Dec 21, 2009, and the winter solstice is upon us, or was just before noon today. You might think the shortest day of the year would also have the latest sunrise and 
the earliest sunset. But not exactly, the reason is because of the earth's tilt, curvature, the shape of our planet's orbit, and the fact that the earth is whipping through space around the sun at almost 20 miles a second! If you were standing in the middle of the US and took a picture of the sun at its highest point throughout the year, say three times a month or so, it would make a sort of lopsided, leaning figure eight called an analemma, like the one to the left near Athens, Greece. The shortest day occurs when the sun is nearest the horizon at its highest point of the day.
But because of the earth's curvature, motion, tilt, all affecting the sun's apparent motion during the year, the earliest sunset occurs a couple of weeks or so before the solstice and the latest sunrise a couple of weeks or so after. In short, the analemma is lopsided and the bottom is rounded, and so the sun moves a moves little from the left to the right of its lowest position in relation to a curved horizon, resulting in slightly different positions of sunsets and sunrises which in turn determine the earliest sunset and latest sunrise.
Sounds complicated, but this stuff was worked out by many neolithic societies even before the advent of agriculture. And for good reason, the sky was the prime time entertainment of their day. The patterns and changes in the stars, phases of the moon, along with sunrises and sunsets were terribly important. Afterall, if the sun is creeping lower and lower in the sky, it might not stop! What if it keeps going lower until it's gone and never comes back? Even stone age humans knew that would be a disaster. The day it stopped going lower and reversed course heading to sweet, gentle spring and summer was greeted with relief and celebration. Later settlements had to know when to plant, or take the flock to greener spring pastures. So the sky was mighty important and events were often encoded in dramatic stories that served these people better than Dancing with the Stars or American Idol serves us today. Astrology, the date of Christmas, and some pagan rites are three distant descendants of this early astronomical interest that survive into modern times.
Here in Austin the earliest sunset was on Dec 1 - Dec 7, at 5:30 PM. The latest sunrise will be Jan 7 - Jan 11 at 7:29 AM. Happy Holidays and Happy Winter Solstice!
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