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    Re: 1st lunar attempt
    From: George Huxtable
    Date: 2003 Apr 26, 00:03 +0100

    Thanks to Herbert Prinz for making some very valid points, which I missed.
    I agree with almost all he says, but I think he is being over-pessimistic
    in saying-
    
    >You probably missed your chance this month.
    
    I agree with his figure of 56 deg for the separation between Sun and Moon
    on Saturday. With that angle between them, the Moon will appear very pale
    in the sky, once the Sun is up, but with clear air it should be possible to
    pick it out with the naked eye, and if so, it should show clearly in a
    sextant's telescope. It's worth a try, especially as Doug is an experienced
    celestial observer. Both bodies should be in position to be observed, from
    a couple of hours after dawn until mid-afternoon. I agree with Herbert that
    by Sunday the Moon and Sun will be so close together that the Moon will be
    too pale and it would be very, very hard to see in daytime. If we get any
    clear skies here over the weekend, I will see what's visible from here.
    
    I have presumed a geographical location for Doug of N 35, W 120, simply
    because they are round numbers and because I think I recall Doug stating
    that he was based in the West.
    
    From that position, at around 12:30 GMT, the Moon will be at a usable
    altitude of 14 deg, with the Sun somewhere between civil and nautical
    twilight, so brighter stars should then be well visible. There should be a
    reasonable choice of stars West of the Moon, or Mars, as Herbert suggests.
    But there's a very narrow time-window on Saturday, which would shrink to
    almost nothing on Sunday (as Herbert points out).
    
    I would agree, then, that there's little or no chance of getting any value
    out of Sunday. But I suggest Doug would find it worthwhile to get up well
    before dawn on Saturday, and if the sky is clear enough to reveal the Moon
    and also Mars or identifiable stars near its path (shown by the line of
    symmetry bisecting the Moon between its horns), to get cracking with his
    sextant. Similarly, after sun-up, if he can make out the Moon in the sky,
    he could take a shot at a Sun-lunar. I think it's worth a try. otherwise,
    he will have to wait until about 6 May.
    
    And it points up an interesting lesson about one of the big limitations of
    lunars: how unavailable lunars were to early navigators who relied on them,
    for quite long periods around new moon. And shows up a big advantage of
    chronometers, available all the time (though a view of the sky was still
    needed, to use a chronometer). The optimum arrangement was to have a
    chronometer on hand for use when needed, but to check it against a lunar
    when lunars were possible.
    
    Herbert may be correct in his assessment of the difficulties in measuring
    altitudes with a "makeshift" reflector under Saturday's conditions. But it
    seems to me unsatisfactory to rely on a watch to provide GMT for
    calculating altitudes, when your whole aim, in measuring a lunar, is to
    obtain GMT. However, that process does allow an estimation of how much
    error occurred in the lunar-distance measurement. Whenever possible, I
    suggest that altitudes of the bodies should be measured, as part of a lunar
    observation that is satisfyingly complete.
    
    George.
    
    ================================================================
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
    
    

       
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