At 12:04 p.m. UT (5:04 a.m. Mountain US Time) on December 21, the Sun will reach its most southerly declination. Over the past six months, for people in the northern hemisphere, every day the peak of the Sun's arc across the daytime sky has been getting lower. Well, at 12:04 UT it reaches its lowest point, and now every day the Sun will be a bit higher in the sky at local noon. It will peak on June 21 at 05:45 UT, and be at the highest point it can get. Then the process reverses.
We call those points in time the solstices. Despite the fact that half the Earth has its seasons reversed, tonight is the Winter Solstice, and June's is the Summer Solstice. If you live south of the Equator, sorry, but that's what you get for living your lives standing on your head.
Anyway, the Winter Solstice has always been a time of celebration, because ancient people -- more closely tied with the skies due to agriculture and no light pollution -- knew that it meant the Sun was coming back up, and spring was coming. We celebrate it in modern times by going into debt. We're far more sophisticated these days.
So given the weather here in Boulder the past few weeks, even though I love the snow, I'll be glad to see the Sun up a little it longer every day.
Next on the trip 'round the Sun: perihelion, January 4th!