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    Re: Time of meridian passage accuracy
    From: George Huxtable
    Date: 2009 Sep 26, 23:41 +0100

    Douglas Denny wrote-"No need for retractions I think."
    
    No, I thought that might be too much to expect.
    
    "If stationary - as I was with my observations (and which you ignored) -
    then culmination of the apparent (true) Sun is by definition Local Apparent
    Time of the (true) Sun."
    
    Perhaps "culmination" was an unfortunate word for us to be using between us,
    because it can be ambiguous.
    The Oxford English Dictionary states- "Of a heavenly body: To reach its
    greatest altitude, to be on the meridian.", as though these two events
    coincided. But for the Sun, they differ, even for a stationary observer.
    That is the very nub of our discussion. So let's avoid "culmination".
    
    If we rephrased Douglas' statement, above, to read-
    "Local Apparent Time is, by definition, 12 hours at the instant when the
    true Sun crosses the observer's meridian", then I suggest both of us might
    accept those words
    
    But that is not the instant of greatest altitude of the Sun; not even for a
    stationary observer, because of the Sun's changing declination. The
    difference may never be much more than a quarter of a minute of time, but it
    isn't zero (except at the solstices).
    
    (For the Moon, because its declination changes much faster, the time
    difference between meridian passage and maximum altitude can amount to many
    minutes.)
    
    In many situations, such as deciding on the best moment to observe for a
    noon Sun latitude, that time difference can indeed be ignored, for an
    observer who is stationary or slow-moving. But when the longitude is to be
    derived from timing the Sun's changing altitude, it must be considered;
    otherwise, it will just add a systematic error to the result, which varies
    with time-of-year. That's what Douglas has neglected to take into account.
    If he is simply claiming that such a correction is too small for him to
    bother with, in view of the limited precision of the observation, then that
    would be fair enough. However, he appears to be denying the case for such a
    correction at all, as I read his postings. Is that his position?
    
    Douglas added-"This is all treated with rigor in Charles H. Cotter's book
    'The Complete Nautical Astronomer' with full formulae for calculation of
    these effects and full derivations."
    
    Thanks for bringing that Cotter book to my attention. I hadn't come across
    it before. Cotter also treats these questions in his "History of Nautical
    Astronomy", (1968), but some aspects are confused, and several of his
    equations are wrong. Perhaps he had got things clearer by 1969.
    
    =============================
    
    I ask again: what happened to the claim, in [9932], that-
    
    "The difference is 0.000737068 degrees,  or 0.04422408 minutes,  or 2,6
    seconds of arc.
    
    Utterly  negligable "
    
    Does Douglas wish to defend that claim, or perhaps withdraw it? What does it
    mean?
    
    George.
    
    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.
    
    
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