Shortening days and falling temperatures are heralding the arrival of winter which occurs at one instant on the shortest day of the year, today at 11:28 am ET. It is when the sun is at its lowest, or most southern position in the sky. The transition happens at the same moment around the world, though its local time depends on which of the 24 times zones you call home. Starting tomorrow the days will grow longer, though only a few minutes a day, until we celebrate the longest day of the year, the summer solstice. And so it goes, one season at a time.