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. ||||| The winter solstice is upon us: Wednesday, December 21, will be the shortest day of 2016 for anyone living north of the equator (and Tuesday night was the longest night of the year). If pagan rituals are your thing, this is probably a big moment for you. If not, the official first day of winter is neat for other reasons, too. 
 
 Below is a short scientific guide to the solstice and the longest night of the year (though not, as we’ll see, the longest night in Earth’s history — that happened back in 1912). 
 
 1) Why do we have a winter solstice, anyway? 
 
 Most people know this one. Earth orbits around the sun on a tilted axis (likely because our planet collided with some other massive object billions of years ago, back when it was still being formed). 
 
 So between September and March, Earth’s Northern Hemisphere gets less exposure to direct sunlight over the course of a day. The rest of the year, the north gets more direct sunlight and the Southern Hemisphere gets less. It’s the reason for the seasons: 
 
 In the Northern Hemisphere, "peak" sunlight usually occurs on June 20, 21, or 22 of any given year. That’s the summer solstice. By contrast, the Southern Hemisphere reaches peak sunlight on December 21, 22, or 23 and the north hits peak darkness — that’s our winter solstice. 
 
 Technically speaking, the winter solstice occurs when the sun is directly overhead the Tropic of Capricorn, or 23.5° south latitude. Like so: 
 
 In 2016, the winter solstice will occur at exactly 5:54 am EST on Wednesday, December 21. 
 
 2) How many hours of sunlight will I get on Wednesday? 
 
 That depends on where you live. The further north you are, the less sunlight you’ll get during the solstice — and the longer the night will be. Alaska-based climatologist Brian Brettschneider created this terrific guide for the United States: 
 
 Washington DC will get about 9.5 hours of daylight — and 14.5 hours of glorious, glorious darkness. 
 
 On the off chance you live near the Arctic Circle, you’ll barely get any daylight during the solstice (if you live north of the circle, you’ll get none). The time lapse below shows the eerie scene in Fairbanks, Alaska, which only gets three hours of sunlight on the solstice — the sun basically skims the horizon for a a brief while and then vanishes: 
 
 Brettschneider has more charts on the solstice, including the differences in daylight between winter and summer solstices; his entire post is worth your time. 
 
 3) Is the winter solstice the earliest sunset of the year? 
 
 Not usually. Just because December 21 is the shortest day of the year for the Northern Hemisphere, it doesn’t mean every location has its earliest sunset or latest sunrise on that day. 
 
 Brettschneider also created this map showing earliest sunset/latest sunrise for different parts of North America: 
 
 If you live in Washington, DC, you missed the earliest sunset — it happened back on December 6. But you can still catch the latest sunrise on January 6. If you like sleeping in, that’s arguably the most exciting day of the winter. No annoying sun bothering you in the morning. 
 
 4) Is the winter solstice the coldest day of the year? 
 
 Also not usually! It’s true that the Northern Hemisphere gets the least direct sunlight on the winter solstice (which is officially the first day of winter). But the coldest months are yet to come — usually in January or February, depending on where you live. 
 
 A big reason for this “seasonal lag” is that the Earth’s massive oceans absorb much of the sun’s energy and release it slowly, over time. So there’s a delay between when there’s the least sun and when the air temperatures are actually coldest. The same thing happens in summer — there’s a delay between when solar insolation is at its maximum (the summer solstice in June) and when the hottest months are (usually July or August). 
 
 This seasonal lag varies greatly from place to place — during the summer, it’s pretty extreme in San Francisco, which is surrounded by water on three sides, and where temperatures don’t typically peak until September. Likewise, places distant from large bodies of water, like Iowa, can often see sharper swings in temperature than places like Rome that are surrounded by ocean. 
 
 5) What does all this have to do with Stonehenge? 
 
 No one knows for sure why Stonehenge was built some 5,000 years ago (at least I don’t, sorry). But one strong possibility is that it was used to mark solstices and equinoxes. That’s because the structure is directly aligned toward the sunset during the winter solstice. (The sun also rises directly over the Heel Stone during the summer solstice.) 
 
 Why was the winter solstice a big deal? Here’s Teresa Wilson of the American Astronomical Society to explain: “While the summer solstice draws a larger crowd, the winter solstice may have been more important to the ancient builders. At this time, cattle were slaughtered so the animals did not need to be fed through the winter, and wine and beer made previously had finally fermented.” 
 
 Nowadays, humans still gather to pay homage the winter solstice at Stonehenge — they just use modern technology, like so: 
 
 The Wikipedia entry on Stonehenge is absurdly detailed, so read up on that if you want more. 
 
 6) Was Tuesday the longest night in Earth’s entire history? 
 
 Probably not, although it's close. And the reason why is quite interesting. My colleague Joseph Stromberg did a fantastic deep dive into this topic a few years back, but here’s the two-minute version. 
 
 Ever since the Earth has had liquid oceans and a moon, its rotation has been gradually slowing over time due to tidal friction. That means — over very, very long periods of time — the days have been getting steadily longer. About 4.5 billion years ago, it took the Earth just six hours to complete one rotation. About 350 million years ago, it took 23 hours. Today, of course, it takes about 24 hours. And the days will gradually get longer still. 
 
 Given that, you'd think the winter solstice of 2016 would be the longest night in all of history. But while it's certainly up there, it doesn't quite take top honors. 
 
 That's because tidal friction isn’t the only thing affecting the Earth’s rotation — there are a few countervailing factors. The melting of glacial ice, which has been occurring since the end of the last Ice Age 12,000 years ago (and is now ramping up because of global warming) is actually speeding up Earth’s rotation very slightly, shortening the days by a few fractions of a millisecond. Likewise, geologic activity in the Earth’s core, earthquakes, ocean currents, and seasonal wind changes can also speed up or slow down the planet's rotation. 
 
 When you put all these factors together, scientists have estimated that the longest day in Earth’s history (so far) likely occurred back in 1912. That year’s summer solstice was the longest period of daylight the Northern Hemisphere has ever seen. And, conversely, the 1912 winter solstice was the longest night we’ve ever seen. 
 
 Eventually, the effects of tidal friction should overcome all those other factors and Earth’s days will get longer and longer as its rotation keeps slowing (forcing timekeepers to add leap seconds to the calendar periodically). Which means that in the future, there will be plenty of winter solstices that set new records as the "longest night in Earth's history." 
 
 7) I clicked this article accidentally and really just want a cool picture of the sun 
 
 I got you: 
 
 The image above was taken by NASA’s Solar Dynamics Observatory, a spacecraft launched in 2010 to better understand the sun. Caption: "A full-disk multiwavelength extreme ultraviolet image of the sun taken March 30, 2010. False colors trace different gas temperatures. Reds are relatively cool (about 60,000 Kelvin, or 107,540 F); blues and greens are hotter (greater than 1 million Kelvin, or 1,799,540 F)." 
 
 That star of ours is nothing to mess with. Here’s an in-depth piece I wrote about what happens when the sun erupts and sends space weather our way to wreak havoc on Earth. Happy solstice! ||||| Astrophotographer Anthony Lopez captured this photo of the full moon taken in Juarez, Chihuahua, Mexico on May 26, 2013. Scientists say they now know how the moon got its lemon shape. 
 
 Earth's powerful gravity tugged the moon into its oddball shape long ago, shortly after both bodies formed, a new study suggests. 
 
 Tidal forces exerted during the early days of the solar system can explain most of the moon's large-scale topography, including its slight lemon shape, reports the study, which was published online today (July 30) in the journal Nature. 
 
 The new findings could help scientists tackle longstanding lunar mysteries, such as why the moon's near side is dominated by dark volcanic deposits, while the far side is not, researchers said. [How the Moon Formed: A Timeline (Gallery)] 
 
 "What is the origin of that asymmetry?" said study lead author Ian Garrick-Bethell, of the University of California, Santa Cruz. "Chipping away at this problem of the shape of the moon can give us insight into those types of fundamental geology problems," he told Space.com. 
 
 Scientists think the moon formed from debris blasted into space when a mysterious planet-size body slammed into the young Earth about 4.5 billion years ago. The moon was born hot, and it came into existence quite close to our home planet. (The moon has been slowly spiraling away ever since.) 
 
 The newborn moon was thus primed to be sculpted by Earth's gravity, and that's exactly what happened, researchers say. 
 
 Indeed, scientists have posited for more than a century that tidal forces helped shape the molten moon, causing bulges that froze into place when Earth's natural satellite cooled down and solidified. But the new study provides a much more detailed understanding of how this likely happened. 
 
 Garrick-Bethell and his team studied topographic data gathered by NASA's Lunar Reconnaissance Orbiter and information about the moon's gravity field collected by the agency's twin GRAIL (Gravity Recovery and Interior Laboratory) spacecraft. Though the researchers took a global view of the moon, they focused on areas outside of the body's biggest impact craters, which can complicate such analyses. 
 
 The data strongly implicate tidal effects as a key shaper of the moon, researchers said. For example, tidal forces pulled on the lunar crust, stretching it out and heating it up in places. This process thinned out the crust at the lunar poles and thickened it in the regions that lined up with Earth, helping sculpt the moon into a lemon with two small bulges (one on the side facing our planet, and one on the side directly opposite). 
 
 Such tidal heating could have occurred only when the moon's crust was floating on a sea of molten rock, largely decoupled from the rest of the body, Garrick-Bethell said. 
 
 "This happened a long time ago, when the moon was not completely solid," he said. "This was in the first 100 to 200 million years of lunar thermal evolution." 
 
 Also contributing to the moon's overall shape were more straightforward tidal deformations, which Garrick-Bethell likened to squeezing the lemon with your hands, and rotational forces, which cause spinning bodies such as the moon to flatten at the poles and bulge out near the equator. 
 
 When the moon cooled, the changes wrought by all of these processes were frozen in place. 
 
 Interestingly, the long axis of the moon doesn't point directly toward Earth as it likely did long ago; instead, it's offset by about 30 degrees. This probably happened when volcanic activity, impact cratering and other events made the moon's interior a much less homogeneous place, researchers said. 
 
 "Internal density anomalies have formed, and they've kind of canted the moon," Garrick-Bethell said. "At some point in lunar history, you have these events that took place that caused these density anomalies and shifted the density axis away from the shape axis." 
 
 The new results could have applications beyond the moon, potentially helping researchers better understand any celestial body strongly affected by tidal forces, Garrick-Bethell said. 
 
 "This idea was inspired by Europa," he said, referring to the huge moon of Jupiter. Europa is similar today to Earth's moon long ago, he added, in that it harbors a solid shell (of ice in Europa's case rather than rock) sitting atop an ocean layer (which consists of liquid water rather than magma). 
 
 The study could even yield insights about the evolution of faraway alien planets, Garrick-Bethell said. 
 
 "Tides are so ubiquitous; they're everywhere across the galaxy," he said. "So understanding tidal processes is always important." 
 
 Follow Mike Wall on Twitter @michaeldwall and Google+. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.
– 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.)