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    Re: Dava Sobel
    From: Frank Reed CT
    Date: 2006 Apr 30, 06:47 EDT

    Alex Eremenko, you wrote:
    " "the ship's  capitan learned his longitude in
    the comfort of his cabin, by comparing his  pocket watch
    to a constant clock that told him the correct time at home  port".
    No other explanation is given in a book which is  called
    "Longitude" and is written for "broad audience"!
    I could not  believe my eyes... and this is was a bestseller...
    are we really living in  "Dark age"?"
    First of all, that isn't true. I had to look it up to see.  You are perhaps a
    victim of a cursory reading... The sentence you've quoted is a  TEASER at the
    very end of one chapter intended to draw the reader on into the  rest of the
    book. In fact, the essential principle of comparing local time with  absolute
    time is explained in the opening chapter of the book --and ya know,  it's not
    rocket science --it's a fairy simple concept. Sobel does an admirable  job
    explaining the overall topic. No, there's no spherical trigonometry and she
    doesn't go into the details of taking time sights, for example, but this
    diminishes the value of the book only slightly and only for that small segment  of the
    market that can follow such discussions. As for this book being evidence  of
    a "Dark Age".... Come on. That's absurd. There are millions and millions of
    people who read this book, and they learned a lot from it. I have personally
    met  many people who can explain the basic concept of lunar distances simply
    because  they read the book! The topic of longitude and celestial navigation
    generally is  known far more widely than it has been in decades, and it's
    primarily because of  Sobel's fine little book.
    As for Shovell's shipwreck, there are only a  few sources. W.E. May in his
    'History of Marine Navigation' wrote that he had  examined in detail all of the
    surviving logbooks from the fleet, and the  positions are not that far off in
    general. He sees the real problem as a  charting failure. The Scillies had
    been charted some ten to fifteen miles too  far north for a long time, leaving
    Shovell with the mistaken impression that he  had more room to maneuver.
    In the era, the wreck of the fleet was  popularly blamed on a failure to know
    longitude, but that may have been more of  an attention-grabbing move on the
    part of the people who had a vested interest  in solving the longitude problem
    (and there were many such people). As for  "Longitude", it makes no mention
    of the latitude factor or other issues that  might have caused the shipwrecks
    and follows the historical theory --blaming it  on longitude foremost, and
    Shovell's arrogance secondarily. But in "The  Illustrated Longitude", you'll find
    that Andrewes's caption notes that the  incident probably had as much to do
    with errors in latitude as errors in  longitude. Naturally: If you need to sail
    towards Portsmouth from west of  Ushant, you might set a course around 060
    degrees true. An error in either  latitude or longitude will get you in trouble.
    But if you are confident you're  in the latitude of Ushant, and you are not
    confident of your longitude, then you  might pick a course closer to north for a
    first leg. If it then turns out that  your latitude was really wrong instead,
    you're in big trouble. Be aware also  that Sobel did not invent the idea that
    this tragedy was a result of the failure  to know longitude (and primarily
    longitude) --that popular theory lasted for  more than a century and has been
    widely told.
    What was really lacking in  1707 was scientific dead reckoning navigation.
    The tools that might have saved  Shovell's fleet were not available until almost
    200 years later. Imagine how  different things might have been (not in time
    for Shovell & co., of course)  if the Board of Longitude had spent some of its
    vast monies on some fast system  for taking soundings "on the fly", and maybe
    also a little money for tools and  techniques for improving the accuracy of
    compasses. But that's not how  astronomers and mathematicians think...
    42.0N 87.7W, or 41.4N  72.1W.

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