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    Time and N.A.
    From: Herbert Prinz
    Date: 2001 Jul 10, 8:43 AM

    George Huxtable wrote:
    
    > However, I have since found that the lunar distance predictions in the
    > Nautical Almanac switched from apparent time to mean time somewhere in the
    > early 1800s.
    
    Not just the lunar distances, but the argument of the entire ephemerides used
    to be apparent time up to the early 19th century. (An interesting exception are
    the eclipses of the satellites of Jupiter, which were tabulated in mean time.
    E.g. in Blunt's edition of  Nautical Almanac and Astronomical Ephemeris for
    1819.)
    
    Apparently, Portugal (Coimbra) and Prussia (Berlin) were the first to change
    over to mean time. I don't know the exact year. The English followed in 1834,
    in compliance with a recommendation of the Astronomical Society of London. (See
    Explanatory Supplement to the Astronomical Almanac, 1992, p.611.) Schumacher's
    distance tables for 1833 are still for apparent time. I am not sure when Paris
    switched over, all I know is that the Connaissance des temps for 1845 is
    already tabulated in mean time. For a while, the French had an interesting
    twist on the equation of time: L'equation de l'horloge, which is NOT identic
    with EoT, but offset from EoT such that it would be zero on Nov. 1 and positive
    for the rest of the year.
    
    It's a good idea to always check carefully what is being used as argument, when
    using almanacs of that time. By the way, since this has not been mentioned
    before in the context of this discussion, I would also remind you all that
    there is another pitfall concerning time: For the American Nautical Almanac,
    until 1924 (inclusive) the astronomical way of reckoning of time from noon in
    Greenwich was used. From 1925 on time was reckoned from midnight. Again,
    different almanacs switched at different times (if at all). I seem to remember
    vaguely having read a story about a sailor (Slocum, Chichester?) giving up on a
    lunar that did not work out because he was not aware of  the shifted time
    reckoning. Does this ring a bell for anybody?
    
    Best regards
    
    Herbert Prinz (from 1368950/-4603950/4182550 ECEF)
    

       
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