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    Re: Almanac errors
    From: Frank Reed
    Date: 2008 Mar 11, 13:53 -0400

    Bill Noyce, you wrote:
    "Was that caused by the mix-up with leap year, that Nathaniel Bowditch
    made such a big deal about?
    Even though 1800 is divisible by four, it wasn't a leap year."
    I've been meaning to mention that the "almanac errors" we've been talking
    about are not errors, as in mistakes, but inaccuracies in the model of the
    Moon's motion. As theory improved and observational data accumulated, the
    ability to predict the Moon's position has gotten better and better. In
    1803, the position was off by 30 arcseconds on average (in terms of the
    absolute error in lunar distances). These were not mistakes. That was simply
    the best they could do with the science then available. These almanac
    "errors" only affected the "longitude by lunars". Jusr a dozen years later
    the inaccuracy had been reduced to about 7 arseconds on average. By 1860, it
    was down to 3 arcseconds.
    The leap year problem that Bowditch and others noticed was in editions of
    Moore's "New Practical Navigator" from the 1790s. This was an actual mistake
     --Moore should not have listed 1800 as a leap year. It was fixed by 1800,
    both in Moore and in Bowditch's edited version of Moore, which eventually
    became the "New American Practical Navigator", so the only navigators
    affected by this error were those who were using an older edition of Moore.
    It caused a real error in the Sun's declination and therefore the latitude
    by Noon Sun which was a primary method of navigation for the vast majority
    of vessels in that period. It was said back then that some vessels were lost
    because of this error in Moore's Navigator. I think that may be more legend
    than reality --and it didn't hurt sales of Bowditch's revised book to repeat
    the story in any case!
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