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    Re: Advice concerning sextants
    From: George Huxtable
    Date: 2010 Dec 31, 00:28 -0000

    Patrick Goold wrote-
    "I began shooting ten minutes before clock noon to be sure to get the LAN"
    Yes, that would certainly have worked, in the circumstances, but "clock 
    noon" isn't what governs the moment of maximum altitude.
    What matters is the time of Local Apparent Noon, which is affected by two 
    factors. One is how far he is displaced from the midpoint of his time zone, 
    which is at 75�W. As it happens, his location is some 78 arc-minutes 
    further West than that, so the Mean Sun would pass his meridian about 12:05 
    pm. But then, at that time of year, the Apparent Sun, which is what 
    matters, is lagging behind the Mean Sun by about 2min 30 sec, so the Sun 
    wouldn't have passed his meridian until nearly 12:08 pm. So he didn't need 
    to start his observing so early, though it did no harm.
    I don't have an Almanac for 2010 to hand, but wonder whether the Sun 
    declination was taken from the Almanac at the time of Greenwich noon, 
    rather that at the time of the observation, 5 and-a-bit hours later, as it 
    should have been. If so, that would give rise to an error of some 0.6 
    arc-minutes: not a very serious matter, because dec. is changing only 
    slowly at this time of year, but worth allowing for nevertheless.
    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ----- Original Message ----- 
    From: "Patrick Goold" goold@vwc.edu
    *After this experience and after reading Gary's advice, I did things a 
    differently today.
    I used the lower limb observation instead of superpositioning.
    I used oil in the horizon rather than water, to reduce rippling
    I put the horizon outside an hour before noon so there would be no fogging
    I began shooting ten minutes before clock noon to be sure to get the LAN
    The result was a big improvement over yesterday's!
    Sextant reading   59� 26.6'
    No index correction
    Halved   29� 43.3'
    Lower limb correction +14.6'
    HO = 29�  57.9'
    ZD = 60� 02.1'
    declination = 23� 09.2'
    latitude = 36� 52.9'
    gps latitude = 36� 50.01
    The improvements to the artificial horizon plus a drop in the wind were
    probably the most important difference.  Taking the lower limb probably had
    an effect, too.  Finally, beginning to shoot early, I was able to follow 
    sun to its zenith and then detect the moment when it began to fall away 
    I look forward to using a real horizon soon.   Frank's observations about
    local geography put me in mind of a number of possibilities.

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