Figuring out how far north or south you are, even when at sea, is fairly easy. You just need to find the height of the sun at noon (i.e., when the sun is due north or south of your position) and apply a corrective factor based on the time of year. That is, all that's required to determine your latitude is a compass, a sextant, and a single table—all of which sailors would have had easy access to.
On the other hand, figuring out how far east or west you are is considerably more difficult, because of the rotation of the earth. If you want to determine your longitude, you have to contend with the fact that, if you're observing something in the sky now, someone several hundred miles to your east could have observed almost precisely the same thing an hour ago. On the other hand, the rotation does have an advantage: if you know what time it is in some fixed location, and know what time it is locally, you can tell how far you are east or west of that fixed location. Prior to a genuine solution, the general practice was to sail to the correct latitude then go East or West until you hit land, but that has certain drawbacks: not only is it remarkably inefficient, it makes it difficult to avoid any nautical hazards that are at the same latitude as your desired destination.
Historically, there have been two forms of attempted solution to this problem:
- Observe celestial phenomena which vary rapidly and are visible simultaneously worldwide, and compare local time when you see it to the predicted time of occurrence at a fixed location. There were two ways of doing this: observing the eclipses of the moons of Jupiter or observing the angular distance from the Moon to some particular star:
The moons of Jupiter eclipse behind their primary very regularly and very frequently, making this method fairly easy to use; it was used to find the longitudes of places on land more or less from the time Galileo discovered them and the pattern of their eclipses. Unfortunately, finding the moons on the deck of a moving ship proved to incredibly impractical.
Measuring the angular distances of the Moon to the fixed stars—or the Sun, when they're both up—was much easier to do, requiring only equipment that all seamen already had (i.e. a sextant and a compass), particularly when using the Sun (big giant thing in the sky that it is, seamen preferred to use it). However, this measurement was completely useless on its own. The method relies on the fact that the Moon's path across the sky moves very regularly (half a degree per hour), and that the Moon's position relative to the fixed stars is the same no matter where you're looking from. Thus, you can tell what time it is in a fixed location (e.g. in London)...if an astronomer has already done the maths and published a table predicting the position of the Moon as observed from London for each hour of each day of that year. A daunting task. And then the navigator would have to do a bit more maths to actually use the tables properly. Despite all the work it took, this method was standard from the late 18th through the early 19th century, and for the early period of chronometers (see below), this method was often used to make slight corrections to the clock.
- Have a very accurate clock. If you carry around a clock set to some fixed time, you can compare local time to time at that fixed location, and thereby determine how different your longitude is from its longitude. On the other hand, constructing a clock that keeps good time while on a moving ship is itself a very difficult problem. The first chronometer capable of keeping sufficiently accurate time while on a ship was made by John Harrison in 1761; for quite some time, nobody actually believed that a mere clock could solve the problem astronomers had struggled with for centuriesnote , and even once people did listen they were prohibitively expensive for years, but eventually they became cheap enough to be the standard method of determining longitude.
Neither of these methods were viable until the middle of the 18th century; thus, any ships out of sight of land before that time would have had no good way of knowing where they were, leading to many navigational mishaps as well as wasted time at sea. For the British, this problem became intolerable and led to a massive 20,000 pound reward (on the order of ten million of today's dollars). The money was enough for an entire generation of bright minds to seek a solution.
In the modern-day era of GPS and radio signals broadcast by atomic clocks, this is almost never an issue.
Another kind of "longitude problem" that may show up as a plot device in works of fiction (as in the The Adventures of Tintin
example below) is the fact that, for centuries, there was no universally accepted prime meridian (a meridian arbitrarily defined as the zero longitude meridian). Various conventions have been used or advocated in different regions and throughout history. Therefore, longitude coordinates that you find in old texts can be tricky if there is no mention of the prime meridian used. You could be mistaken if you infer that it was the Greenwich meridian.
For more details, see The Other Wiki
Invocations of the longitude problem in media include:
Film - Live-Action
- In The Adventures of Tintin album Red Rackham's Treasure, the protagonists originally thought that the coordinates on Francis Haddock's parchments had the Greenwich meridian as the prime meridian. Tintin correctly figures that the French naval officers under Louis XIV were using the Paris meridian as their reference point.
- In The Bounty, Fletcher Christian's inability to determine how far away Pitcairn Island is (because he can't get a fix on longitude) nearly causes his crew to mutiny for a second time before they finally reach the island.
- In The Baroque Cycle, the problem is brought up a number of times, with the English government offering a cash prize if the problem can be solved. A number of natural philosophers take up the challenge, but the problem remains unsolved by the end of the series.
- The Island of the Day Before by Umberto Eco contains a number of different attempts to solve the longitude problem, including one that uses Sympathetic Magic (the theory is that a wounded dog is taken on the ship; the sympathetic magic is performed on the dog every night at midnight in Paris; by watching the dog's reaction and noting the local time, you can figure out your longitude much as with the "clock" method).
- In Devil of a Fix by Paliser Marcus, the MacGuffin is a secret almanac containing a solution to the longitude problem.
- The Medici Emerald by Martin Woodhouse and Robert Ross has Leonardo da Vinci investigate the death of a friend of his, a merchant captain renowned for getting where he wanted to go at unheard-of speed. The captain had done a favor for an Arab mathematician who built a very accurate clock ... in the 1460s or '70s.
- Shows up occasionally (as to be expected) in the Wooden Ships and Iron Men Aubrey-Maturin series of novels by Patrick O'Brian, usually when one of the ship's clocks has some kind of breakdown and makes navigation very difficult. Watches and clocks are rightly shown as being extremely important for any maritime venture, though not only for navigation.
- It was to the point that no ship carried only one chronometer onboard; mention is made of Jack consulting three to get the average.
- The focus of the book Longitude by Dava Sobel, who traces the various attempts to solve the problem but primarily focuses on clockmaker John Harrison.
- Adapted into a television mini-series staring Michael Gambon as Harrison, shown on A&E in North America, and Channel 4 in the UK.
- The novel Mason & Dixon by Thomas Pynchon talks a lot about the Longitude Problem. This is reasonable when your protagonists are surveyor-astronomers tooling around the British Empire in the 18th century. Mason and Dixon test chronometers and work out moon-observation tables in the course of their careers.
- The gamebook Sail With Pirates has the reader stranded on a small island in the Caribbean with some sailors. There's an argument when the sailors try to determine their latitude and the reader suggests figuring out their longitude. This is immediately shot down as impossible. Which is decidedly odd, because figuring a longitude of a fixed place, instead of moving ship, is relatively easy, if a somewhat long process. They may have meant that none of them knew how to do it, or were missing the necessary tables/equipment.
- Gullivers Travels: When he's in Laputa fantasizing about what he could do if he were immortal, one of the problems he imagines being able to solve is "the discovery of the longitude". He meant, of course, the quick discovery. Discovering longitude on land wasn't a major problem in hos time — sure, it required several careful astronomic observations and complicated calculations, but with some time it could be done very precisely. Sailors simply hadn't the time to do so, because their ship would certainly change its place between observations, and they needed a quick, even if somewhat less precise, way of figuring the longitude, instead of a very precise one that took a week to complete.
- The drama/documentary Longitude (see Literature above) is all about this problem; it mentions the "sympathetic magic" thing, together with other crackpot schemes like having dozens of ships anchored at fixed longitudes to provide reference points. Even after John Harrison builds several accurate chronometers, he finds himself opposed by Obstructive Bureaucrats who prefer the lunar method.
- In the second episode of James May's Man Lab, James tests the 'sympathetic magic' theory described above by attempting to sail to France using a dog as a navigational aid. Not surprisingly, it fails.
- 7th Sea incorporates this problem into the game setting. There's one country that's solved it - the country with teleportation magic among its nobility. Nobody's invented a clock that's accurate and robust enough to survive a sea voyage, but by bringing a noble aboard, having him teleport home, and then having him come back with the time, Montaigne has developed the most accurate charts in all of Theah.