Why December Has The Longest Days

MinutePhysics explains why the holiday season is more complicated than you might think.

Weekly Photo Challenge: Goodbye, Gulf of California

Here’s a shot of the very tip of the Gulf of California (AKA Sea of Cortez), shortly before crossing over Baja and the border back into the United States. It’s always interesting seeing major landmarks from the air. Large version viewable here.

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Happy Winter Solstice!

Hey, folks. Happy Winter Solstice!

*Crickets chirping*

…Yeah, okay. It’s not exactly Christmas. But the Winter Solstice is still one of the most important days on the calendar. If you live north or south of the equator, your December 21st was abysmally short or exceptionally long respectively. Why? It’s a little thing called the axial tilt. Earth doesn’t just orbit around the sun; it does it at an angle. It’s like if you put a marshmallow on a stick, constantly spun the stick around, and held it over a campfire while walking in a circle. If you’ve got a globe handy (Does anyone actually still have one of those on their desk?), you can check it out yourself. You’ll find that it’s tilted to 23.5 degrees, just like the real thing. Oh, and while you’re looking, try finding the Tropic of Cancer and the Tropic of Capricorn. They mark the northern and southernmost latitudes at which the sun can appear directly overhead. The summer and winter solstices are basically the sun reaching these lines, in this case being the Tropic of Capricorn.

It’s kind of important.

Knowing the sun’s location was essential to the development modern civilization. It was the basis of our calendars and how we measure years. You know the calendar on your phone you probably never use? Yeah, they didn’t have an app for that 2,000+ years ago. A guy named Hipparchus had to do that stuff the old fashioned way: strict observations and copious amounts of trigonometry. Even that wasn’t enough; it took until 1627 for Kepler to figure out exactly how long a year lasted. And it’s still being perfected now! Since the measurements are based upon our observations of the sun, any new technologies associated with it can help get the numbers more accurate. Considering how the seasons and our calendars may not match up a few thousand years from now, it’d be nice to get things as cleared up as possible.

And you thought Daylight Savings Time was complicated.

Timekeeping aside, why is the location of the sun so important? Well, it’s basically the 15th century version of Google Maps. If you have a compass and the tools to measure the sun’s position relative to the horizon, you could better understand your position and navigate the world more safely. You think having less daylight to do your Christmas shopping is bad? Try sailing across the Atlantic Ocean. But if you’re stuck at home, the sun’s location is a huge factor in something even more essential: Food. You know all that stuff you bought for your holiday meals? A lot of it needed sunlight. And even that’s after all the agricultural advancements made in the past hundred years or so. If you have a garden, you know how this goes; I am longing for my home-grown tomatoes. Just imagine what it was like for ancient cultures who knew the seasons were changing and their food supply was going to be running low. It’s probably why there are so many rituals and festivals associated with this time of year; months of preparation and harvesting just to make sure the winter could be survived. What better way to celebrate life than to do so in the face of death?

And what better day to do it than with the Winter Solstice?

Oh, and one last thing. The Tropic of Cancer and Tropic of Capricorn? They got cool names for a reason: “Tropic” refers to the Greek tropos, or turn. Cancer and Capricorn refer to the constellations in which the sun appeared in during the respective solstices. Thus, the sun turns back from Cancer or Capricorn during the summer and winter solstices respectively. However, Capricorn is a technically outdated name; after 2,000 years of slow shifts in the Earth’s axis, the sun enters the Sagittarius constellation now. Is it time to change the maps yet?