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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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