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    Re: Two reckonings
    From: George Huxtable
    Date: 2011 Jan 3, 19:53 -0000

    John Huth wrote-
    
    | George -
    |
    | That's truly fascinating, but I guess I shouldn't be surprised by the
    | problems of standards.   What surprised me the most in your post is that
    the
    | circumference of the Earth  was in dispute so late in the game.
    |
    | As I recall Abu Rayhan Biruni had an exceedingly accurate measurement of
    the
    | circumference of the Earth way back in 1000 AD.   He used a dip-angle
    | technique of all things.   Perhaps it was not widely accepted.   I'm
    | actually interested in al-Biruni's technique, as I'm having difficulties
    | tracking this down.   It's widely known that he used dip angle from a
    | mountain in the Punjab, but the precise technique is something I've found
    | elusive.  If I were to do this measurement, I'd probably use a trough of
    | water for my horizontal, but then I'd also have to have a fairly well
    | calibrated device to sight the dip angle.    If anyone out there knows
    about
    | this measurement, I'd be grateful for the details.
    |
    | I'm assuming that Snell and Gunter did a more standard
    astronomical/survey
    | measurement.
    
    ================
    
    To deal with John's last point first, it was Snell's measurement in
    Holland, and Norwood's from London to York, that were important. Gunter's
    role was more as a teacher and a persuader.
    
    I hadn't come across Abu Rayhan Biruni, except as a name, but since reading
    your post I've Googled him. It strikes me that to achieve sufficiently
    precise measurements of the small angles involved, to reach the claimed
    precision for the Earth's radius (except accidentally) an astrolabe , which
    won't measure such angles better than to a quarter of a degree or so, is
    insufficiently precise. So I'm a bit sceptical about that article.
    
    We have to take account of another possibilty. Biruni's observation, in the
    end, depended on some baseline, which may have been just paced-out or
    checked with some measuring cord, in what was called "cubits". Were
    standards of linear measurement, in the 11th century Muslim world, like
    they were in the West, to the 18th century and beyond, when each state, and
    often each city within that state, had its own set of weights and measures,
    quite incompatible with everyone else's, and altering from one era to
    another? The French, Spanish, German, Dutch, and English leagues differed,
    considerably. How well is al-Biruni's cubit independently known today, in
    terms of modern metres? I ask, because it could be that the al-Biruni
    conclusion, in terms of cubits per degree of latitude, may have been the
    basis for modern scholars to assess the true length, in metres, of the
    cubit he used. In which case, his conformity with  modern values of
    Earth-radius would have been a self-fulfilling circular argument.
    
    Let me make it clear that I have no knowledge, at all, about whether such a
    state of affairs actually existed. But it might have done.
    
    Similar problems arose with Eratosthenes' measurement, between Alexandia
    and Syene, around 200 BC, in that we know just what he did. We know what he
    measured, it stades, and the angle it subtended at the Earth's centre, but
    we don't know what value of the stadion he used, as they came in different
    local varieties.
    
    George.
    
    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    | On Sun, Jan 2, 2011 at 7:26 PM, George Huxtable  wrote:
    |
    | > John Huth wrote-
    | >
    | > "The local curator of the Museum of Historical Scientific Instruments
    told
    | > me that the sand-glasses were 28 or 29 seconds, rather than 30.   Could
    | > this be to also create an in-built over-estimate of distance run?
    | >
    | > I guess this doesn't completely cover problems like what Cook
    encountered.
    | >
    | > ============
    | >
    | > John is operning up an intriguing can o'worms here. This matter became
    very
    | > complex.
    | >
    | > The English Log, which came into use in the 16th century, was the first
    | > decent way of assessing a ship's speed in numerical terms. And mariners
    | > came to realise that the answers it was giving them were failing to
    | > correspond with their observed latitude changes, even in simple
    North-South
    | > travel.
    | >
    | > The reason was, mainly, a misunderstanding of the size of the Earth,
    and
    | > therefore the length of an arc-minute of latitude, expressed in feet.
    Over
    | > the 17th century, as a result of the work of Snell (same man as in
    Snell's
    | > law) in Holland, and Gunter and particularly Norwood in England, the
    | > assessed sea-mile increased from 5000 feet to 6000 feet and then 6120
    feet.
    | > Mariners had to adjust their logs accordingly. It wasn't a trivial
    | > business, increasing the spacing between the well-embedded knots along
    600
    | > feet or so of line, and it was much simpler to take a bit of the sand
    or
    | > eggshell out of the timing glass, to reduce its period. So a whole
    range of
    | > different time-period glasses came into use. But then, some log-lines
    were
    | > re-knotted, to conform with the new understanding. And just as you
    might
    | > expect from Sod's law, these were not kept together in associated
    pairs, so
    | > that over time, some vessels would have a short glass but with long
    knots,
    | > and others vice versa. It was complete chaos.
    | >
    | > These problems had afflicted Halley's voyage in 1699-1700 to measure
    | > magnetic variation in the Atlantic, as described in his journal of the
    | > Paramore.
    | >
    | > So much so that in 1763, (around the date of Cook's Atlantic voyages)
    | > Maskelyne devoted a 6-page appendix of his "British Mariner's Guide" to
    | > "Some remarks on the proper length of the log-line". Its first
    paragraph
    | > ended as follows-
    | >
    | > "...for while one ship has a line of 42 feet between knot and knot to a
    | > glass of 28 seconds; another, one of 42 feet to a half-minute glass;
    and
    | > another, one of 48 feet to a glass of 28 seconds; all which proportions
    are
    | > very commonly used, their accounts must differ as much from one
    another, as
    | > most of them do from the truth; for, as only one can be right, all the
    rest
    | > must consequently be wrong.". I relish Maskelyne's pithy prose.
    | >
    | > George.
    | >
    | > contact George Huxtable, at  george{at}hux.me.uk
    | > or at +44 1865 820222 (from UK, 01865 820222)
    | > or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    | >
    | >
    | >
    | >
    | >
    |
    |
    | --
    | Keeping up with the grind
    |
    
    
    
    
    

       
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