dipper@utastro.UUCP (Debbie Byrd) (12/11/84)
Astronomers often have to wait a LONG time to prove their theories. More -- in a minute. December 11 The Long View In the late 1600s, the Greenwich Observatory in England was founded by royal decree. Its original purpose was to "perfect the art of navigation" -- by finding a way to determine the longitude of ships at sea. The second Royal Astronomer was appointed in 1720 -- Edmond Halley, now famous for Halley's Comet. He wanted to put the moon to work in finding longitude. The moon's position relative to the background of stars changes over the hours as the moon moves eastward on the dome of the sky. Navigators could use the apparent position of the moon among the stars to help navigate their ships. But many observations were needed to predict where the moon should be -- and produce tables that sailors could use for navigation. The moon's motion is cyclical in obvious ways -- and it's cyclical in subtle ways. One subtlety is that the moon changes its path across our sky relative to the sun and stars in a cycle of 18 years, eleven and three-tenths days. The Royal Astronomer decided that the best way to get the data on the moon would be to personally record its position throughout one of these 18 year periods. He was 66 when he set himself this task -- one that would not be completed until he was 84. He made it too -- Edmund Halley didn't die until the age of 86. If Halley had lived sixteen more years, he would have seen another long-term project come true -- the return of the comet that bears his name -- predicted by Halley to revisit the Earth every 76 years. Script by Diana Hadley. (c) Copyright 1983, 1984 McDonald Observatory, University of Texas at Austin