GPS
History of GPS and the "longitude problem"
For every 15 degrees that one travels East, the local time moves ahead one hour; and for every 15 degrees traveled West time moves back one hour. Therefore, if the local times in two places on Earth are known it would be possible to use their differences to calculate longitude. This was a major concern for sailors in the 17th century. Time could be determind by observing the Sun, but navigation required that sailors also know their time at some known reference point(Greenwich, England). This was not possible due to the fact that accurate pendulum clocks of the time were offset by the weather at sea an by the rocking motion of the ship.
So in 1675, King Charles II founded the Royal Observatory(seen below) to solve the longitude problem. They wanted to accurately catalogue the position of the stars and Moon. If such a map of the heavens could be made then the motion of the Moon could be used as a natural clock onboard ships. This was known as the Lunar Distance Method of finding longitude. It never really proved as accurate as needed. In 1714, the British Government offered a large prize to anyone that could device a way to provide longitude within half-a-degree(two minutes).
This prize would eventually be awarded to John Harrison, a working class carpenter with little education. Harrison took on the great scientific minds of his time and claimed his prize through astonishing determination and technical insight. To solve the problem, he devised a portable clock so accurate and sea worthy it could stand up to the government's regulations.
Time is still the method of finding longitude today and it has been taken to the skies in the form of extremely accurate atomic clocks. These clocks are what make the Global Positioning System possible.
both from www.rog.nmm.ac.uk/index.html
John Harrison
The Royal Observatory in Greenwich