dipper@utastro.UUCP (Debbie Byrd) (11/20/84)
Astronomers sometimes use a special kind of time called Julian Time. More on what that is -- right after this. November 20 Julian Time Cycles in astronomy help us measure time. The interval of one month is based on the orbit of the moon -- and a year is the time Earth takes to orbit around the sun. Those are definite cycles -- but generally you can measure time any way you want. That's why astronomers can use a system called Julian Time. This system was invented in 1582 by an Italian scholar, who named it Julian for his father. The idea is that there's a Julian Day 1, set arbitrarily to be January 1 in the year 4713 B.C. The next day was Julian Day 2, then Julian Day 3, and so on, one day at a time, until the present. Each Julian Day starts at noon, Greenwich Time -- that's local time in Greenwich England. Julian Time lets you count days one after another, without the bother of months and years. That's important in astronomy. For example, suppose you knew a certain variable star reached its peak brightness one year on January 31 -- then the next year on March 1 -- and you wanted to find the number of days between the two peaks in order to predict the next one. Well, instead of counting days in the months, you could just subtract one Julian date -- one single, rather long number -- from another. Julian dates are used mainly by professional astronomers as an aid to their research. But it's a fun system for us amateurs, too. It's fun to known your Julian birthday, for example -- mine's J.D. 2,431,485. And of course every day has its own Julian date. At noon Central Standard Time today, the Julian date is 2,446,025.25. Script by Deborah Byrd (c) Copyright 1983, 1984 McDonald Observatory, University of Texas at Austin