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On August 21st, the great American eclipse will descend upon the United States, casting its shadow from coast to coast for the first time since 1918. It’s perhaps one of the biggest astronomical events of the decade. And we are here to help you prepare for it. Whether you’re traveling to an optimal viewing place or staying put where you live, here’s what you need to know about the big day. Total solar eclipses are the only time that the sun’s corona — the gaseous outer ring of the sun — is visible to the naked eye on Earth. This image includes color overlays that provide information about the temperature and composition of material in the sun’s outer regions. S. HABBAL / M. DRUCKMÜLLER (fivethirtyeight.com) I HAVEN’T BEEN ON THE INTERNET FOR A WHILE. WHAT IS AN ECLIPSE AGAIN? An eclipse is the serendipitous alignment of the Earth, the Moon, and the Sun. Around every 18 months or so, the Moon passes directly between the Earth and the Sun on its orbit around our planet. It’s a relatively rare occurrence because the Moon doesn’t orbit in the same plane as the Earth and Sun. But when the three bodies line up just right, the Moon covers up the disc of the Sun, and those in the direct path of the Moon’s shadow — called the path of totality — will see the Sun go dark. “WE’RE AT A UNIQUE POINT WHERE THE MOON CAN PERFECTLY COVER THE DISC OF THE SUN. You may be thinking, “Hey, the Moon is way smaller than the Sun. How can it cover it up?” Yes, the Sun is roughly 400 times the size of the Moon, but the Moon is 400 times closer to Earth. So they appear about the same size in the sky. “The Moon is small compared to the Sun, but it’s much closer, so we’re at a unique point where the Moon can perfectly cover the disc of the Sun,” Noah Petro, a planetary geologist at NASA’s Goddard Space Flight Center, tells The Verge. Not all eclipses are the same: sometimes the Sun is totally covered, called a total solar eclipse, and other times the Moon only partially covers the Sun, which is a partial eclipse. And since the Moon’s orbit isn’t perfectly circular, sometimes it’s slightly farther away from the Earth when it passes in front of the Sun. When that happens, it’s an annular eclipse. That’s when the Moon appears slightly smaller than the Sun in the sky and doesn’t completely cover up the solar disc, resulting in a “ring of fire.” There are also hybrid eclipses when an eclipse switches from an annular one to a total one, or vice versa. This month’s upcoming eclipse is a total solar eclipse, so the Sun will be completely covered. Continue reading on NEXT page... SO IF ECLIPSES HAPPEN EVERY 18 MONTHS OR SO, WHY IS THIS ONE SUCH A BIG DEAL? It’s because this eclipse is all American: the shadow of the eclipse will pass from the west coast of Oregon all the way to the east coast of South Carolina. It’s the first time a total solar eclipse has passed from one coast to the other in 99 years. SO WHERE CAN I SEE THIS TOTAL ECLIPSE? Here’s a trusty map, courtesy of NASA: The path of the total solar eclipse. Image: NASA That shaded band cutting across the US is the path of totality — which is about 70 miles wide. That’s the area where the Moon’s shadow will track across the planet’s surface, which is the result of the Moon moving in its orbit and the Earth rotating. Anyone within this region will see the Sun completely covered up for a short period of time. The shape of the Moon’s shadow won’t be completely smooth. That’s because the Moon isn’t a symmetrical sphere, but is covered in mountains, craters, and valleys. NASA can accurately predict the exact shape of the shadow, thanks to NASA’s Lunar Reconnaissance Orbiter and Japan’s SELENE lunar orbiter — both currently in orbit around the Moon. These spacecraft have taken elevation data of the lunar surface to calculate the shape of Earth’s satellite. Click NEXT to continue.... WHAT IF I'M NOT IN THE PATH OF TOTALITY? If you’re in the US, you’ll see some form of the eclipse no matter what. Anyone within the continental 48 states will see the majority of the Sun covered up. Those living in Hawaii and Alaska will see the Moon cover only a small part of the Sun. Vox has a very nifty map of how much coverage you’ll see depending on your area code. IF YOU’RE IN THE UNITED STATES, YOU’LL SEE SOME FORM OF THE ECLIPSE Even if you aren’t in the US, you may not be out of luck: anyone in North America, the northern parts of South America, and even areas of Europe and Africa will see some form of the eclipse. WHEN WILL THE ECLIPSE START AND HOW LONG WILL IT LAST? The eclipse will begin in the middle of the Pacific Ocean around 11:46 AM ET, when the Moon will start creeping in front of the Sun. In the continental US, the total solar eclipse will start on the coast of Oregon around 12:05 PM ET (9:05 AM PT). The shadow will then travel across the path of totality at a whopping 1,500 miles per hour, “leaving” the coast of South Carolina at 4:06 PM ET. More info NEXT... The length of the eclipse depends on your location: if you’re in the path of totality, the Moon should take about an hour and a half to cut across the Sun before completely covering up the solar disc for a few minutes; it will then take another hour and a half to exit the disc. (Vox’s map can also help you figure out how long the eclipse will last in your area.) The length of totality — when the Sun is completely covered — also depends on where you are on the path. Here’s a closer look at the eclipse’s path over Idaho, for instance: The path of totality over Idaho. Image: NASA The red line running through the middle of the eclipse path is known as the centerline. The closer you are to the centerline, the longer totality will last. NASA estimates that totality will max out in southern Illinois, lasting more than two minutes and 40 seconds. WHAT CAN I EXPECT TO HAPPEN DURING THE ECLIPSE? Short answer: a black orb slowly creeping across the Sun. Long answer: a lot of unique phenomena — at least for those in the path of totality. As the Moon begins to conceal the Sun, the sunlight will brighten on one side of the Lunar's edge, creating the so-called diamond ring effect. That’s because it looks like a bright diamond attached to a ring of light, which is the Sun’s atmosphere shining around the Moon. Because the Moon isn’t exactly smooth, you can also witness an effect known as Baily’s Beads. Just before totality, specks of sunlight will pop out between the various valleys and indentions in the lunar crust, creating beads of light that sparkle around the edge of the Moon. Photo: National Astronomical Observatory of Japan / Getty Images Finally, totality will occur, and that’s when you’ll be able to see the Sun’s atmosphere, known as the corona. It will look like delicate threads of light emanating from the solar disc. This is a perfect time for NASA and other researchers to study the corona, which serves as the origin for solar flares and charged particles that stream outward from the Sun. “It’s poorly understood because it’s difficult to observe,” says Petro, “but the eclipse gives us a nice opportunity to observe the solar atmosphere.” Meanwhile, things may get interesting on the ground: birds, for instance, will stop chirping during totality, since they’ll think it’s nighttime. And people may feel a slight drop in temperature, since our main heat source is covered up. NASA has a few tips for how to measure this temperature drop, if you’re interested. WHAT DO I NEED TO WATCH THE ECLIPSE? Staring at the Sun is bad for your eyes. So in order to view the Moon’s encroachment on the Sun, you’ll need to pick up a pair of solar filter sunglasses, which block more than 99.99 percent of sunlight, as well as ultraviolet and infrared radiation. These glasses make the Sun appear as a muted orange or white disc in the sky. NASA and the American Astronomical Society have a list of manufacturers who make safe, standardized glasses for viewing the Sun. Be wary before purchasing your glasses on Amazon — not all manufacturers are selling products that are up to code. Photo by Jeff J Mitchell / Getty Images More Info NEXT... CAN I LOOK AT THE SUN WITH MY NAKED EYES JUST BEFORE AND AFTER TOTALITY? IT’S MOSTLY BLOCKED THEN. No, you really need to wait. If you look at direct sunlight long enough, you can do irreversible damage to your eyes. Here’s a terrifying informative article from NASA about what could happen if you aren’t properly protected. And even a little bit of light peeking out from behind the Moon is enough to hurt. “Until that point, the Moon completely blocks out the Sun, just a little bit of sunlight is still dangerous to your eyes,” says Petro. IS THERE ANY TIME I CAN LOOK AT THIS THING WITH MY OWN TWO EYES? Yes! Once the Sun is completely covered, those in the path of totality can take off their solar filter glasses and view the corona directly. But remember, totality is not going to last very long, so make sure to snap all your pics quickly and just let the view soak in. WILL IT DAMAGE MY SMARTPHONE CAMERA IF I TAKE A PICTURE OF THE ECLIPSE BEFORE AND AFTER TOTALITY? That’s up for debate, but NASA says it’s possible the lens is too small to be damaged by the Sun. If you’re really worried, you can put your solar filter glasses over the lens and take a picture of the eclipse that way. And during totality, you won’t need any filter. SMARTPHONES AREN’T GOING TO BE THE BEST WAY TO GET A PICTURE Smartphones aren’t going to be the best way to get a picture because they don’t zoom in very far; the Sun will look pretty small and grainy. You’re going to want to bust out the DSLR camera for this event if you have one. We’ll have some tips on how to take a good eclipse photo later this week. DO YOU HAVE ANY SUGGESTIONS FOR GOOD ECLIPSE-VIEWING PLAYLISTS? Anything but “Total Eclipse of the Heart.” Click NEXT to continue... I WON’T BE IN THE PATH OF TOTALITY BUT STILL WANT TO SEE IT. ARE THERE GOING TO BE ANY LIVE STREAMS? Yes, definitely. NASA will be hosting an “Eclipse Megacast” on August 21st for about four hours. The show will broadcast on NASA TV, the agency’s own channel, as well as Ustream and YouTube; it will likely be picked up by other TV stations, as well. And expect NASA to have some incredible views of this event: the agency is going to observe the eclipse with numerous satellites. Plus, the astronauts onboard the International Space Station will be passing over the eclipse three times on their orbit, taking photos. Meanwhile, on Earth, dozens of telescopes will be aimed at the eclipse, and pilots flying NASA’s WB-57F research jets will be chasing the eclipse shadow with telescopes mounted on the planes’ noses. There are other live stream options, as well: the Eclipse Ballooning Project — a collection of students from colleges, high schools, and other groups — will be sending up 55 high-altitude balloons during the eclipse. The balloons will be carrying various instruments, including some cameras that will live stream the event here. And of course, there are apps you can download, including the Total Solar Eclipse 2017 App from the Exploratorium museum in San Francisco. Heck, you can even watch this thing in VR with CNN’s live stream. But if you’re in the United States, maybe just go outside for a minute, too. WHAT IF I’M OUT OF TOWN AND MISS THIS ECLIPSE? CAN I SEE ANOTHER? Yes! The next total solar eclipse is slated for 2019, though that one will track mostly over the Pacific Ocean and the southern portion of South America. But in seven years, another total solar eclipse will pass over the US again. This event, slated for April 2024, won’t cross from coast to coast, but it will travel up through Mexico and cover a significant portion of the eastern US. Still, that’s a while from now. For a more instant eclipse fix, it may be best to stay in the US on the 21st. (SOURCE: THE VERGE) If you LIKE this story, please SHARE this story!