dipper@utastro.UUCP (Debbie Byrd) (09/22/85)
A good day to celebrate -- the first day of autumn. More on the September equinox -- when we come back. September 22 The September Equinox Today would have been a special day to the ancient stargazers -- and it's still a good excuse to celebrate. It's the September equinox -- one of two days each year when the sun crosses the celestial equator. The celestial equator is really just an imaginary line drawn around the sky -- directly above the equator of the Earth. Today the sun crosses this imaginary line, which means the sun today is seen directly above our planet's equator. After today, the sun will continue to shift its path across the sky toward the south. Its rays will strike the northern hemisphere less directly -- and winter will come to our half of the globe. Meanwhile, on the southern half of the Earth, people today are celebrating the first day of spring. The equinox was an important day to people long ago. They didn't have calendars. Instead they watched the skies for signs of the changing seasons. Once they understood the equinoxes and solstices, the ancients had the rudiments for making the first calendars. The two equinoxes and two solstices were observed by the ancients -- but they weren't understood as we understand them today. Today we know that the sun shifts its path across the sky because Earth orbits the sun -- and because Earth tilts on its axis by some 23 and a half degrees. As we journey around the sun, the tilt of the Earth causes the path of the sun to appear to shift toward the north -- then toward the south in our sky. Script by Deborah Byrd. (c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin