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    Re: Lunars
    From: Fred Hebard
    Date: 2007 Sep 25, 07:13 -0400

    On Sep 24, 2007, at 10:13 PM, frankreed@HistoricalAtlas.net wrote:
    > Fred H, you wrote:
    > "I fail to understand what the date of invention of the quadrant has
    > to do with the accuracy set for the longitude prize."
    > Ok, Fred. Consider: how would those folks who recommended the prize
    > have
    > known back in 1714 that there would be a device invented, twenty
    > years in
    > their future, that could reliably measure angles to about 1' of arc?
    Newton would have made his secret presentation to the Royal about
    then.  He worked on the moon orbit problem.
    > "It would seem
    > that John Karl's explanation may be correct, in that the advisors to
    > Parliament may have recommended setting the accuracy to 30' because
    > they figured that lunars would be the method chosen and that 1' of
    > arc was about the limit of accuracy for such a measurement,
    > regardless of the instrument."
    > Hmmm. Who in the world suggested that 1' of arc was the limit of
    > accuracy
    > for such measurements?
    > The 30' level for longitude determination was chosen because "30
    > minutes
    > accuracy in longitude is as good as anyone would ever need" (today
    > that
    > sounds a bit like saying "no one would ever need more than 640k of
    > computer
    > memory").
    You say this as if you know.
    > If you can sail from Europe to the Indies and arrive with your
    > longitude known within 30', from the masthead you will be able to
    > see all
    > but the most insignificant, low-lying islands. It gets you there. Once
    > you've arrived, greater accuracy in longitude is little more than a
    > luxury
    > since you're in piloting waters. That close you're going to sail with
    > soundings and an excellent chart or with the aid of a local pilot.
    > At least
    > that's how it would have seemed in 1714.
    > Additionally, the limits for the prize were not "pre-set" so that
    > lunar
    > distances would win. In fact, the prize was probably arranged so that
    > Whiston and Ditton could win. They were the ones promoting the
    > cause, and
    > they suggested that Newton (and others) should set the rules. Isaac
    > Newton
    > was at least somewhat sympathetic to Whiston, his successor as
    > Lucasian
    > Professor of Mathematics at Cambridge (a chair now occupied by Stephen
    > Hawking), after Whiston was booted out of Cambridge on the quaint
    > old charge
    > of religious heresy. Intentions aside, by the time the prize was
    > finally
    > approved, the proposal of Whiston and Ditton was already the
    > subject of
    > scathing satire. It was Jonathan Swift of Gulliver fame who noticed
    > that
    > "Whiston and Ditton" rhymed with the scatalogical "p--sed on and s--
    > t on".
    > And thus the first longitude drinking song was born.
    > A random thought on sextant accuracy:
    > Something that's hard to notice from our perspective in the year
    > 2007 is
    > that sextants stopped getting better over 200 years ago. Sure,
    > there were
    > modest improvements, neat little features like the micrometer that
    > made them
    > easier to use, but fundamentally the best sextants today can
    > measure angles
    > with about the same accuracy, about a quarter of a minute of arc,
    > as the
    > best sextants from 200 years ago. And who could have guessed that?!
    > With
    > significant, continual impovement in angle-measuring devices during
    > the 18th
    > century, surely the logical prediction would have been continued
    > improvement. One of Chauvenet's stated reasons for developing a
    > novel method
    > for clearing lunars c.1850 was his belief that better instruments for
    > measuring angles might be developed one day soon. But that didn't
    > happen...
    Because its hand held.
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