December 21 might not have brought the apocalypse that some were expecting, but an important celestial event did occur around 6 am Eastern time: the winter solstice. People across the Northern Hemisphere can look forward to longer and longer days as we make the inexorable march towards spring.
On Friday, the northern half of our planet is tilted as far away from the sun as it gets, and the sun appears to hang extremely low in the sky at noon.
The reason for the season is axial tilt. The Earth doesn't spin around an axis that points straight up and down relative to its orbit; it rotates on a bit of an incline, with the North Pole pretty much constantly pointing towards the north star, Polaris. Axial tilt means that during certain parts of our orbit around the sun, the Northern Hemisphere is tipped more towards Earth than the Southern Hemisphere, making it summer for half the planet; then, in another part of the cycle, we're tilted back away from the sun.
Winter solstice coincides with the longest night of the year.
“But that’s a good thing!” astronomer Phil Plait wrote for Slate on Friday. “Every day for the next six months, we’ll slowly round the Sun and have our axis point more toward it. The Sun will get higher, the days longer and warmer.”
As we progress towards the summer solstice, we'll pass through the vernal equinox on March 20, the point in spring when day and night will be about the same length.
Throughout human history, the winter solstice has been a time of celebration and significance, especially for agricultural communities more at the mercy of nature. Stonehenge, for instance, appears to have been constructed in part to observe celestial events like the summer and winter solstices.
The solstice also brings the peak of the Ursid meteor shower, which happens as Earth passes through the stream of debris left by Tuttle's Comet. The Ursids, which appear to radiate from the constellation Ursa Minor, aren't show-stoppers like the recent and much more active Geminids, and it'll be harder to see them thanks to the waxing moon.
December 21 also marks another special celestial event, as the sun appears to get close to Sagittarius A*, the center of our Milky Way galaxy. Despite what some conspiracy theorists might say on YouTube, the sun isn't going to align perfectly with the center of the galaxy.
"The winter solstice Sun does not get closer than 3 degrees to the center of the Milky Way Galaxy," the California-based Griffith Observatory says. "This is equal to six full moons [from our perspective], a very large discrepancy, even for the unaided eye."
Some doomsday seers and profiteers have stoked fears that a so-called Galactic Alignment means Earth will be sucked into some kind of catastrophe, but since the alignment happens every year, there's no reason to expect 2012 will be any more eventful than the last few millenia.
“Enjoy the solstice, by all means, and don't let the Dark Rift, alignments, solar flares, magnetic field reversals, potential impacts or alleged Maya end-of-the-world predictions get in the way,” NASA says.