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    Re: Historical Lunars : take in account 'delta-T' or ignore it ?
    From: George Huxtable
    Date: 2009 Dec 12, 00:04 -0000

    Antoine asks a lot of interesting questions about delta-T and lunar 
    I'm not sure I can answer them, but let me have a try at some aspects, to 
    make sure that I am following his reasoning.
    He wrote (section 2)-
    "how should we best proceed if we are to reprocess / re-compute historical 
    Lunars with to-day's computation tools?
    I would like to offer the following comments and will certainly be very glad 
    to hear feed-back from the NavList community, especially if view points are 
    different from the one I am detailing here-after.
    -*-*- First, we can state that, when they cleared their Lunars, the 18th, 
    19th and (if any) the early 20th centuries Navigators were not aware of the 
    "necessity" of using 2 time scales. In other words - and unless they used 
    data amended with 'observed corrective terms'(see �6 here-above) - they 
    (unknowingly) cleared all their Lunars with the Value delta-T=0.0 seconds of 
    As I see it, that was exactly the right thing to do. Mayer's predictions 
    were based on the contemporary observations of his epoch, the length of the 
    day (then) and the length of the month (then). They were related to the 
    observed instantaneous position of the Sun and Moon at some moment, and 
    predicted what their values would be subsequently, for some coming years. 
    Until many years had passed, and the slowing Earth rotation increased the 
    day-length, and so give rise to a significant change in the time-base, there 
    was no delta-T to take into account. So the navigators of the 1760s didn't 
    need to worry about any delta-T correction. Just like the way we use an 
    almanac for 2009 today, indeed.
    There is no absolute value for delta-T. We only use the value we do, of 65 
    seconds or whatever, because Simon Newcomb worked it all out in around 1893, 
    and chose to set delta-T to have a zero value in (I think) 1895. There's 
    nothing magic about the actual value of delta-T; all that matters is the 
    CHANGE in delta-T, between the epoch for which predictions are made, and the 
    date of the observations to which those predictions are being applied, or 
    compared. If you like, you can think of it as delta-delta-T, between those 
    If we wanted to check predictions made by Mayer, say, or Maskelyne, to 
    determine their accuracy, by comparing with the modern JPL or 
    Chapront-Touzet ephemeris, then we should work out what the modern ephemeris 
    predicts for that date and time in the 18th century, and then apply the 
    change in delta-T, between this date and that one.
    If we were foolish enough to check Mayer's predictions by extrapolating them 
    to the present day, that would be doomed to fail, because those predictions 
    were nowhere near accurate enough to take such a strain. But, in theory at 
    least, if we did so, we would then have to allow for the change in delta-T, 
    in the same way.
    Because the Moon is the object that moves fastest in the sky, with respect 
    to the stars, its position is by far the most affected by changes in 
    delta-T. For that reason, study of the history of such changes involves 
    looking at the dating and timing of ancient eclipses and conjunctions, and I 
    suppose lunar distances too, made by observers whose longitude can be known.
    I wonder if this is addressing the questions Antoine is asking? Perhaps 
    he'll say.
    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. 
    NavList message boards: www.fer3.com/arc
    Or post by email to: NavList@fer3.com
    To unsubscribe, email NavList+unsubscribe@fer3.com

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