them, but to get useful
    data.  In any case, here is the list of exotic sights that I’ve taken
    in order of difficulty

    1)      High altitude sights of the moon (Ho >89 degrees)
    2)      High altitude sights of the sun
    3)      Amplitudes of stars/planets
    4)      Amplitudes of the moon
    5)      Nighttime (by moonlight) star fix
    6)      Lower transit of a star (due mostly to low altitude)
    7)      Lunar distances

    I’d be interested in hearing what other sights have proven troublesome
    to navigators out there.

    Jeremy

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