dipper@utastro.UUCP (Debbie Byrd) (11/09/85)
The stars are so regular you could set your watch by them. More on sidereal time -- after this. November 9: Sidereal Time "Sidereal" comes from a Latin word that means "star." Sidereal time is simply star time -- time measured by the stars. Here's how it works. The stars appear fixed relative to one another. They really are moving through space -- but they're so far away they don't appear to move -- even over a lifetime. But, meanwhile, the Earth rotates once each day beneath the stars. To measure a sidereal day, you first pick out a star that passes overhead. Then you wait until you see the same star again in exactly the same place. When you do, you'll know that the Earth has turned once on its axis, with respect to the stars. One sidereal day will have passed. There's only one problem. A sidereal day isn't 24 ordinary hours long. Instead, as measured by your watch, it lasts only 23 hours, 56 minutes, and 4 seconds. The reason it isn't 24 standard hours is that Earth is doing more than just spinning. It's also moving in orbit around the sun. Our world's movement in orbit makes the stars all rise a little earlier each night with respect to the sun. If you set your watch by the stars, it would gain four minutes each day. After a week, it would be nearly half an hour fast. After six months, it would be l2 hours fast. Only astronomers and navigators use sidereal time. Because the rest of us like to get up when the sun does, not when Arcturus or Sirius or Betelgeuese or any other star does, we use solar time -- time measured by the sun. Script by Deborah Byrd and Harlan Smith. (c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin