Welcome to winter.
While it’s tempting to think of everything after Thanksgiving as “wintertime” – especially if you’ve already been shoveling snow for months – the official first day of winter in the Northern Hemisphere actually falls in late December. This year, Thursday, Dec. 21 is that day.
Why the 21st? The start of winter is determined by the winter solstice – that is, the day with the fewest hours of daylight, when the North Pole reaches its farthest tilt away from the sun. The solstice usually lands on Dec. 21 or 22, but on rare occasions can occur on Dec. 20 or 23. (If you’d like to witness the next Dec. 23 winter solstice, you’ll need to wait until 2303, according to timeanddate.com.)
What’s so special about the winter solstice? Two things: First, from the Earthling point of view, the solstice marks the day the sun reaches its southernmost position of the year. After seeming to hover in place for a few days (“solstice” is derived from the Latin for “sun stands still”), it will start tracking back north.
Second, after the solstice, the days (technically the “daylight periods”) start getting longer for those of us above the Equator. This was a huge deal for past civilizations, and remains so for those of us who hate driving home from work in the dark.
How much darkness are we talking? That depends on where you live. Here in Denver, we’re looking at 14 hours, 39 minutes between sunset and sunrise. Our friends in Anchorage, A.K., will enjoy 18 hours, 33 minutes of night. (Though for them, the day will seem even darker since they’re up north and the sun, as you’ll recall, is way down in the southern sky. But that’s a whole other blog…)
So if we’re getting more sunshine in January and February, why doesn’t it get warmer during the first part of the year?
Seasonal lag. As any weather forecaster will tell you, the sun isn’t the only factor in our temperatures. For one thing, oceans and other bodies of water take a while to heat up. So while we may be getting a bit more sun, the Northern Hemisphere as a whole is still losing more solar energy than it’s gaining. Things won’t start heating up until we see a net gain in solar energy some weeks from now. But we will see longer days.
So while winter is just beginning, things are already starting to get brighter. Not a bad way to close out a crazy year.