# Essay On Greenwich Time

771 Words4 Pages
For every 15 degrees that one travels eastward, the local time moves one hour ahead. Similarly, traveling West, the local time moves back one hour for every 15° of longitude. Therefore, if we know the local times at two points on Earth, we can use the difference between them to calculate how far apart those places are in longitude, east or west. This idea was very important to sailors and navigators in the 17th century. They could measure the local time, wherever they were, by observing the Sun, but navigation required that they also know the time at some reference point, e.g. Greenwich , in order to calculate their longitude. Although accurate pendulum clocks existed in the 17th century, the motions of a ship and changes in humidity and temperature would prevent such a clock from keeping accurate time at sea. King Charles II founded the Royal Observatory in 1675 to solve the problem of finding longitude at sea. If an accurate catalogue of the positions of the stars could be made, and the position of the Moon then measured accurately relative to the stars, the Moon's motion could be used as a natural clock to calculate Greenwich Time. Sailors at sea could measure the Moon's…show more content…
Harrison worked on three portable clocks, H1, H2 and H3, filled with features of his great ingenuity. But he eventually won the coveted prize with H4. The H4 is completely different from the other three timekeepers. Just 13 cm in diameter and weighing 1.45 kg, it looks like a very large pocket watch. Harrison's son,William, set sail for the West Indies with H4 aboard the ship Deptford on 18 November, 1761. They arrived in Jamaica on 19 January, 1762, where the watch was found to be only 5.1 seconds slow! It was a remarkable achievement, but it would be some time before the Board of Longitude was sufficiently satisfied to award Harrison the