Although it may seem like winter started back in November, the first day of winter is actually December 21!
The arrival of winter (and all seasons) is a result of Earth tilting 23.5 degrees on its spin axis together with the 365 day orbit around the sun. When the North Pole is tilted towards the sun, this is summer for the North Pole and when the North Pole is tilted away from the sun, this is winter for the North Pole.
December 21 is also the winter solstice. On this day, daylight is at its shortest for the year in the Northern Hemisphere. This is why ancient civilizations built calendars around sunlight, so that they could determine when to harvest.
Learn more about winter at Arizona Science Center’s Snow Week, December 26-January 1.