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    Re: Lunar Distance in Wikipedia
    From: Fred Hebard
    Date: 2007 Aug 2, 08:42 -0400

    Two minor edits.
    
    On Aug 2, 2007, at 6:53 AM, George Huxtable wrote:
    
    substituting "per" for "in an" in the sentence below.
    
    > A lunar distance changes with time at a rate of roughly half a
    > degree, or 30
    > arc-minutes, per hour.
    
    Move the sentence on calculation precision to the end of this
    paragraph, to dovetail with the discussion of overall error.  This
    also sets up a discussion of the various methods of clearing, which
    all aimed to simplify the clearing process, hence Bowditch's claim to
    fame.
    
    > In the early days of lunars, predictions of the Moon's position
    > were good
    > only to half an arc-minute, so to obtain the required overall
    > precision of
    > one arc-minute, only half a minute could be allowed for errors in
    > measuring
    > the lunar distance, and in calculation, combined. The best sextants
    > could
    > indicate angle to one-sixth of a minute, but in practice at sea,
    > actual
    > errors were somewhat larger, good observers typically achieving
    > overall
    > accuracy within half a minute in favourable conditions. From the
    > unstable
    > deck of a small vessel, in bad weather, with the sextant elevated
    > and tilted
    > at an awkward angle to get both objects simultaneously in view in
    > its small
    > mirrors, and with much of the sky obscured by square sails, it
    > became a
    > severe test of a navigator's skill.  Because of the half-arc-minute
    > errors in both the moon's position predicted by almanacs and in the
    > measurement, great precision was required in the calculation of
    > Greenwich time.  The calculations, which were complex, called for
    > five-figure log tables of trig functions.
    
    
    
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