The day is also the summer solstice, or longest day of the year, in the southern hemisphere.
Likely older than Stonehenge, the opening of this enclosed, mound-like structure is designed so that as the sun dawns on the morning of the winter solstice, light shoots down a tiny, cramped passage at a flawless angle to light an interior chamber. The axis is the line through the planet that Earth spins around, connecting the north and south poles.
During the winter solstice, the sun will be quite low on the sky, appearing to set right in the place where it would rise. But the axis isn't quite perpendicular to the path of its orbit, instead it is tilted on its side about 23 degrees, with the axis always pointing the same way into space.
This position is the southernmost the sun will reach throughout the entire year, marking the exact moment when the winter solstice will occur. But astronomically, that's a bit of an oversimplification: Technically, the solstice this month falls on December 21 at 4:28 PM Universal Time, or 9:28 PM Eastern time. The summer solstice is when the sun's rays are directly over the Tropic of Cancer.More news: This Chinese startup's electric SUV is a lot cheaper than Tesla's
It would make sense that the shortest day of the year would also be the coldest, but that's actually a bit of a misnomer.
While the shortest day can be kind of a bummer, it has a little silver lining: It means that from here, sunlight will stick around longer and longer. Here are six things to know about the longest night of the year. Basically, the farther above the equator a country is located, the colder will be its temperature over winter solstice. And that's why, for millennia, humans have celebrated the solstice-for the return of the sun that it marks.
This year's event is even more noteworthy as it comes on the 50th anniversary of the monument's solar alignment being first documented in 1967 by archaeologist Michael J O'Kelly.