text: 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. 
   
   I got this wrong. The Earth's rotation is gradually slowing on an extremely long timescale, but on a shorter year-to-year basis, geologic factors can alter the speed as well. 
   
   Data indicates that the rotation speed has actually sped up slightly over the past forty years (due to all sorts of factors, such as melting of ice at the poles and the resulting redistribution of the Earth's mass), and before that, the trend was up-and-down for most of the 20th century — so, as far as we know, the longest night in Earth's history likely occurred in 1912. I apologize for the error. Thanks to Steve Allen and Ryan Hardy for pointing it out. 
   
   Read: No, this winter solstice wasn't the longest ever. Scientists explain how we got this wrong. 
   
   Today, you might already know, is the winter solstice. That means for people living in the Northern Hemisphere, it's the longest night of the year. 
   
   However, as science blogger Colin Schultz points out, tonight will also be the longest night ever. 
   
   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. 
   
   Why this night will be the longest ever 
   
   The reason is that the rotation of the Earth is slowing over time. Every year, scientists estimate, the length of a day increases by about 15 to 25 millionths of a second. 
   
   It may be a truly tiny amount (and it means that even in your entire lifetime, the length of a day will only expand by about two milliseconds), but it forces official timekeepers to add a leap second every few years. 
   
   The main reason Earth's rotation slowing down is the moon. Shortly after the formation of Earth, it was impacted by a planet-sized object. This enormous collision threw off the material that would eventually coalesce into the moon, and also sent Earth spinning quite rapidly. 
   
   In the four-plus billion years since, that spinning has slowed down pretty significantly (with an Earth day going from about six hours to 24 hours as a result) because of the moon's gravity. 
   
   The moon's gravity pulls ocean water slightly toward and away from it, causing tides. But because of the alignment of the two bodies, the resulting bulge of water is slightly ahead of the spot on Earth that's directly under the moon. 
   
   As a result, the Earth encounters just a bit of friction from this bulge of water as it rotates, slowing it down slightly. 
   
   The phenomenon — called tidal acceleration — also allows the moon to drift slightly farther away from Earth over time. (It's also what's led the same face of the moon to always faces Earth as it rotates around us, and eventually, if things went on long enough, the same face of Earth would always face the moon as well, a phenomenon called tidal locking.) 
   
   There are a few other things that contribute to Earth's slowing down, but their contributions are minor. One is that the moon's gravity similarly causes Earth's crust to flex, like its water, leading to some friction as well. 
   
   Why winter solstice is the longest night of the year 
   
   This one is much simpler. 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. This is what accounts for the changing of the seasons. 
   
   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. On June 21 or 22, they get the longest days and shortest nights. 
   
   Meanwhile, everything is reversed for locations in the Southern Hemisphere — they have their longest days in December, and longest nights in June. 
   
   Related: The year we just lived through, in one video. ||||| Winter weather hit many parts of the U.S. early this year. Yet officially (using the astronomical and traditional definition), the new season begins with the winter solstice at 6:03 p.m. ET today. 
   
   Why are the shortest days not the coldest? And why was our earliest sunset already two weeks ago? Here are five questions (and answers) that explain the winter solstice… 
   
   1. Why are the days so short? 
   
   On the December solstice, the Northern Hemisphere is at its maximum tilt away from the sun and experiences significant amounts of darkness. (timeanddate.com) 
   
   On the December solstice, the sun appears directly overhead at 23.5 degrees south latitude, along the Tropic of Capricorn. While the Southern Hemisphere enjoys its longest day of the year, we here in the Northern Hemisphere see the sun follow its lowest and shortest path across the southern sky. For the next six months, the days will gradually lengthen as our hemisphere begins to tilt back toward the sun. 
   
   2. What time is sunrise and sunset on the winter solstice? 
   
   Sunrise and sunset times in major U.S. cities on the December solstice. (Justin Grieser, data from timeanddate.com) 
   
   For the Northern Hemisphere, the winter solstice marks the shortest day and longest night of the year. The nation’s capital sees under 9 and a half hours of daylight on the solstice – but it could be worse. Northern cities like Seattle and Minneapolis get less than 9 hours of daylight, while the sun is up for barely 4 hours in Fairbanks, Alaska. 
   
   If the sun is shining on December 21 or 22, your shadow at local noon will be the longest of the year. The higher the latitude, the lower the sun appears in the sky, which means your shadow will be significantly longer in Seattle than in Atlanta or Miami. 
   
   3. Where does the sun rise and set along the horizon? 
   
   Position of sunrise and sunset in Washington, D.C. on the winter solstice. (suncalc.net) 
   
   Thinking about photographing the solstice sunset? Look to the southwest! On the December solstice, all locations on Earth (outside of the polar regions) see the sun rise and set at its southernmost point along the horizon. 
   
   The map of Washington, D.C. (above) shows sunrise and sunset occur within 120 degrees from due north along the horizon – well to the southeast and southwest. The higher your latitude, the closer sunrise and sunset appear to due south on a compass. Around the Arctic Circle, the location of sunrise and sunset converges in the southern sky, until eventually the sun never makes it above the horizon. 
   
   4. Why are the earliest sunset and latest sunrise not on the solstice? 
   
   Date and time of the earliest sunset and latest sunrise in major U.S. cities. (Justin Grieser) 
   
   The winter solstice marks the shortest daylight period, but it’s not the day of the latest sunrise or earliest sunset. In the mid-latitudes the earliest sunset occurs in early December, while the latest sunrise is not until early January. This misalignment occurs because of a discrepancy between “clock time” (which is based on 24 hours), and “solar time” (the time it takes for the sun to appear in the same position in the sky from one day to the next). 
   
   The Earth’s axis is tilted 23.5 degrees, and our orbit around the sun is elliptical (non-circular). In December, these two factors combine in such a way that our days are actually a few seconds longer than 24 hours – as seen by the amount of time it takes the sun to cross our local meridian (longitude) from one day to the next. In effect, this pushes the time of solar noon several minutes later during December, advancing both sunrise and sunset times even as the days continue to shorten until December 21. 
   
   Above we see that Washington, D.C. had its earliest sunset on December 8, while the latest sunrise is not until January 5. At higher latitudes, the date of the earliest sunset and latest sunrise occurs closer to the solstice, while closer to the equator (e.g. Miami), they occur more than a month apart. 
   
   5. Why do the coldest days of the year come after the solstice? 
   
   
   
   The coldest day of the year on average across the U.S., based on 1981-2010 climate normals. (NCDC) 
   
   Even though daylight slowly increases after the solstice, many places don’t see their coldest days until mid-January. This happens because the Northern Hemisphere continues to lose more heat than it gains for several more weeks. The oceans – which take longer than land to heat up and cool down – play a role in this seasonal temperature lag. Only after the Northern Hemisphere starts to receive more solar energy than it loses do average temperatures begin their upward ascent. 
   
   Recently, the National Climatic Data Center published a map showing when the U.S. sees its coldest temperatures of the year, based on 30-year climate averages. Above, we see that the intermountain West tends to see its coldest low temperatures closer to the winter solstice, while the coldest days in the Northeast usually don’t arrive until late January. The “average” coldest day of the year depends on several factors, including proximity to water and the timing and amount of snow cover. 
   
   Whether you love or hate winter, the winter solstice offers a ray of light for everyone: Snow lovers can rejoice that the coldest days are still upon us, while for warm weather fans, the lengthening days mean spring is just a few more months away. 
   
   Read more: 
   
   Winter Solstice 2013 
   
   Winter Solstice 2012 
   
   Winter Solstice 2011
target: – 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.)