Bad news for those who like daylight: we won't have much on Friday.
Dec. 21, 2012, is the winter solstice in earth's Northern Hemisphere. The days have been getting a little shorter since the summer solstice last June.
As you may remember from your grade school science lessons, the seasons and the changing lengths of the day and night throughout the year are a result of the Earth's axial tilt.
Try to visualize the Earth's orbit around the sun as an elliptical path on an imaginary plane in space. As the Earth rests in that plane, its north and south poles—the ends of its axis—do not point straight "up" and "down." The axis is instead about 23.4 degrees off the "vertical."
As a result, the northern and southern hemisphere do not receive equal amounts of sunshine throughout the year. Right now, the northern hemisphere is "leaning" away from the sun. From tonight until the summer solstice next June 21, as the Earth continues around the sun, that tilt in the planet's axis will be "leaning" our hemisphere more towards the sun each day.
If not for the tilt of the Earth's axis, we would not have seasons. The day and night would be exactly the same length, year round. The northern and southern hemispheres would share the sun's light equally. Right now, that only happens on the days of the spring and fall equinoxes (March 20 and September 22, 2013).
The sun's path will reach its southern most high noon position in the sky, directly over the Tropic of Capricorn, according to An Almanac Minute: The Winter Solstice. The short video is attached to this article.