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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...

-FER
42.0N 87.7W, or 41.4N  72.1W.
www.HistoricalAtlas.com/lunars

   

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