dipper@utastro.UUCP (Debbie Byrd) (12/01/84)
The earliest sunsets of the year take place around now. More on the earliest sunset -- right after this. December 1 Earliest Sunset From the latitude of Austin, Texas, the earliest sunsets of the year happen this week. That doesn't mean the shortest day of the year is this week. The shortest day comes on the winter solstice, on December 21. On the winter solstice, the sun makes its lowest arc across the sky as it journeys from sunrise to sunset. That day has the fewest total minutes of daylight. But today is the earliest sunset for about 30 degrees north latitude -- our latitude here in Austin. So it must be that the sun rises earlier today than it will on the day of the solstice -- to make this a slightly longer day overall. For higher latitudes, say about 40 degrees north latitude, the earliest sunsets of the year come later, around December 5. The earliest sunsets of the year vary somewhat with latitude. But they always come before the shortest day on the solstice. Likewise, the latest sunrises of the year aren't on the day of the solstice -- they always come a few weeks afterwards. So the sequence is: earliest sunset in early December, shortest day on the solstice, and finally the latest sunrise, in early January. The dates are different because our clocks don't jibe precisely with the sun. The sun can get ahead or behind clock time, because Earth's orbit around the sun isn't a perfect circle, and because Earth tilts on its axis. If you checked, you'd find that sunrise and sunset times slip back and forth by a few minutes against our smoothly running clocks. Script by Deborah Byrd. (c) Copyright 1983, 1984 McDonald Observatory, University of Texas at Austin