At 11:30 p.m. tonight (Wednesday, December 21) the sun will be directly over 23.5° South latitude, also known as the Tropic of Capricorn. This is the southernmost extent of the sun in the sky relative to the earth's surface. For us it means the sun travels from southeast horizon to southwest horizon during the day but very low in the sky. This low sun angle creates minimal warming. Welcome to the winter solstice.
The winter solstice is the astronomincal beginning of winter. I qualify it as astronomical winter because meteorological winter begins December 1 and runs for three months inclusive. This is for the convenience of weather record keeping.
Many people will claim that the day of the winter solstice is the shortest day of the year. That is incorrect on two counts. First, a day is 24 hours....well, actually it is 23 hours, 56 minutes, and 4 seconds. It can't get any longer or shorter. The more correct way to describe the winter solstice is the shortest amount of daylight. But even that wouldn't be entirely correct.
Here in Milwaukee, our minimal amount of daylight is 9 hours exactly, and that occurs not just on one day but on several. In fact, between December 19 and December 26, daylight hours in Milwaukee fluctuate between 9 hours and 9 hours and 1 minute. After December 26 the daylight hours get longer again, much to the relief of most people who are sick of later sunrises and earlier sunsets this time of year.
Which brings up another oddity. The dates of the latest sunrise and earliest sunset do not correspond to the winter solstice. In fact, the earliest sunset in Milwaukee is 4:17 p.m. from December 4 through December 14. The latest sunrise is 7:23 a.m. from December 29 through January 9.
The reason for this oddity is twofold: the earth orbits the sun in an elliptical orbit, not a circular one; and the earth is tilted on its axis by 23.5° degrees. Those two effects mean that "solar noon" (when the sun is directly south of us in the sky) is different than local noon on the clock.
Earth's elliptical orbit around the sun means it travels faster near Perihelion (early January) compared to Aphelion (early July).
An elliptical orbit means the earth is closest to the sun this time of year and farthest from the sun in early July. At closest approach, the earth is moving faster in its orbit than it does in summer. In the northern hemisphere, winter is 3 days shorter than summer due to this orbital speed.
To read more about earliest sunset/latest sunrise not occuring on the winter solstice, check out a detailed explanation HERE.