A total solar eclipse is unlike anything you've seen in your life. As totality approaches, you will see the astonishing sight of day turning to night and the Sun's corona blazing in the sky. This is truly a great American eclipse because totality will sweep the nation from the Pacific to the Atlantic. Nearly everyone in the US can reach this total solar eclipse within one day's drive. An eclipse is a cosmic billiard shot — the Sun, Moon, and Earth line up to reveal the Sun's atmosphere, it's corona. Eclipses on Earth occur only because of an amazing celestial coincidence. ||||| Editor's note: The Great American Solar Eclipse was on Aug. 21, 2017. It swept across the continental United States from Oregon to Georgia. See our full coverage. Another total solar eclipse will be seen across the United States in 2024. 
 
 A solar eclipse occurs when the moon gets between Earth and the sun, and the moon casts a shadow over Earth. A solar eclipse can only take place at the phase of new moon, when the moon passes directly between the sun and Earth and its shadows fall upon Earth’s surface. But whether the alignment produces a total solar eclipse, a partial solar eclipse or an annular solar eclipse depends on several factors, all explained below. 
 
 The fact that an eclipse can occur at all is a fluke of celestial mechanics and time. Since the moon formed about 4.5 billion years ago, it has been gradually moving away from Earth (by about 1.6 inches, or 4 centimeters per year). Right now the moon is at the perfect distance to appear in our sky exactly the same size as the sun, and therefore block it out. But this is not always true. 
 
 The next solar eclipse is a partial eclipse that will take place on Jan. 5-6, 2019. It will be visible from northeast Asia, the Pacific Ocean and parts of Alaska. A total solar eclipse will occur on July 2, 2019 in South America. To find out about upcoming solar eclipses and how to see them, check our guide. 
 
 How Solar Eclipses Work: When the moon covers up the sun, skywatchers delight in the opportunity to see a rare spectacle. See how solar eclipses occur in this Space.com infographic Credit: Karl Tate, SPACE.com Contributor 
 
 There are four types of solar eclipses: total, annular, partial and hybrid. Here’s what causes each type: 
 
 Total solar eclipses 
 
 These are a happy accident of nature. The sun's 864,000-mile diameter is fully 400 times greater than that of our puny moon, which measures just about 2,160 miles. But the moon also happens to be about 400 times closer to Earth than the sun (the ratio varies as both orbits are elliptical), and as a result, when the orbital planes intersect and the distances align favorably, the new moon can appear to completely blot out the disk of the sun. On the average a total eclipse occurs somewhere on Earth about every 18 months. 
 
 There are actually two types of shadows: the umbra is that part of the shadow where all sunlight is blocked out. The umbra takes the shape of a dark, slender cone. It is surrounded by the penumbra, a lighter, funnel-shaped shadow from which sunlight is partially obscured. 
 
 During a total solar eclipse, the moon casts its umbra upon Earth's surface; that shadow can sweep a third of the way around the planet in just a few hours. Those who are fortunate enough to be positioned in the direct path of the umbra will see the sun's disk diminish into a crescent as the moon's dark shadow rushes toward them across the landscape. 
 
 During the brief period of totality, when the sun is completely covered, the beautiful corona — the tenuous outer atmosphere of the sun — is revealed. Totality may last as long as 7 minutes 31 seconds, though most total eclipses are usually much shorter. 
 
 On Jan. 4, 2011, the moon passed in front of the sun in a partial solar eclipse - as seen from parts of Earth. Here, the joint Japanese-American Hinode satellite captured the same breathtaking event from space. The unique view created what's called an annular solar eclipse. Credit: Hinode/XRT 
 
 Partial solar eclipses 
 
 A partial solar eclipse occurs when only the penumbra (the partial shadow) passes over you. In these cases, a part of the sun always remains in view during the eclipse. How much of the sun remains in view depends on the specific circumstances. 
 
 Usually the penumbra gives just a glancing blow to our planet over the polar regions; in such cases, places far away from the poles but still within the zone of the penumbra might not see much more than a small scallop of the sun hidden by the moon. In a different scenario, those who are positioned within a couple of thousand miles of the path of a total eclipse will see a partial eclipse. 
 
 The closer you are to the path of totality, the greater the solar obscuration. If, for instance, you are positioned just outside of the path of the total eclipse, you will see the sun wane to a narrow crescent, then thicken up again as the shadow passes by. 
 
 Annular solar eclipses 
 
 An annular eclipse, though a rare and amazing sight, is far different from a total one. The sky will darken ... somewhat; a sort of weird “counterfeit twilight” since so much of the sun still shows. The annular eclipse is a subspecies of a partial eclipse, not total. The maximum duration for an annular eclipse is 12 minutes 30 seconds. 
 
 However, an annular solar eclipse is similar to a total eclipse in that the moon appears to pass centrally across the sun. The difference is, the moon is too small to cover the disk of the sun completely. Because the moon circles Earth in an elliptical orbit, its distance from Earth can vary from 221,457 miles to 252,712 miles. But the dark shadow cone of the moon’s umbra can extend out for no longer than 235,700 miles; that’s less than the moon’s average distance from Earth. 
 
 So if the moon is at some greater distance, the tip of the umbra does not reach Earth. During such an eclipse, the antumbra, a theoretical continuation of the umbra, reaches the ground, and anyone situated within it can look up past either side of the umbra and see an annulus, or “ring of fire” around the moon. A good analogy is putting a penny atop a nickel, the penny being the moon, the nickel being the sun. 
 
 Hybrid solar eclipses 
 
 These are also called annular-total (“A-T”) eclipses. This special type of eclipse occurs when the moon’s distance is near its limit for the umbra to reach Earth. In most cases, an A-T eclipse starts as an annular eclipse because the tip of the umbra falls just short of making contact with Earth; then it becomes total, because the roundness of the planet reaches up and intercepts the shadow tip near the middle of the path, then finally it returns to annular toward the end of the path. 
 
 Because the moon appears to pass directly in front of the sun, total, annular and hybrid eclipses are also called “central” eclipses to distinguish them from eclipses that are merely partial. 
 
 Of all solar eclipses, about 28 percent are total; 35 percent are partial; 32 percent annular; and just 5 percent are hybrids. 
 
 Eclipses do not happen at every new moon, of course. This is because the moon’s orbit is tilted just over 5 degrees relative to Earth’s orbit around the sun. For this reason, the moon’s shadow usually passes either above or below Earth, so a solar eclipse doesn’t occur. 
 
 But as a rule, at least twice each year (and sometimes as many as five times in a year), a new moon will align itself in just such a way to eclipse the sun. That alignment point is called a node. Depending on how closely the new moon approaches a node will determine whether a particular eclipse is central or partial. And of course, the moon’s distance from the Earth — and to a lesser degree, Earth’s distance from the sun — will ultimately determine whether a central eclipse is total, annular or a hybrid. 
 
 And these alignments don’t happen haphazardly, for after a specific interval of time, an eclipse will repeat itself or return. This interval is known as the Saros cycle and was known as far back as the days of the early Chaldean astronomers some 28 centuries ago. The word Saros means “repetition” and is equal to 18 years, 11⅓ days (or a day less or more depending on the number of leap years that have intervened). After this interval, the relative positions of the sun and moon relative to a node are nearly the same as before. That third of a day in the interval causes the path of each eclipse of a series to be displaced in longitude a third of the way around Earth to the west with respect to its predecessor. 
 
 For example, on March 29, 2006, a total eclipse swept across parts of western and northern Africa and then across southern Asia. One Saros later, on April 8, 2024, this eclipse will recur, except instead of Africa and Asia, it will track across northern Mexico, the central and eastern United States and the Maritime provinces of Canada. 
 
 You should never look directly at the sun, but there are ways to safely observe an eclipse. See how to safely observe a solar eclipse with this Space.com infographic Credit: Karl Tate, SPACE.com Contributor 
 
 As a solar eclipse approaches, the mainstream media often will provide a variety of warnings and advisories against looking at the sun with bare eyes, as blindness could ensue. This has given most people the idea that eclipses are dangerous. 
 
 Not so! 
 
 It’s the sun that is dangerous — all the time! The sun constantly emits invisible infrared rays that can damage your eyes. Ordinarily, we have no reason to gaze at the sun. An eclipse gives us a reason, but we shouldn’t. 
 
 There are safe ways, however . . . 
 
 By far, the safest way to view a solar eclipse is to construct a “pinhole camera.” A pinhole or small opening is used to form an image of the sun on a screen placed about 3 feet (or about 1 meter) behind the opening. Binoculars or a small telescope mounted on a tripod can also be used to project a magnified image of the sun onto a white card. The farther away the card, the larger you can focus the image. Look for sunspots. Notice that the sun appears somewhat darker around its limb or edge. This method of solar viewing is safe so long as you remember not to look through the binoculars or telescope when they are pointed toward the sun; put another way, never look directly at the sun when any part of its blindingly bright surface is visible. 
 
 A variation on the pinhole theme is the “pinhole mirror.” Cover a pocket-mirror with a piece of paper that has a ¼-inch hole punched in it. Open a sun-facing window and place the covered mirror on the sunlit sill so it reflects a disk of light onto the far wall inside. The disk of light is an image of the sun’s face. The farther away from the wall is the better; the image will be only 1 inch across for every 9 feet (or 3 centimeters for every 3 meters) from the mirror. Modeling clay works well to hold the mirror in place. Experiment with different-sized holes in the paper. Again, a large hole makes the image bright, but fuzzy, and a small one makes it dim but sharp. Darken the room as much as possible. Be sure to try this out beforehand to make sure the mirror’s optical quality is good enough to project a clean, round image. Of course, don’t let anyone look at the sun in the mirror. 
 
 If you’re around leafy trees, look at the shadow cast by them during the partial phases. What do you see? Is it worth a photograph? You will see scores of partially eclipsed suns projected through pinhole gaps between the leaves. This is caused by diffraction, a property of light. According to Vince Huegele, an optical physicist at the NASA Marshall Space Flight Center, the light rays do not shoot straight by the rim of the gaps, or a pinhole, but bend around the edge. This wave effect creates a pattern of rings that resembles a bull's eye. 
 
 Acceptable filters for unaided visual solar observations include aluminized Mylar. Some astronomy dealers carry Mylar filter material specially designed for solar observing. Also acceptable is shade 14 arc-welder’s glass, available for just a few dollars at welding supply shops. Of course, it is always a good idea to test your filters and/or observing techniques before eclipse day. 
 
 Unacceptable filters include sunglasses, old color film negatives, black-and-white film that contains no silver, photographic neutral-density filters and polarizing filters. Although these materials have very low visible-light transmittance levels, they transmit an unacceptably high level of near-infrared radiation that can cause a thermal retinal burn. The fact that the sun appears dim, or that you feel no discomfort when looking at the sun through these types of filters, is no guarantee that your eyes are safe. 
 
 There is one time when you can safely look directly at the sun: during a total eclipse, when the sun's disk is entirely covered. During those few precious seconds or minutes, the magnificent corona shines forth in all its glory surrounding the darkened sun; a marvelous fringe of pearly white light. It differs in size, in tints and patterns from eclipse to eclipse. It is always faint and delicate, with a sheen like a pale aurora. It has a variable appearance. Sometimes it has a soft continuous look; at other times, long rays of it shoot out in three or four directions. It may stand out from the disk in filmy petals and streamers. But when the sun begins to again emerge into view, the corona quickly disappears and you’ll need to protect your eyes once again. 
 
 This photo of the partial solar eclipse of Sept. 13, 2015, was snapped by astrophotographer K.J. Mulder from his home in South Africa. Credit: K.J. Mulder/Worlds in Ink 
 
 As best as we can determine, the earliest record of a solar eclipse occurred over four millennia ago. In China, it was believed that the gradual blotting out of the sun was caused by a dragon who was attempting to devour the sun, and it was the duty of the court astronomers to shoot arrows, beat drums and raise whatever cacophony they could to frighten the dragon away. 
 
 In the ancient Chinese classic Shujing (or Book of Documents) is the account of Hsi and Ho, two court astronomers who were caught completely unaware by a solar eclipse, having gotten drunk just before the event began. In the aftermath, Zhong Kang, the fourth emperor of the Xia dynasty ordered that Hsi and Ho be punished by having their heads chopped off. The eclipse in question was that of Oct. 22 in the year 2134 B.C. 
 
 In the Bible, in the book of Amos 8:9, are the words, “I will cause the sun to go down at noon, and I will darken the Earth in the clear day.” Biblical scholars believe this is a reference to a celebrated eclipse observed at Nineveh in ancient Assyria on June 15, 763 B.C. An Assyrian tablet also attests to the event. 
 
 A solar eclipse even stopped a war. 
 
 According to the historian Herodotus, there was a five-year war that raged between the Lydians and the Medes. As the war was about to move into its sixth year, a Greek sage, Thales of Miletus foretold to the Ionians that the time was soon approaching when day would turn to night. On May 17, 603 B.C. the sun faded away just as Thales had alluded that it would. So believing that it was a sign from above, the combatants called a truce, which was cemented by a double marriage, for, as Herodotus wrote: “Without some strong bond, there is little of security to be found in men’s covenants.” 
 
 And giving new meaning to the term, “Scared to death,” is the timid emperor Louis of Bavaria, the son of Charlemagne, who witnessed an unusually long total eclipse of the sun on May 5, A.D. 840, which lasted for over five minutes. But no sooner had the sun begun to emerge back into view, Louis was so overwhelmed by what he had just seen that he died of fright! 
 
 Astronomers have learned much by studying eclipses and by the 18th century, observations of solar eclipses were recognized as providing veritable treasure troves of astronomical information, though sometimes getting that information wasn’t easy. 
 
 Samuel Williams, a professor at Harvard, led an expedition to Penobscot Bay, Maine, to observe the total solar eclipse of Oct. 27, 1780. As it turned out, this eclipse took place during the Revolutionary War, and Penobscot Bay lay behind enemy lines. Fortunately, the British granted the expedition safe passage, citing the interest of science above political differences. 
 
 And yet in the end, it was all for naught. 
 
 Williams apparently made a crucial error in his computations and inadvertently positioned his men at Islesboro — just outside the path of totality — likely finding this out with a heavy heart when the narrowing crescent of sunlight slid completely around the dark edge of the moon and then started to thicken! 
 
 During a total solar eclipse, a few ruby-red spots may seem to hover around the jet-black disk of the moon. Those are solar prominences, tongues of incandescent hydrogen gas rising above the surface of the sun. During the total eclipse of Aug. 18, 1868, the French astronomer Pierre Janssen trained his spectroscope on the prominences and discovered a new chemical element. Two English astronomers, J. Norman Lockyer and Edward Frankland, later named it “helium,” from the Greek helios (the sun). The gas was not identified on Earth until 1895. 
 
 And because sunlight is blocked during a total eclipse, some of the brighter stars and planets can be observed in the darkened sky. Under such conditions astronomers were able to test part of Einstein’s now-celebrated general theory of relativity. That theory predicted that light from stars beyond the sun would bend from a straight path in a certain way as it passed the sun. The positions of stars photographed near the sun’s edge during a total eclipse on May 29, 1919, were compared with photographs of the same region of the sky taken at night; the results strongly supported Einstein’s theory. 
 
 Our modern technology now allows astronomers to make most of the observations that once had to await an eclipse. But a total eclipse of the sun will always remain among the most impressive of natural spectacles and is a sight that will always be remembered. Be sure to put it on your bucket list; you will not be disappointed. ||||| Two years from today, on Aug. 21, 2017, North Americans will get an opportunity to observe nature's greatest sky show. 
 
 Picture this: The day starts off bright and sunny. Then, a bit later, you begin to notice that, although it is still sunny, the day doesn't seem quite so bright. And still a little while later, it almost seems like some big storm is brewing. Then, suddenly, and without any warning, the midsummer day turns strangely dark. 
 
 A few stars come out. Birds and animals become confused and quickly head home to sleep. Night insects begin to chirp. All around the horizon, there is a strange yellow-orange glow resembling a weird sunset. And meanwhile, up in the sky where the sun should be, there appears instead a jet-black disk surrounded by a softly glowing halo. [Amazing 2015 Solar Eclipse Photos] 
 
 Then, just as suddenly, the sky brightens up. The stars disappear, birds and animals awaken, and the sun returns. 
 
 What you have just witnessed is a total eclipse of the sun. 
 
 This total solar eclipse of 2017 will be the first time in nearly four decades that such an event will be visible so close to home. "Close," of course, is a relative term. But for most Americans, this spectacular phenomenon will occur literally in their own backyards. 
 
 Contrary to popular belief, totalsolar eclipses are not particularly rare. Astronomers predict 68 to take place during the present century — one about every 17.6 months. On such occasions, the moon casts its dark, slender cone of shadow (called the umbra) upon the Earth's surface. [How Solar Eclipses Work (Infographic)] 
 
 The track traced by the moon's umbra can run for many thousands of miles, but it's also very narrow — at most, about 167 miles (268 kilometers) wide. For this reason, it has been calculated that, on average, a total eclipse of the sun is visible from the same spot on Earth only once in about every 375 years. 
 
 In recent years, for instance, assiduous eclipse chasers had to travel to remote locations such as Ellesmere Island in Nunavut, Canada (2008), Easter Island (2010) or the Norwegian archipelago of Svalbard (2015). 
 
 Map showing the path of totality across the United States for the total solar eclipse of Aug. 21, 2017. Credit: Fred Espenak/NASA GSFC 
 
 But Aug. 21, 2017, will mark the first time this century, and the first time since 1979, that a total solar eclipse will cross the contiguous (lower 48) United States (Alaska had its turn in 1990; Hawaii in 1991). And for the very first time, the shadow track — better known as the "path of totality" — will sweep only over the United States and no other country, leading some people to refer to this upcoming event as "The Great American Eclipse." 
 
 Many Americans taking full advantage of this event's close proximity to their homes will have a golden opportunity to witness firsthand one of the most beautiful and most exciting of nature's sky events. The total eclipse will be seen by an estimated 12 million people who fortuitously live within the totality path. However, the number of people who are within just one day's drive of the totality zone is probably around 200 million. [How to Safely Photograph the Sun (Photo Guide)] 
 
 Not since 1970 has there been an opportunity to see a total solar eclipse in such easily accessible and widespread areas of the United States. There have been a couple of limited opportunities, such as in 1972 (Quebec and the adjacent Canadian Maritime Provinces) and 1979 (the Pacific Northwest and Northern Plains), but the areas of visibility were either limited or somewhat difficult to reach. And not until April 2024 will there be another opportunity comparable to that offered on Aug. 21, 2017. 
 
 Also, for the first time since 1918, the moon's dark shadow will sweep across the U.S. from coast to coast. At local sunrise on this third Monday in August, the shadow of the moon will first touch the Earth at a point in the North Pacific Ocean, about 1,500 miles (2,414 km) northwest of the Hawaiian Islands. Then, for 3 hours and 13 minutes, the shadow will race first east-northeast, then east and finally southeast, along the way darkening a narrow strip of the North American continent. 
 
 Initially, the shadow will traverse nothing but wide-open ocean for 28 minutes. Finally, it will make landfall along the coast of Oregon at the community of Lincoln Beach. Traversing the United States, the total eclipse will be visible within a path of darkness stretching from Oregon and then on through Idaho, Wyoming, Nebraska, Kansas, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina and, finally, South Carolina. 
 
 The path will average 67 miles (108 km) in width, but while moving through western Kentucky, about 12 miles (19 km) northwest of the town of Hopkinsville, the shadow's size will widen to a maximum of 71 miles (114 km). 
 
 Because the moon's shadow is moving at such a tremendous speed, totality cannot last very long in any one place. The duration of the total phase is always longest along the center of the shadow's path. As an observer moves away from the center, the duration of totality time decreases, becoming zero along the path's edges. [Video: How Total Solar Eclipse Photos Help Astronomers] 
 
 Right at the Oregon coastline, totality lasts 1 minute and 59 seconds, as the shadow will be traveling at more than three times the speed of sound (2,400 mph, or 3,900 km/h). Heading southeast along the center line, the totality time slowly lengthens, reaching a maximum of 2 minutes and 40.3 seconds at a spot in southern Illinois about a dozen miles (about 20 km) southeast of the city of Carbondale. 
 
 The shadow — which, at this point, has slowed to 1,450 mph (2,330 km/h) — then begins to increase in speed, and the duration of totality subsequently begins to diminish. Indeed, when it arrives at the South Carolina coastline, the duration of totality will have dropped to 2 minutes and 34 seconds. The shadow then exits out to sea, finally leaving the Earth 75 minutes later at local sunset in the North Atlantic Ocean, 390 miles (627 km) to the southwest of the island nation of Cape Verde. 
 
 Notable cities inside the totality path include Idaho Falls, Idaho; Casper, Wyoming; Grand Island, Nebraska; Lincoln, Nebraska; Columbia, Missouri; Nashville, Tennessee; Columbia, South Carolina; and Charleston, South Carolina. The metropolitan areas of Kansas City, Missouri, and St. Louis are right on the edge of the totality path. 
 
 Warning: Never stare directly at the sun, either with your unaided eye or through telescopes and binoculars, without proper protection. Serious eye damage can occur. Solar eclipse observers and astronomers use special filters to safely observe the sun. 
 
 Weather experts have performed studies to help observers pick the best opportunity for cloudless skies in and near the path of totality on Aug. 21, 2017. 
 
 The best chances for a clear look appear to be in the area running from western Oregon through Wyoming, where, on average, the odds are on the order of about 70 percent. Going farther east, the odds of clear skies gradually diminish; from Kentucky to the Atlantic coast, it's just about a coin toss for favorable weather: just 50-50. 
 
 Starting the week before, the National Weather Service will provide increasingly reliable forecasts, which will enable people to choose a location where the chances of a cloudy sky are low. 
 
 Surrounding the dark umbra is the penumbra, or partial shadow. It will also be conical but much larger — nearly 6,000 miles (9,700 km) in diameter. The penumbra is simply the half shadow that lies outside every deep shadow, whether it is cast by the moon or a house. Wherever the penumbra falls, a partial eclipsewill occur. 
 
 All of North America will be inside the penumbra, causing a rather large partial eclipse for much of the U.S. and portions of southwest Canada. The closer you station yourself to the path of totality, the greater the magnitude of the partial eclipse will be. When watching the partial phases, observers should take precautions while viewing the still blindingly bright sun. 
 
 A number of large cities will lie just outside the totality zone and will see the sun cut down to an exceedingly thin sliver of light. These include Portland, Oregon; Omaha, Nebraska; Chattanooga, Tennessee; and Atlanta. From these cities, a strangely weird twilight effect will fall for a couple of minutes. 
 
 But a partial solar eclipse pales in comparison to a total one, even when 99 percent of the sun's disk is obscured. The great show can only begin at the moment when the last speck of sunlight is extinguished. And only when it's in total eclipse is the sun perfectly safe to look at; only inside the totality path itself can you witness the full grandeur of the eclipse. For that reason, many people will likely journey to position themselves within the path. 
 
 Many of us take the sun for granted, giving it little thought until it scorches our skin or gets in our eyes. But our star is a fascinating and complex object, a gigantic fusion reactor that gives us life. How much do you know about the sun? Start the Quiz 0 of 10 questions complete 
 
 Solar Quiz: How Well Do You Know Our Sun? Many of us take the sun for granted, giving it little thought until it scorches our skin or gets in our eyes. But our star is a fascinating and complex object, a gigantic fusion reactor that gives us life. How much do you know about the sun? 0 of questions complete 
 
 Without a doubt, the "All American Eclipse" will be one of the big news stories of 2017. 
 
 The major news networks will certainly deploy teams of cameras and reporters along the totality path and will follow the moon's umbra during its 90-minute sweep across the contiguous United States. Media coverage — at least for this one day — could be reminiscent of the coverage accompanying the first flights of the space shuttle. 
 
 In short, the third Monday in August 2017 will be an amazing day. Mark your calendars! 
 
 2017 ECLIPSE WEBSITES: 
 
 http://eclipse.gsfc.nasa.gov/SEgoogle/SEgoogle2001/SE2017Aug21Tgoogle.html 
 
 http://www.greatamericaneclipse.com/eclipse-2017/ 
 
 http://www.eclipse2017.org/2017/path_through_the_US.htm 
 
 http://aa.usno.navy.mil/data/docs/Eclipse2017.php 
 
 http://www.shadowandsubstance.com/2017/2017e.html 
 
 Editor's note: If you have a skywatching photo that you'd like to share for a possible story or image gallery, send images and comments in to Space.com at spacephotos@space.com. 
 
 Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmer's Almanac and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com. ||||| WASHINGTON, Aug. 22 (UPI) -- Mark your calendars for Aug. 21, 2017. That's when all of North America will get a once-in-a-lifetime glimpse at a total solar eclipse. 
 
 The Great American Eclipse, as its been dubbed, will be visible to millions across the United States for the first time in nearly four decades. It will mark the first time this century a total solar eclipse will cross through the contiguous United States. It's also the first time the shadow track, called the path of totality, will sweep across the U.S. and no other country. About 200 million people will be able to reach the path of the eclipse, which will sweep the nation from South Carolina to Oregon, within a one-day drive. 
 
 Officials in Hopkinsville, Ky., are calling it the largest event to hit the town of 32,000. The town will experience the longest period of totality of any city at 2 minutes and 40 seconds. City leaders are anticipating up to 50,000 visitors will bring some $23.6 million into the area. They recently unveiled the city's logo for the event and are planning for days-long festivities. 
 
 "It is a unique opportunity and it's our great excuse to roll out the red carpet and be the best version of Hopkinsville we can possibly be," said Mayor Carter Hendricks. "Science has given us the best viewing location just north of Hopkinsville. It is up to us to promote properly so people want to come and be here."
– Ever dreamed of seeing the sky darken and the stars twinkle in the middle of a summer's day? Then mark your calendar for Aug. 21, 2017, because if you're in the US the chances are good that you'll be among the 200 million or so people within a day's drive of the country's first total eclipse of the sun since 1979, reports Space.com. While total solar eclipses are more common than many realize, with 68 this century, this eclipse will be the first since 1918 where the moon's shadow—the so-called "path of totality"—will sweep across the US from coast to coast. It's being called the Great American Eclipse because it will be visible from no other country on the planet. Total eclipses are technically "a fluke of celestial mechanics and time," adds Space.com, because right now the moon, which is gradually moving away from Earth, happens to be the perfect distance to appear equal in size to the sun. The moon's shadow will totally block the sun for the longest duration, two minutes and 40 seconds, just outside of Hopkinsville, Ky., and is being hailed as the largest event to ever hit the town of 32,000 people, reports UPI. But the best bet for clear viewing is said to be in Oregon, where totality will only last 2 minutes but there's a 70% chance of clear skies, as opposed to just 50% from Kentucky east through South Carolina. The path of totality will average 67 miles in width, with a few notable cities in its path, including Lincoln, Neb., Columbia, Mo., and Nashville, Tenn. To those outside the path of totality but in the continental US, a partial eclipse should still be visible, though the sky will be considerably less dark. (The last total solar eclipse was in March.)