It’s been a while since I read Longitude by Dava Sobel, but I happened to catch another documentary on the story recently. It reminded me again about this amazing story of science and perseverance. As an engineer, I can’t help but admire John Harrison for his scientific method. It’s also a description of an iterative method. Granted the time periods are years vs. weeks, but the idea is the same.
Before I describe why this is such an amazing story, there needs to be some background on longitude and the longitude problem. Using latitude and longitude, you can describe any location on earth. Both latitude and longitude are expressed in degrees — as an angle from a reference line. Latitude is a vertical position relative to the earth’s equator, and north or south is added to differentiate. Longitude is expressed east or west from the prime meridian, which runs vertically through Greenwich, London.
Latitude is relatively easy to find and calculate. Very simply, if you measure the angle between the horizon and the sun at its highest point (noon,) you can figure out your latitude. A sextant is usually the tool of choice here. Remember, I’m talking about the time before GPS devices.
Longitude is another matter, however. Until John Harrison came around, there wasn’t a practical way to calculate longitude from a ship. Most often, navigators would combine frequent speed measurements and a hourglass. Speed was usually measured by dropping a rope over the stern with a wooden plate on the end. This rope had knots tied at precise distances. Using a 30 second sandglass, a sailor would count the number of knots pulled out over the 30 seconds. This count was reported as “knots.” Hence, you get n knots as a nautical speed (1 knot = 1.85166 km/h.) Using the number of turns of the hourglass and the frequent knot readings, the navigators would attempt to calculate their longitude. Unfortunately, neither of these pieces of data were very accurate. As a result, a lot of ships found themselves aground or sunk due to bad longitude information. This is where the Longitude Prize came from.
The British government offered the prize for a simple and practical method of determining a ship’s longitude. It was a HUGE prize in its day — enough to retire. The prize was administered by the Board of Longitude, and this is where the problem came in. The Board of Longitude was made up mostly of astronomers who were convinced that the sky was the key. John Harrison’s realization was that if you had an extremely accurate clock, calculating longitude was pretty simple. If you had a clock set onboard set to noon at your home port, you can figure out noon on your ship at sea using the sun. If it’s noon on the ship, and your clock reads 2:00pm, you know precisely where you are (think time zones which the same idea.) The trouble was that no one knew how to build a clock accurate enough, especially one that would run on a ship that rocked and was often very humid.
John Harrison worked with wood, so his first few clocks were made of wood with no training in clock making. Some of these clocks still work today — without lubrication due to the natural oils in the wood. At the time, these clocks were considered the most accurate built to date. These clocks were pendulum clocks and of no use on a ship. Still, he invented several techniques and mechanisms still in use today.
Moving from wood to metal and putting himself into the running for the Longitude Prize started him on a 40+ year quest to retrieve the prize. No one ever officially received the prize, but John Harrison did eventually receive a monetary award from Parliament for his achievements. What it took to get there is a fascinating story. It’s one of those stories that you can’t make up.
If you can, check out the book or any of the documentaries. Not only is it a great story, but it’s a great description of an iterative, scientific process to solve a very difficult problem.