A chronometer is actually just a time keeping device, a small clock or watch that is so accurate that it enables a mariner (with the help of some astronomic tables) to determine his precise location on the water while in the middle of the ocean. A very simple example of how this can be done: if a sailor knows the precise time at a location, say Greenwich, England, then he can observe a celestial occurrence that should happen at a certain time, based on the rotation of the Earth, say local noon. If local noon (the point at which the sun reached the highest point in the sky) were observed where the ship was located, but the chronometer says that the time in Greenwich is 3:00 p.m. the sailor would know that the Earth had gone through one-eighth of its rotation since it had passed overhead in England. Since the Earth is 360 degrees around, he only needed to calculate one-eighth of 360 degrees to find his longitude, 45 degrees west longitude.
Latitude could be calculated even more easily simply by reckoning the declination of the sun off the horizon. Based on the curvature of the Earth and its expected tilt toward or away from the sun this manner could be fairly accurate and was done for centuries before mariners could calculate longitude.
Because early mariners were only able to calculate latitude, they were prone to making great errors in figuring their positions. Ships frequently met disaster, running on obstacles they assumed they were not even near. There was a huge disaster off the Scilly Isles in 1713, when 4 ships sank and around 2000 sailors drown simply because the ships could not calculate their longitude. This prompted the British Government to offer a prize to anyone who could create a clock that would be accurate enough at sea to make the necessary calculations for longitude.
There actually were clocks on land that were accurate enough to calculate longitude. But at sea they became unreliable. The ships motion was only one among many factors that made time keeping at sea difficult. Pressure, temperature and humidity changes could also wreak havoc on a time piece.
Although many people worked assiduously to solve the clock problem it was not until the mid-1700s that the first accurate, ocean-going chronometers were developed. John Harrison substantially produced the requisite time piece and was rewarded by Parliament for his work in 1773. Yes, Harrison's chronometer still ran on gears, but it made an amazing advancement that compensated for temperature change, this is now called a bi-metallic strip. Wherever temperature could prove a problem, he introduced this innovation of brass and steel welded back to back. This would prevent metallic parts from expanding or contracting unduly. He also invented the ball bearing to deal with friction for one of his prototypes and the movement of the ship was compensated for by making the chronometer in the manner of a watch.
The chronometers that were finally adopted for use by mariners, indeed, resembled large pocket watches. They would increase the accuracy of navigation on the world's oceans and were a common fixtures on ships until the advent of satellite navigation.
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