The marine chronometer was designed to solve the problem of determining longitude at sea, which was the cause of many major shipwrecks. This problem eventually prompted the English parliament to pass an act allowing a reward of up to 20,000 pounds to be awarded to the first person to develop a practical solution.

Various methods had previously been developed for measuring longitude on land. One of these involved using a telescope to observe the orbits of Jupiter’s moons. This information was used to determine the time at a specific location, e.g. Paris, Greenwich, etc., which was compared to the local time to determine the longitude. Using this method, many land-based observations were made to determine the longitudes of many towns. However it was an impractical technique on board a ship. The ship’s rocking prevented the accurate measurements required. And so, a new system was needed for determining longitude a sea.

The sextant was developed for this purpose. Charts were developed which told the time at which the moon would be at certain positions. The sextant was used to measure this and then determine the time. The problem was that the charts were not very accurate.

The easiest way to determine the longitude would be to carry a clock on the ship. Clocks at that time, however, used pendulums to keep time and would never have been accurate enough on a rocking ship. This is why John Harrison began working on a better clock in 1730. He completed his first clock in 1735. It weighed 72 pounds, used a balance spring instead of a pendulum, and lost only about three seconds in 24 hours.

However, after creating this clock, commonly known as the H1, John Harrison began work on more accurate clocks. The H2 and H3 were all very similar in appearance to the H1. However, the fourth clock, the H4, was very different. It was only about 5 inches across, and looked like a large pocket watch. In fact, pocket watches were based on this design. It was found to be much more accurate than the reward required, loosing only five seconds after a two month trip. However, it took another two years for John Harrison to receive his award, and only after the intervention of King George III himself. John Harrison built only one more clock, the H5, which was similar to his previous one, the H4.

The invention of the marine chronometer was a major achievement. It allowed for the first time accurate and simple determination of longitude. It revolutionized marine navigation, and was not equaled until radio waves were used to broadcast the time.

*Pictures from:
Medwin, Jonathan. "An Historical Account of Maritime Navigational Practice and the subsequent Invention of the Chronometer". 4 Jan. 2004. <http://rubens.anu.edu.au/student.projects97/naval/home.htm>

Medwin, Jonathan. "An Historical Account of Maritime Navigational Practice and the subsequent Invention of the Chronometer". 4 Jan. 2004. <http://rubens.anu.edu.au/student.projects97/naval/home.htm>

• This website describes the problem of determining longitude and the history of the marine chronometer.