The Marine Chronometer and its use on board ships

The Marine Chronometer and its use on board ships
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The Elusive Longitude

Before the marine chronometer was invented, Latitude could be found quite accurately with celestial navigation and older instruments like the astrolabe. However, longitude could only be estimated, because an accurate determination of longitude is made by comparing time differences.

Older clocks could lose upto 10 minutes a day, which meant a possible daily error in ship’s position of at least 150 miles! The determination of Longtitude was such a problem that the ‘Longitude Prize’ was offered by the British government in 1714, when an Act of Parliament was passed for this purpose. A total prize of 20000 GBP, a huge sum in those days, was offered to anybody who could calculate longitude within a 30 mile accuracy.

Harrison’s Chronometer

John Harrison’s Solution

The more accurate pendulum clocks of the time were useless at sea as ships always roll in heavy seas, making pendulums unworkable. In the race for the invention of an accurate timekeeping device, John Harrison, a carpenter in Yorkshire, invented a spring based clock in 1764.

His invention was the most important advance to marine navigation in the three millennia that mariners had been going to sea. In fact, fifteen years later- in 1779- British explorer explorer Captain James Cook used Harrison’s chronometer to circumnavigate the globe. On his return, it was found that his calculations of longitude based on the chronometer were accurate to within 8 miles! Harrison won a total of 14500 GBP over a period of time for his invention. The nature of navigation had undergone a sea change.

Long By Chron

The simplest way of determining longitude at sea using celestial navigation is called, even today, the ‘longitude by chronometer’ or ‘long by chron’ method. This involves, simply, taking two ‘sights’ (altitudes) of the sun with a sextant, one around 9am and the other around noon. A simple ‘running fix’, which transfers the first position line (Literally, a line on a chart. The ship can be on any point on this line) obtained to the ship’s approximate position at noon then determines latitude (obtained solely from the noon sight) and longitude at noon.

Care of the Marine Chronometer

Today a cheap quartz watch keeps accurate time, and many chronometers on ships are battery operated. However, even thirty years ago before these were easily available, the daily routine at sea involved treating the chronometer with great care- daily careful winding, protection against ship’s rolling to avoid damage and a careful tallying of the chronometer time with a time signal on the radio to confirm the accuracy of the marine chronometer on a daily basis.


Harrison’s Chronometer Image -