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| - 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. Historically, there have been two forms of attempted solution to this problem: In the modern-day era of GPS and radio signals broadcast by atomic clocks, this is almost never an issue.
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| abstract
| - 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. 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: 1.
* Observe celestial phenomena which vary rapidly. Generally the idea here was to make very precise observations of either the moon or the moons of Jupiter. Unfortunately, finding the moons of Jupiter while on the deck of a moving ship proved impractical. Observations using the moon, on the other hand, were possible in theory, but required extraordinarily complex computations in practice. Nevertheless, well-designed calculation tables made this the standard method for a while in the late 18th and early 19th century. 2.
* 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 centuries, 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. For more details, see The Other Wiki, here.
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