So, it’s officially Winter.

But why is it Winter today? Why not yesterday or tomorrow or next Tuesday?

It has to do with the tilt of the Earth’s axis relative to the Sun. Specifically, when the angle reaches 23° 26′, Winter begins.

The Winter Solstice occurs on the day with the shortest sunlight hours and the longest night. Up to this point, the days have been getting shorter and the nights longer. From now until the Summer Solstice, we’re going to start seeing more sunlight each day – something that I’m quite happy about!

Many cultures have traditions based on the winter solstice. As I wrote a couple of years ago, the Norse celebrated Yule from the 21st of December through January, giving us our 12 days of Christmas. The ancient Greeks celebrated the festival of Cronus, Kronia, which the Romans rebranded as Saturnalia. Wikipedia has an extensive list of the various cultural celebrations based on the winter solstice that’s worth checking out.

In the mean time, Gledelig Jul!

A tip o’ the hat to NASA for the graphic by Dr. David P. Stern. You can find out when the various solstices, equinoxs and other astronomical events happen on the USNO’s website.