The obvious and again simple answer is that he takes an accurate clock with him, which he sets to home time before leaving. The great flaw in this ‘simple’ theory was – how does the sailor know time back home when he is in the middle of an ocean?ĭ. Then the difference between the obtained time and local time served for calculation in longitude from Greenwich. Time corresponding to Greenwich Time was determined using the nautical almanac. The sailors also had to calculate the Moon’s position if seen form the centre of Earth. The angular position of Moon and other bright stars was recorded in three-hour intervals of Greenwich Time. In order to determine longitude, sailors had to measure the angle between Moon centre and a given star – lunar distance – together with height of both planets using the naval sextant. Up until the middle of the 18th century, navigators had been unable to determine their position at sea with accuracy and they faced the huge attendant risks of shipwreck or running out of supplies before reaching then destination. A comparison with your local time (easily found by checking the position of the Sim) will then tell you the time difference between you and home, and thus how far round the Earth you are from home.Ĭ. The key to knowing how far around the world you are from home is to know, at that very moment, what time it is back home. Even when in the middle of the ocean, with no land in sight, knowing this longitude position is very simple in theory. The crew of a given ship was naturally only concerned with how far round they were from their own particular home base. The longitude is a measure of how far around the world one has come from home and has no naturally occurring base line like the equator. Knowing one’s position on the earth requires two very simple but essential coordinates rather like using a street map where one thinks in terms of how far one is up/down and how far side to side.ī. Indeed, most of the scientific community thought such clock impossibility. The reality was that in the 18th century no one had ever made a clock that could suffer the great rolling and pitching of a ship and the large changes in temperature whilst still keeping time accurately enough to be of any use. It was, as Dava Sobel has described a phenomenon: ‘the greatest scientific problem of the age’. Timekeeper 2 Invention of Marine ChronometerĪ.
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