Today, you might already know, is the winter solstice. On June 21 or 22, they get the longest days and shortest nights. At any location in the Northern Hemisphere, in other words, tonight's period of darkness will be slightly longer than any other, ever — at least, since the planet started spinning right around the time it was first formed some 4.5 billion years ago. Here are five questions (and answers) that explain the winter solstice… 
 
 1. The Earth orbits around the sun on a tilted axis, so sometimes, the Northern Hemisphere gets more exposure to sunlight over the course of a day, and sometimes, the Southern Hemisphere does. Correction: The article below said that, due to the rotation of the Earth gradually slowing down over time, this winter solstice would feature the longest night ever. Every year, on December 21 or 22, this tilt means that locations in the Northern Hemisphere get the shortest duration of sunlight they'll get all year, so they experience the shortest day and longest night. Read more: 
 
 Winter Solstice 2013 
 
 Winter Solstice 2012 
 
 Winter Solstice 2011
– The winter solstice officially occurred at 6:04pm EST yesterday, marking the first day of winter and the longest night of the year. But all "longest nights" aren't created equal, and the longest night in Earth's history likely happened in 1912. Here's why: The Earth's rotation has been slowing a little bit every year since the planet formed some 4.5 billion years ago, and we have a phenomenon known as tidal acceleration to thank. The moon's gravity pulls on our oceans, tugging the oceans that face it into a "bulge" (LiveScience digs into why there's a bulge on the other side, too). But as the Earth rotates, that tidal bulge is pushed to a position just ahead of the point of our planet that's directly below the moon. That causes the Earth to experience "just a bit of friction from this bulge of water as it rotates, slowing it down slightly," explains Vox. And as a NASA article explains, as Earth slows, the moon is able to inch away from us. The effects of these things are so tiny it takes atomic clocks to spot them, but they're very gradually making our nights longer—mostly. In the long view, we're slowing, but geological factors (like ice melting at our poles, for instance) can impact the rotational speed, and as this University of California Observatories graph shows, the length of our day has gone infinitesimally up and down over the last century, but the peak—and therefore the longest night in our history—occurred in 1912. As far as the length of last night goes, it depends on where you spent it, notes the Washington Post. Washington, DC, saw about 9.5 hours of daylight yesterday, while Miami got almost 10.5, and Fairbanks, Alaska, saw less than four. (The 2010 winter solstice was remarkable for quite another reason.)