Winter Solstice 2011: Shortest Day of the Year

Dec. 21, 2011 — -- If the early evening gloom is getting to you, take comfort that the days are about to start getting longer.

The winter solstice -- which marks the beginning of winter and the shortest day of the year in the Northern Hemisphere -- comes tonight at 12:30 a.m. EST.

We're assured that the Mayan calendar doesn't really predict that the world will end a year from today, but there's no fighting the laws of orbital mechanics, which have been known in some form since before Stonehenge was built (more about that in a moment).

A quick reminder of what's happening: The Earth, turning on its axis as it circles the sun, is tilted at an angle of 23.5 degrees relative to its orbit. Whatever the season, the axis points the same way, with Polaris, the North Star, hovering over the North Pole. (It actually shifts slowly in a 26,000-year cycle, but that's an issue for another day.)

Tonight is the night that the axis, as seen from the north, points as directly away from the sun as it will all year. So Chicago, for instance, will get just 9.1 hours of daylight tomorrow, Atlanta and Los Angeles will get 9.9 hours (they're closer to the equator) and everything north of the Arctic Circle will experience 24 hours of darkness.

We've known all this for a long time, which conveniently brings us back to Stonehenge, which was built between 3000 and 1600 BC. Modern scientists figured out long ago that it was, among other things, an ancient astronomical calendar, and that on the morning of the summer solstice -- usually June 21, when northern days are at their longest -- the sun rises over the famous heelstone at one end.

But they have continued to wonder how the ancient builders of Stonehenge built the great circle, and where they got the rock for it -- none of which is commonly found on Salisbury Plain, west of London.

Now, researchers from Leicester University and National Museum Wales report they've made a match. They say rocks at Craig Rhos-y-felin, an outcropping in south Wales 100 miles away, are 99 percent similar in mineral composition and texture to the rock at Stonehenge -- more similar than any other stone found. The researchers said they were able to match Stonehenge rock to an area just a couple of hundred feet across.

"Being able to provenance any archaeologically significant rock so precisely is remarkable," said Rob Ixer, a Leicester minerologist, in an interview with the BBC.

Tomorrow morning the sun won't rise over Stonehenge until 8:08 a.m. local time, and it will set at 4:02 p.m. But take comfort that the shortest day of the year will now be past.