Fun Research Rec--Longitude
Sep. 29th, 2003 08:07 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
dmarleySobel, Dava. Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time. New York: Walker and Co, 1995. (Also available in an Illustrated Edition, which I recommend. The A&E network also made a very good four-hour movie based on the book, which you might be able to find for rent or at a library.)
This lovely little book chronicles the story of John Harrison, the man who finally created the first reliable marine timekeeper, and thus solved the problem of finding longitude at sea. Along the way, Sobel gives a concise yet detailed picture of the "longitude problem" as it stood in the 1700s, managing to convey the enormity of the issue, and the effect it had on the lives of every sailor who braved the seas. Very good for getting a feel for the methods of navigation that weren't possible in the 1720s.
For those who want the condensed version, here's an extremely sketchy summary of the longitude issue as presented in Sobel's book, provided more as a "this is what they didn't have in Pirates of the Caribbean" than a precise history of longitude.
There are two sets of imaginary lines that crisscross the globe, forming a grid that navigators, cartogrophers, and explorers use to pinpoint their location. The horizontal lines mark latitude, and run parallel to each other in concentric circles. The vertical lines mark longitude, and each line crosses both poles, making the distances between them vary as one travels north and south.
Finding latitude is relatively easy. There are a number of ways to calculate latitude, including measuring the length of the day or the elevation of the sun. But the measurement of longitude depends on knowing both the local time at a ship's current location and the local time at the ship's point of departure. Only by comparing the two and calculating the difference can a sailor know how far east or west he has travelled. Before the 1770s, there was no truly reliable method of gauging longitude. Captains of sailing ships would simply sail in a straight line along the latitude of their destination until they hit something. Often literally. Another drawback of this technique was that ships were forced to use well-known shipping lanes for travel, which were consequently well-known to pirates, too.
It's something of a statement on the problem that one of the most-often used methods to estimate longitude was "dead reckoning." A log would be tossed overboard, tied to a line that had knots at fifty-one-foot intervals. As the ship sailed away from the log, the sailors would count how many knots passed over the rail in thirty seconds. From that, they would calculate their rate of speed based on numerous variations of wind and weather, and thus estimate how far they had travelled. Needless to say, there was an enormous potential for miscalculation.
It's worth noting that Galileo actually did find a way of measuring longitude in the 1620s, based on the movements of Jupiter's moons. Unfortunately, the method was unusable at sea due to the difficulty of making telescopic observations aboard a moving ship, but after Galileo's death his method became the accepted means of finding longitude on land.
One extremely popular solution for finding the time at sea was to track the movements of the moon. Astronomers theorized that if they could produce accurate enough lunar tables, recording the moon's position in relation to various stars at various times and locations, then captains could calculate the time using the charts and his own local observations. There were, however, several major obstacles to this solution.
First, astronomers simply didn't have an accurate enough map of the heavens, and it would be many years before enough astronomical data was amassed to publish accurate lunar charts in the 1760s. Second, making accurate celestial observations aboard a pitching ship was a tricky proposition at best, and it wasn't until the invention of the octent in 1731, quickly followed by the more familiar sextant, that there was a practical method of making such observations. Third, the lunar observations relied on the moon being visible, which was not always the case even when weather didn't intervene. And last, the first lunar tables which were published in the 1760s required some four hours of calculations before the longitude could be computed. Even when Nevil Maskelyne published a more concise method in the 1780s, the math still required half an hour, and was open to rampant human error.
Even though Harrison completed his final version of a marine timekeeper in the 1760s, it wouldn't be until the 1780s that such chronometers would become cheap enough to be in wide use. Many sailors still used the lunar tables because, despite their shortcomings, they were reliable enough and more affordable than the expensive chronometers. But by the turn of the century, watches had supplanted the lunar tables, and the problem of longitude was solved in such a way that we now take it completely for granted.
This lovely little book chronicles the story of John Harrison, the man who finally created the first reliable marine timekeeper, and thus solved the problem of finding longitude at sea. Along the way, Sobel gives a concise yet detailed picture of the "longitude problem" as it stood in the 1700s, managing to convey the enormity of the issue, and the effect it had on the lives of every sailor who braved the seas. Very good for getting a feel for the methods of navigation that weren't possible in the 1720s.
For those who want the condensed version, here's an extremely sketchy summary of the longitude issue as presented in Sobel's book, provided more as a "this is what they didn't have in Pirates of the Caribbean" than a precise history of longitude.
There are two sets of imaginary lines that crisscross the globe, forming a grid that navigators, cartogrophers, and explorers use to pinpoint their location. The horizontal lines mark latitude, and run parallel to each other in concentric circles. The vertical lines mark longitude, and each line crosses both poles, making the distances between them vary as one travels north and south.
Finding latitude is relatively easy. There are a number of ways to calculate latitude, including measuring the length of the day or the elevation of the sun. But the measurement of longitude depends on knowing both the local time at a ship's current location and the local time at the ship's point of departure. Only by comparing the two and calculating the difference can a sailor know how far east or west he has travelled. Before the 1770s, there was no truly reliable method of gauging longitude. Captains of sailing ships would simply sail in a straight line along the latitude of their destination until they hit something. Often literally. Another drawback of this technique was that ships were forced to use well-known shipping lanes for travel, which were consequently well-known to pirates, too.
It's something of a statement on the problem that one of the most-often used methods to estimate longitude was "dead reckoning." A log would be tossed overboard, tied to a line that had knots at fifty-one-foot intervals. As the ship sailed away from the log, the sailors would count how many knots passed over the rail in thirty seconds. From that, they would calculate their rate of speed based on numerous variations of wind and weather, and thus estimate how far they had travelled. Needless to say, there was an enormous potential for miscalculation.
It's worth noting that Galileo actually did find a way of measuring longitude in the 1620s, based on the movements of Jupiter's moons. Unfortunately, the method was unusable at sea due to the difficulty of making telescopic observations aboard a moving ship, but after Galileo's death his method became the accepted means of finding longitude on land.
One extremely popular solution for finding the time at sea was to track the movements of the moon. Astronomers theorized that if they could produce accurate enough lunar tables, recording the moon's position in relation to various stars at various times and locations, then captains could calculate the time using the charts and his own local observations. There were, however, several major obstacles to this solution.
First, astronomers simply didn't have an accurate enough map of the heavens, and it would be many years before enough astronomical data was amassed to publish accurate lunar charts in the 1760s. Second, making accurate celestial observations aboard a pitching ship was a tricky proposition at best, and it wasn't until the invention of the octent in 1731, quickly followed by the more familiar sextant, that there was a practical method of making such observations. Third, the lunar observations relied on the moon being visible, which was not always the case even when weather didn't intervene. And last, the first lunar tables which were published in the 1760s required some four hours of calculations before the longitude could be computed. Even when Nevil Maskelyne published a more concise method in the 1780s, the math still required half an hour, and was open to rampant human error.
Even though Harrison completed his final version of a marine timekeeper in the 1760s, it wouldn't be until the 1780s that such chronometers would become cheap enough to be in wide use. Many sailors still used the lunar tables because, despite their shortcomings, they were reliable enough and more affordable than the expensive chronometers. But by the turn of the century, watches had supplanted the lunar tables, and the problem of longitude was solved in such a way that we now take it completely for granted.