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    Re: Sun Moon Lunars to 155 degrees
    From: George Huxtable
    Date: 2010 Mar 30, 22:53 +0100

    Brad wrote-
    | Rather than turning LAT into LMT and then adjusting LMT for the
    difference in longitude from
    | the central meridian, just use LAT directly.
    WHY? Where has equation of time got to?
    LAT was determined by observation of the Sun, so apparent time.
    Frank's lunar predictor provides GMT, Greenwich Mean Time.
    Before you look at the difference between those two times, you have to put
    them on the same time base. Apparent time and mean time differ, through the
    year, by a variable amount, of up to a quarter of an hour either way, the
    equation of time. If you don't allow for the equation of time, you will get
    the wrong answer.
    The other question, that Kermit keeps mentioning, relates to delta-T, due
    to slowing of the Earth's spin. Bayly's own calculations were correctly
    based on the almanac of his day, and required no correction for delta-T.
    Modern back predictions of the positions of astronomical bodies should
    always include an allowance for the slowing of the spin, and should say so.
    If they don't allow for delta-T, they should say so, too. I presume that
    Frank's lunar calculator allows properly for past delta-T, which is not
    hard to do. The difficuly, with delta-T, is when you want to predict into
    the future.
    As I see it, Frank's lunar predictor for 1773 is intended to provide GMT
    values that relate to the time shown by a clock running on Mean Time at
    Greenwich, in 1773. Bayly's local apparent time is to be converted to local
    mean time, and the difference between those mean times is his longitude. No
    allowance has to be made for delta-T.
    Please argue back if you disagree.
    contact George Huxtable, at  george@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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