Tonight (December 21, 2011 at exactly 11:30pm) marks the Winter Solstice, or the official first day of Winter. It looks like someone forgot to give Mother Nature the memo, we’ve been having some great weather so far.
The winter solstice occurs exactly when the axial tilt of a planet’s polar hemisphere is farthest away from the star that it orbits. Earth’s maximum axial tilt to our star, the Sun, during a solstice is 23° 26′. More evidently from high latitudes, a hemisphere’s winter solstice occurs on the shortest day and longest night of the year, when the sun’s daily maximum elevation in the sky is the lowest. Since the winter solstice lasts only a moment in time, other terms are often used for the day on which it occurs, such as midwinter, the longest night or the first day of winter.
The seasonal significance of the winter solstice is in the reversal of the gradual lengthening of nights and shortening of days. Depending on the shift of the calendar, the winter solstice usually occurs on December 21 to 23 each year in the Northern Hemisphere, and June 20 to 23 in the Southern Hemisphere.
