On Aug. 21, a total solar eclipse will sweep across America, casting millions of people into temporary darkness. It will be the biggest astronomical event America has seen in years, watched by millions of people from within the path of totality and tens of millions more who are outside it. One astronomer has said it will be the “most photographed, most shared, most tweeted event in human history.”

Some eclipse enthusiasts have spent years preparing for this solar spectacle, the first eclipse to cross the entire continental United States in almost a century. But even if you are just finding out about the eclipse, it's not too late to plan for the big event. Here's what you need to know:

A total solar eclipse occurs when the moon passes between Earth and the sun, blocking our view of the sun. If you are standing in the moon's shadow on Earth, you will see the sky darken and feel the temperature drop. The place where the sun should be will look like a black circle in the sky. You will be able to view the sun's atmosphere, called the corona — a halo of exceedingly hot gas that’s invisible under normal circumstances. Mike Kentrianakis, the solar eclipse project manager for the American Astronomical Society, calls it “the most gorgeous natural wonder you will ever see.”

“If it strikes you hard enough,” he promises, “you will never be the same.”

[Q&A: Last-minute eclipse questions with NASA solar physicist Ryan Milligan]

The eclipse will occur across the continental United States on Aug. 21. The moon's shadow first hits land north of Newport, Ore., at 10:15 a.m. Pacific time. It will then make its way southeast through Idaho, Wyoming, Nebraska, Kansas, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina and South Carolina. The shadow will leave the continental United States close to Charleston, S.C., at about 2:49 p.m. Eastern time.

The 70-mile-wide, 3,000-mile-long swath that lies directly in the shadow of the moon is called “the path of totality.” For this eclipse, it will start off the coast of Oregon and sweep across the country to South Carolina. Because the moon orbits around Earth so quickly (at a pace of 2,100 miles per hour), each spot on the path will experience only about two minutes of totality. There is no way to chase the shadow around the country — it will cross the entire United States in about 90 minutes, faster than the speediest jetliner.

The shadow of the moon, known as the “umbra,” will cross a relatively small swath of land. But the moon also casts a lighter shadow, called the “penumbra.” People in this region, which will cover all of North America, will experience a partial eclipse. They will see the sun partly covered by the moon — like a cookie with a bite taken out of it. The degree to which the sun is covered depends on your proximity to the path of totality — the closer you are, the less of the sun you will see. People in the Washington area can expect to see about 80 percent coverage of the sun, starting just after 1 p.m. and peaking around 2:42 p.m. If you are watching a partial eclipse, you must wear protective glasses for the entire event, or you will risk severe eye damage. (More on that below.)

Veteran eclipse chasers say that a partial eclipse isn't nearly as beautiful as totality. “It's a completely different phenomenon,” said Kentrianakis, of AAS. “It shouldn’t even be called an eclipse. It should be called something else.” He recommends that everyone who is interested in the eclipse make their way to the path of totality for the big event.

Clouds would be a bummer. If the sky is overcast during totality, it will still get dark, but you will not be able to see the moon cover the sun or the glow of the corona. Eclipse experts recommend checking the weather forecast for your area in the days before Aug. 21 to ensure that you watch the event from a spot where skies will be clear.

This is the first total solar eclipse to occur solely in the United States since the country was founded. For most Americans, this is the best chance to see a solar eclipse we will have in our lifetimes. An estimated 12 million people live in the path of totality, and as many as 7 million more will migrate to the path for the big event. It's likely to be a tremendous astronomical experience, and you don't want to miss it.

This eclipse is also a huge opportunity for scientists. The corona, which becomes visible when the moon covers up the sun, is the object of intense scientific interest — it emits sprays of hot, ionized particles that can damage electrical grids and satellites and harm astronauts in space. Because this eclipse will move across thousands of miles of mostly inhabited landscapes, rather than hard-to-reach wilderness or open seas, it will be within sight of scientists for almost the duration of totality. That means that researchers positioned at various locations along the path of totality can film the event and piece their clips together to create an unprecedented 90-minute video of the corona in action.

Anyone within the path of totality will be able to see the moon cover up the sun. But several cities along the path are putting on a special show for the occasion. Here's a look at some of the festivals taking place across the country on the big day.

It's also worth considering where you are least likely to experience clouds. There, The Washington Post's Capital Weather Gang has you covered. Here are the cities with the best chance of clear skies during totality.

If you're here in Washington on Aug. 21, head to the National Air and Space Museum. The Smithsonian will have solar telescopes set up for viewing the event. You can also snag a pair of eclipse-viewing glasses at the museum, or learn to build a pinhole camera that will let you safely observe the sun. Eclipse enthusiasts can also head to the Udvar-Hazy Center in Chantilly for similar programming.

In the path of totality, Columbia, S.C., is planning a full weekend of festivities in the days leading up to Aug. 21. Free eclipse-viewing glasses will be available many places in the city, including the University of South Carolina campus, which is an official NASA viewing location.

Yes! Except during the brief phase of totality, you must wear eclipse-watching glasses the entire time you are looking at the sun. This is also true if you are watching the eclipse from outside the path of totality. If you attempt to look at the eclipse without protective lenses, you risk severe damage to your eyes. Ultraviolet rays from the sun can literally give your eyeballs a sunburn.

Only specially designed solar filters will do the trick. Regular sunglasses are not good enough! NASA has identified several manufacturers of eclipse glasses and solar filters that meet international safety standards. Eclipse glasses are also available at many libraries.

To recap: If you are outside the path of totality, you must wear glasses the entire duration of the eclipse. If you are within the path of totality, wear your glasses until the moon completely blocks the sun. Then you can take your glasses off to see the spectacle and the sun's corona. But before totality ends (after about 2 minutes, depending on your location), put your glasses back on to watch the rest of the event.

We are not trying to scaremonger, but c'mon, guys. Be smart, wear your glasses. Your optometrist will thank you.

The next total solar eclipse visible from planet Earth will occur in July 2019 over Argentina and Chile. And the United States is set to see another total eclipse on April 8, 2024, when the moon will cast a shadow across Mexico, Texas, Oklahoma, Arkansas, Missouri, Illinois, Kentucky, Indiana, Ohio, Pennsylvania, New York, Vermont, New Hampshire, Canada and Maine.

We experience total solar eclipses because of the cosmic coincidence that the moon is 400 times closer than the sun and 1/400th of its size. This means that when Earth, the moon and the sun line up perfectly, the moon neatly blocks out the light of our star. But the orbit of the moon is slightly tilted relative to the plane in which Earth orbits the sun, so most of the time it passes below or above the sun from our line of sight. The moon's orbit takes it directly in between Earth and the sun every 18 months or so, resulting in a total solar eclipse.

In addition to being tilted, the moon's orbit is not perfectly circular. Sometimes, the moon is farther away when it passes between us and the sun. In these cases, it appears slightly smaller than the sun, so a ring of the sun's light is still visible during the eclipse. This phenomenon is known as an annular eclipse, and it also occurs roughly every year and a half.

But Earth won't experience eclipses forever. The moon is drifting away from our planet at a rate of about an inch and a half per year. In approximately 650 million years, the moon will be so distant that it can no longer completely block out the sun. Humans will have seen their last eclipse — if we manage to make it that long.

Question submitted by Dave V.

It is always dangerous to look directly at the sun. The star at the center of our solar system emits tremendous amounts of visible light, infrared radiation (i.e., heat) and ultraviolet radiation, all of which can irreversibly damage your eyes. The reason no one talks about not staring at the sun when an eclipse isn't happening is that, well, it's common sense. Saying “don't look at the sun” on an ordinary day is like saying “don't touch fire.” We just assume that you already know that.

But when a total solar eclipse takes place, it's as though nature has stuck something shiny, beautiful and rare among the flames. The desire to watch this incredible phenomenon can make people ... less than prudent. After a 1999 solar eclipse in Britain, several thousand people sought treatment at special clinics or via hotlines, and 14 people suffered serious eye damage, according to a study in the British Medical Journal. This is why doctors, astronomers and journalists are constantly reminding people to use protective glasses when looking at the sun during the Aug. 21 event.

Eclipses also require extra caution because the sun can appear to be less blinding than it is normally. If you were to go outside and look directly at the sun now (please don't do this), the lens of your eye would concentrate the sun's rays on the sensitive neural tissue of your retina, and it would be painful. Luckily, we humans have a reflex to avert our eyes when that happens. But during the partial phase of an eclipse, the visible light from the sun is reduced enough that it is not as painful to watch directly. But heat and ultraviolet radiation are still streaming from the sun into your retinas, so it is still dangerous to observe this phase without protection. The only time it is safe to remove your protective glasses is during the brief period of totality, when the moon has fully eclipsed the sun.

Question submitted by Jere B.

The path of totality will pass several hundred miles south of Washington, through South Carolina. If you'd like to drive down for the big event, your best bet is probably Columbia, S.C. — a (relatively) easy seven-and-a-half hour drive on Interstate 95. Columbia is a large city, so it may still have affordable rooms available, and it is connected to a network of highways, so traffic will not be as bad. That said, millions of people are going to be traveling for this event, so you should be prepared for jams wherever you go. If you do decide to drive down on that day, give yourself plenty of time to reach your destination.

Still have questions?

For more, check out our live Q&As with NASA experts from August 1 and August 15. You can also email your query to eclipse2017@washpost.com.

Explore The Washington Post’s reporting on the eclipse

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Graphics by Denise Lu. Icons by Anniken & Andrew, Anton Anuchin, Erika Baltusyte, Oksana Latysheva, Sergey Demushkin for the Noun Project. Originally published July 21, 2017.

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