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By Root 2781 0

at its cutting edge. As a maritime nation, the British were concerned with one problem in particular: they could make clocks that kept very good time as long as they stayed perfectly still but not when they were shaken about, and particularly not on board a rolling ship. If you wanted to sail, it was impossible to keep a precise record of time. And at sea, if you can’t tell the time, you don’t how far east or west you are. It is relatively easy to calculate latitude – your distance north or south of the equator – by measuring the height of the Sun above the horizon at noon; but this won’t let you calculate longitude – your position east or west.

The problem of accurate timekeeping at sea was finally cracked in the middle of the eighteenth century by John Harrison, who invented a clock – a marine chronometer – which could go on accurately telling the time in spite of fluctuations in temperature and humidity and the constant movement of a ship, thus making it possible for the first time for ships anywhere to establish their longitude. Before a ship set sail, its chronometer would be set to the local time in harbour – for the British this was usually Greenwich. Once at sea, you could then compare the time at Greenwich with the time of noon on board ship, which you fixed by the Sun; the difference between the two times gave you your longitude. There are twenty-four hours in the day so, as the Earth rotates, every hour the Sun apparently ‘moves’ across the sky one twenty-fourth of a complete circle of the globe – that is, 15 degrees. If you are three hours behind the time in Greenwich, you are 45 degrees west – in the middle of the Atlantic. If you are three hours ahead you are 45 degrees east – on the same latitude as Greenwich, you would be somewhere south-west of Moscow.

Harrison’s chronometers were pioneering, high-precision instruments made in tiny numbers and affordable only to the Admiralty. It was not until around 1800 that two London clockmakers managed to simplify the chronometer mechanisms so that virtually any ship – and certainly the larger ships of the Royal Navy – could carry them as routine equipment. Our object is one of those lower-cost chronometers, made in 1800 by Thomas Earnshaw. It is made of brass and is around the size of a large pocket watch, with a normal clock dial showing roman numerals and a smaller dial at the bottom for the second hand. The clock is suspended inside a swivelling brass ring fitted to the inside of a wooden box – this is the key to keeping the chronometer level even in an unsteady ship. The geographer Professor Nigel Thrift assesses the background:

The chronometer is the pinnacle of a long history of clockmaking, and it is very important to realize that clocks have been around since 1283 in England. Everyone talks about Harrison and the fact that he was a genius. He was, but you have to understand the innovative efforts made by hundreds and thousands of clockmakers and general mechanics that, in the end, produced that object. Gradually, all of those things are incorporated into this extraordinarily efficient machine. These kinds of chronometers were phenomenally accurate; for example, one of the first was used by Captain Cook on his second voyage of exploration to the Pacific, and when Cook made final landfall in Plymouth in 1775 after circumnavigating the globe it gave an error of less than eight miles in calculated longitude.

This particular chronometer sailed on many ships – always issued and set, as others were, at Greenwich; but it is famous because in 1831 it was issued to HMS Beagle, the ship that carried Charles Darwin on his great voyage to South America, the Galapagos Islands and on around the world, which ultimately led to his theory of evolution and his great work On the Origin of Species.

The Beagle was on a mission to map the coastline of South America, work which relied on very accurate measurements of longitude and latitude. The chronometer for the first time allowed absolutely accurate charting of the oceans, with all that implied for establishing safe