Longitude, by Dava Sobel

[essentialsaltes]

subtitled: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time

Another of Sobel's great books on history/science.

Before GPS, it wasn't easy to find out where you were at sea. Finding your latitude was relatively straightforward by observing the sun (if not exactly safe -- before the invention of the back-staff, the use of the cross-staff involved staring into the sun -- could be why so many pirates have eye patches.) But longitude was harder to determine and dead reckoning was liable to error. In 1707, a terrible naval disaster, involving the deaths of more than a thousand sailors. A few years later, the British passed the Longitude Act, setting up a huge prize for the first person to 'solve' the problem of longitude, making navigation safer. This is the main story of the book, with some broader discussion of events before and after.

For instance, Galileo developed a clever idea of using the motions of the Galilean satellites of Jupiter. If a certain eclipse would occur at midnight in Milan, but a sailor saw it at 11, he would know he was "one time zone away" or 15 degrees of longitude. It was a bit impractical, since you'd need an observatory on your ship with a trained astronomer. And you'd need timetables of satellite eclipses that didn't exist. But solutions along these lines were in the forefront, making a strange marriage of navies and astronomers. Though now it makes sense that we have Naval Observatories.

In some ways, this caused problems for the eventual winner of the contest. It was overseen by the Royal Astronomer (and others) who preferred their own approach, and the rather simple (and successful in retrospect) solution of just making an accurate clock that would keep time on board a ship seemed impractical. A fair amount of the story is a sad war between our hero, clockmaker John Harrison, and our villain, Astronomer Royal Nevil Maskelyne, who developed his own solution involving lunar measurements and thwarted Harrison through delays, additional hoops to jump through, and possibly manipulation of some of the clocks (held in his possession to 'test' them).

And just a silly little detail of French/English friction: 

"In 1884, at the International Meridian Conference held in Washington, D.C., representatives from twenty-six countries voted to make the common practice official. They declared the Greenwich meridian the prime meridian of the world. This decision did not sit well with the French, however, who continued to recognize their own Paris Observatory meridian, a little more than two degrees east of Greenwich, as the starting line for another twenty-seven years, until 1911. (Even then, they hesitated to refer directly to Greenwich mean time, preferring the locution “Paris Mean Time, retarded by nine minutes twenty-one seconds.”)"