One of the biggest problems encountered by sailors over the centuries was that of finding longitude, the relative east-west position of any point on the earth's surface. Finding the north-south position, the latitude, was a relatively easy matter of observing the sun's altitude and knowing the date.

As early as 1530 the Flemish astronomer and instrument maker, Gemma Frisius, had suggested that longitude could be found using a mechanical clock, provided that such a machine could be made to measure time accurately enough. In simple terms, the earth has a circumference of 360º and one day equals 24 hours, thus one hour of time is equal to 15º of longitude. If you carry the local time of a known location and compare it with the observed local time at the ship's location, a simple calculation between the two gives a time difference and thus a longitude difference. The problem was making a clock which would maintain accuracy over long periods in the harsh conditions of ocean travel. Such was the enormity of the problem that governments offered large monetary prizes for anyone who could make a timekeeper of sufficient accuracy.

It was not until John Harrison's fourth timekeeper H4 was tested on a three-month voyage to Jamaica that the problem was solved. His timekeeper's error was calculated at 5.1 seconds in 62 days. On the return voyage, on arriving at Spithead, the watch had lost only 1 minute 54½ seconds in 147 days proving at last that a timekeeper could establish the longitude. It was not then long before all ships at sea carried marine chronometers for position finding, with the the precision of navigation determined by the accuracy of the chronometer.

Today navigation relies on satellites orbiting the earth which transmit signals for GPS (Global Positioning System) equipment.

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