Longitude by Dava Sobel

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Sailors for hundreds of years could tell their latitude by seeing where it stood at it’s highest point in the sky, and then consulting a table, the further North or South, the lower the sun was at noon.

This meant that although they could know how far to travel “up/down” the globe to get to their destination, there were still many lives lost at sea because they couldn’t tell how close they were to land to the “left/right” of them.

This became the greatest problem of the 18th century to try and solve, and if you solved it, you were promised riches and glory.

Many prominent thinkers of the day tried to solve the longitude riddle, by taking accurate measurements of constellations, but this proved near impossible to do because ships were constantly moving, and having a clear sky to take measurements wasn’t a factor they could count on.

The most elegant solution was by John Harrison, an English clockmaker. It was proposed that if you took a clock on board when you went out to sea, you could calculate the difference between your current time when the sun was at it’s highest, which would be 12:00, and the time that was kept by the clock, which would show the time of “home”, you could tell how many degrees “across” the world you had traveled.

This was a brilliant idea, but had some flaws. The clocks of the day didn’t do well with humidity, salty sea air, change in temperatures, or constant movement by the ship. They typically would rust, lose time, swell, shrink, and the wood would rot. In addition to that, most sailors were incredibly superstitious, and having some contraption on board that they didn’t understand looked like witchcraft, and were man-handled, broken, and improperly used.

This book is story of how John Harrison pioneered the technology that enabled sea-travel to become much safer.  It also shows some insight into the mind of someone that worked for decades to create something that would change the world.

It’s a book everyone should read, if only to understand the problems people faced hundreds of years ago, that now we take completely for granted.


Above: the H4 chronometer, an incredible piece of engineering and ingenuity.