Shortest Day Today: A Signal to Santa?

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Interesting concept, yes? Maybe, way up north at Santa’s workshop, Santa needs a sure-fire method of knowing when it’s time to deliver the toys. You know, in case his watch runs down or the electricity fails or the elves hide the calendar. He certainly can’t use falling leaves or the tides. Would the winter solstice, also known as the shortest day of the year, help Santa? After all, the shortest day of the year typically precedes Christmas by four days. Who needs a calendar when you have the unfailing reliability of nature?

Santa's Workshop on the Winter Solstice. Image: NOAA

Except…the North Pole hasn’t seen daylight since October. Early October. And Santa’s workshop won’t cast a shadow again until March 21, the vernal equinox. Even then, the sun will just crest the horizon. Without daylight for months at a time, it’s tough to know which day is the shortest day when all the days are — well, nights. (And you thought it was the fog that gave Rudolph his big chance!)

So what’s going on? What’s with the shortest day of the year and why is the North Pole cloaked in darkness for months? It’s all about how the Earth rotates around its own axis and how it rotates around the Sun. Part of the answer is the winter solstice, and it’s happening today.

As you know, the Earth takes 365 days to make one complete orbit around the sun. Additionally, it takes 24 hours to make one complete rotation about its own axis. That axis however, is key to everything. It’s tilted, and because the axis is not exactly perpendicular to the sun, the Earth receives unequal heating across its surface. And because the tilt does not change throughout Earth’s one-year journey around the sun, northern and southern hemispheres are on the receiving end of maximum solar radiation for exactly six months at a time. And within that six-month exposure there is a halfway point for each hemisphere: the winter solstice (today in the Northern Hemisphere) and the summer solstice (June 20 in 2012; June 21 in 2013).

Try not to think of it as one six-month long, unchanging exposure. Instead, think of it as three months going “in,” and three months coming back “out.” Indeed, with each day going “in” to the summer season there is progressively more exposure to the sun: each day across the hemisphere growing progressively longer and warmer. At the solstice (and the beginning of the second three-month period), each day begins to receive less daylight and less solar radiation, getting dark earlier and getting cooler. Likewise, the opposite is true in the winter season. Each day going in gets progressively shorter and cooler until reaching the winter solstice — when the process begins to reverse, one day at a time.

Armed with a little knowledge, it should come as no surprise to see that it's night time outside Santa's window as he puts the finishing touches on a few toys. If Santa's workshop was at the South Pole, there would be nothing but sunshine at this time of year. Image: santastoys

Also consider what’s happening to the other hemisphere today. The solstice is the solstice for both hemispheres at the same time, not just one. At the very same moment the northern hemisphere is experiencing its coldest and darkest day, the southern hemisphere is experiencing its longest and warmest day. With that in mind, it’s easy to visualize the Earth tilted in such a way to maximize sun exposure to the south, but minimizing the exposure to the north. And the extreme latitudes in both hemispheres get the short (or long) end of the stick, depending on the time of year. This explains why Santa’s workshop at the North Pole is in total darkness from early October, and won’t see sunlight again until early March. It also explains why relying on the shortest day of the year as a countdown to loading his sleigh is a bad idea. Plus, everybody knows Santa uses his lithium-ion-powered smartphone and Twitter feed to follow the master atomic clock at the Naval Observatory in Washington, D.C. to keep his team on the tightest schedule.

The winter solstice arrived early this morning at 0530 UTC (0030 EST).

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