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

Book by Dava Sobel published in 2005

I try my best to write a short summary/review of the books I read, and this is one of them. I typically publish them on Goodreads, but also sync them to here.

"What are you reading?" "A book about marine navigation between the 16th and 18th century!" The girlfriend was not impressed.. But in all honesty, this is a page-turner. How did sea vessels navigate the great oceans before the modern-day where we simply look at a magical map with a dot showing our current location? Measuring latitude (north-south co-ordinate) is easy and has been well-known since at least the Age of Discovery: measure the angle from the horizon to the North Star, Polaris. The angle is equal to your north-south (latitude) position.

Great, now how do we know how far east-west (longitude) we are? You didn't.. until the mid 1700s. The best you could do to figure out roughly where you were was to periodically throw a log in the water to figure out how fast you were going. Then, look at your compass and log both in your logbook. Imagine doing that accurately when waves are 10 meters high and the weather is terrible. If you do something slightly wrong or don't have the ability to do it for a while, you're good old fashioned lost at sea. This primitive navigation meant that the same trade routes were plied, making piracy easy in the Atlantic, and whalers stumbling upon each other. The biggest sea-going nations at the time: the Spanish, Portuguese, British, and the Dutch were all losing ships left and right: shipwrecks, pirating, and others that just never returned.

The root cause: the inability to figure out where you were. The inability to determine your longitude. Prizes by the nations mentioned earlier went out at the equivalent of a million dollars today to find a reliable way on ships to measure longitude within half a degree of accuracy. This was one of the greatest scientific problems at the time with Newton, Galileo, and others giving it a shot. The first method that worked was Galileo's method of using Jupiter's moons (this was impossible at sea though, because even your heartbeat would throw the telescope off enough to miss the moons -- meant it was OK at land, but not at sea). When Ole Rømer looked into this method to determine longitude, he discovered discrepancies. He finally explained this by realizing that light did not instantly get from A to B, but that light like sound has a speed. Nifty by-product of solving one of the most important problems of his time.

Finally, in the mid 1700s a method was published that relied on moon's movement. You'd measure the angle from the moon to some known celestial body and consult a table published annually to convert the angle into a longitude. This could take up to four hours (!). However, there was another way that no-one thought possible: using time. If you knew the time in, say, Greenwich, England and you knew your time where you are currently -- you could figure out your longitude. Each hour difference is equal to 360/24 = 15 degrees longitude. So, say you're in tumultuous waters off the Azores and would be delighted to know where you are. At high noon you're measuring your latitude, as per usual. Now, imagine you also had a clock synced to Greenwich and you noted the time of the clock at high noon too. The clock shows 14:13:00 when the sun is at the highest altitude where you are. Your longitude then comes out to ((13.00/60) + 2) * 15 = 33.25. To find the latitude, you take out your good ol' sextant and measure the angle to the sun at 37.0 degrees, so your coordinates are (37.0, 33.25). Ta-da! You're now ready to be sent on an Age of Discovery era ship with just a compass, sextant, and pocket watch.

Problem was... no-one could create a sea-faring clock that was accurate enough. To measure within a half degree accuracy (~40km), which was required by the prize, you couldn't drift more than 3 seconds a day. Most clocks of the 1700s drifted by 5-15 minutes a day! Factor in the humidity, temperature, and vibrations at sea -- you wouldn't get even close. However, John Harrison persevered for something like 50 years finally creating a clock (through much drama with the longitude prize board) that was accurate enough. For example, one of the big problems was that some metals would shrink and others enlarge based on temperature. He'd find materials that complemented each other to keep time syncing, if one shrank, another would enlarge in harmony. The clock worked, but was only available to few as it was expensive and took years to create each one. James Cook had one, and found it invaluable to chart out the Pacific. Over the next 60 years other clock-makers made it cheaper and cheaper by decade. By the 1850s, almost everyone was using a chronometer derived from Harrison's and lunar tables went into the pages of history.

I really enjoyed this book. Great science tale of solving a massive problem, being biased toward a particular type of solution (astronomy), discovering useful other things along the way (speed of light), and perseverance.

The book, unfortunately, can't get the 5th star. The reason comes down to how there's not a single picture in the book, which makes the concepts hard to understand. I had to read up on them independently of the book to understand them properly. Dana seems to skate over the significance of the lunar distance tables that predated Harrison's clocks, which is a shame, because they were a revolution themselves despite their complexity.