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In the seventeenth and eighteenth centuries clockmaking was a vital European technology, and London was 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 know 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 ______________ 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 precisely telling the time in spite of 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 twentyfour hours in the day so, as the Earth rotates, every hour the Sun apparently ‘moves’ across the sky one twentyfourth 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.
*chronometer: (천문·항해용) 정밀 시계