WeatherWhys for December 20,2013 from accuweather.com

The winter solstice occurs when the sun is directly over the Tropic of Capricorn, which takes place this year on Dec. 21 at 12:11 p.m. EST. In the weeks that follow, the sun's vertical rays will migrate northward and reach the equator on the first day of astronomical spring. The south-north migration of the sun in the sky is caused by the tilt of the Earth's axis and the movement of the Earth around the sun.