On August 21, a total solar eclipse will cut through the entire continental United States. It’s going to be awesome. If you’re in the bull’s eye center of the moon’s shadow, known as the totality, the sky will go dark for a few minutes in the middle of the day. The temperature will drop, stars will appear, and birds will become confused and start chirping their nighttime songs. And it’s all because of a cosmic coincidence: From the Earth, both the moon and sun will appear to be roughly the same size.

At just 70 miles wide, the path of the totality is narrow. For the rest of the US beyond this slim band, anywhere from 20 to 99 percent of the sun will be covered by the moon. But everyone in every state (even Hawaii and Alaska!) who’s awake and outside will get to experience it. You won’t want to miss it.

Solar eclipses are pretty unusual. You probably have some questions. To clear up any confusion, we’ve complied this comprehensive scientific guide.

The first thing to know: why do we have solar eclipses?

The simple answer: Because the moon occasionally covers the sun in its path across the sky.

The complicated answer: Because of three key conditions that rarely occur all at the same time.

1) There has to be a new moon

One side of the moon is always lit by the sun, but the lit side isn’t always facing the Earth. This is how we get the phases of the moon. For a solar eclipse to occur, it needs to be in its “new moon” phase.

During the new moon, the dark side of the moon is directly facing the Earth.

2) The moon has to cross the plane of Earth’s orbit

So if the dark side of the moon has to be facing the Earth for a solar eclipse to occur, why don’t we have them every new moon?

Because the moon’s orbit isn’t perfectly matched up with the Earth’s.

It’s tilted 5 degrees, like so:

(No one is completely sure why — but it might have to do with how the moon was likely formed: from a massive object smashing into Earth.)

Basically, this means during most new moons, the shadow misses Earth.

But there are two points in the moon’s orbit where the shadow can fall on the Earth. These are called nodes.

For a total eclipse to occur, the moon needs to be at or very close to one of the nodes.

3) The moon’s distance to the Earth

You might remember this from middle school science: The moon’s orbit around the Earth is not a perfect circle. It’s an ellipse.

There’s a point in the orbit where the moon is farthest away from the sun, and where it’s closest. For a total eclipse to occur, the moon needs to be near its closest approach to Earth.

Eclipses can occur when the moon is at apogee (its farthest distance from the Earth), but the moon won’t block out the entire sun. These are called “annular” or “ring of fire” eclipses, because a white-hot band of sun will encircle the darkened face of the moon. They look cool.

When all three conditions line up, you get this: The Earth and sun are aligned with the moon at a node, and a shadow is cast upon the Earth. (The opposite configuration — when the full moon is facing Earth — is when we have lunar eclipses.)

The very darkest part of the shadow is the totality — it’s where the entire disc of the sun is blacked out by the moon. While most of the US will see a partial eclipse on August 21, only those in the 70-mile-wide bull’s-eye of the shadow will see the totality.

How fast does the shadow move across the US?

The totality will reach Oregon at 10:16 am Pacific time, and will end in South Carolina at 2:49 pm Eastern time. That’s an hour and 33 minutes to cross the country. Fast!

But because of the geometry of the Earth and moon, the shadow won’t be traveling at a constant speed. It moves quicker at the beginning of the path and at the end, according to Ernie Wright, a NASA data visualizer. He provided this sketch to demonstrate.

In space, the shadow moves at a constant rate. But it’s “going to cover more ground when it's hitting the Earth at a slant [the red line] than it will when it's more straight-on [the green line],” says Wright. Covering more distance in an equal amount of time means faster speeds.

When the totality hits Oregon, it will be moving at 2,955 mph, according to eclipse2017.org. It will slow down to 1,462 mph as it passes through Kentucky. Then it will speed back up to 1,502 mph by Charleston, South Carolina. After that, the shadow will trail off the edge of the Earth.

So is it easy to predict when an eclipse is going to happen?

The three cycles that conspire to create eclipses — the new moon, the moon crossing the plane of Earth’s orbit, and the moon’s distance to the Earth — all repeat on slightly different time scales.

It takes the moon 27.212 days to return to a node (this is called a draconic month).

And every 27.554 days, the moon returns to its closest approach to Earth (an anomalistic month).

And finally, the moon cycles through all its phases once every 29.530 days (a synodic month).

It takes a while, but three months will come back in sync on an 18-year, 11-day, eight-hour cycle. This is known as the Saros cycle, and it predicts both solar and lunar eclipses.

Saros is ancient: The Babylonians discovered it in the few centuries before the start of the common era. And they worked it all out just by making careful observations of the moon for hundreds of years.

Some 115 years ago, an archeologist was sifting through objects found in the wreck of a 2,000-year-old vessel off the Greek island Antikythera. Among the wreck’s treasures — beautiful vases and pots, jewelry, a bronze statue of an ancient philosopher — was the most peculiar thing: a series of brass gears and dials mounted in a case the size of a mantel clock. Scientists would later realize that the clock had a dial for counting the Saros cycle.

Today, NASA doesn’t have to rely on the Saros cycle alone to predict both solar and lunar eclipses. It has detailed computer models of the orbits of the Earth and moon around the sun.

“It’s pretty easy now to ask directly, where is the sun, where is the moon, where is the Earth, where are they lined up, and you can make a computer do that millions of time,” Wright explains.

Do the eclipses repeat in the exact same spot?

No.

Because the Saros cycle ends on an uneven number of days, eclipses happen in different locations. “The extra 1/3 day displacement means that Earth must rotate an additional ~8 hours or ~120º with each cycle,” NASA explains.

There are many separate Saros series that create eclipses. Learn more about them here.

Do we know where the next total solar eclipse is going to be?

Yes!

There’s a total solar eclipse roughly once every 18 months. The next one will be on July 2, 2019, stretching over a wide swath of the Southern Pacific before passing across Chile and Argentina.

NASA keeps a catalog of all the eclipses (both solar and lunar) that have occurred or will occur from 1999 BC to the year AD 3000. That’s five millennia, mind you.

So that’s why we know, for instance, that on January 27, in the year 2837, a total solar eclipse will pass over southern Mexico (will anyone be around to see it?).

The next solar eclipse over the United States will be in 2024. After that? 2045.

Okay, enough with the astronomy. What am I going to see?

Easy!

My colleagues Casey Miller and Ryan Mark created an interactive to help you out. It grabs your zip code and shows the exact path the moon will take across the sun for your area. You’ll also see when the eclipse will peak (i.e., reach maximum obscuration) in your area. Check it out. It’s very cool.

What happens during a total eclipse?

If you’re in the path of the total solar eclipse, you’re in for some waiting for the big moment.

For most locations, it’s going to be about an hour and a half from the start of the partial eclipse to totality. NASA has a nice interactive map to help you sort out when to watch. (Click on any location in the US and it will tell you when the eclipse starts, when it peaks, and how long totality lasts.)

So the first thing you’re going to see is a partial eclipse: the moon slowly starting to obscure the sun. For most of the country, this is all you’ll see. It’s pretty cool.

During a partial solar eclipse, shadows form eerie crescent shapes.

When totality nears, that’s when the show really begins. There are a couple of awesome phenomena that you can look out for.

Right before totality, the last glimpse of light from the sun will form a “diamond ring” in the sky.

You’ll also be able to see “Baily’s beads” (named after astronomer Francis Baily) — bits of light poking through canyons and craters on the roughed-up surface of the moon.

Then comes totality: The moon is fully covering the sun. This is what you’ve been waiting for.

Depending on your location, this will last for about two minutes (in other eclipses, it has lasted for as long as seven).

What’s so awesome about totality?

In conversations with several “eclipse chasers,” I was told that the photos can’t really capture the awesomeness (in the truest sense of the word) of the experience.

“Anytime you've ever taken a picture of the full moon, it never captures how it felt in your eyes and in your heart, you know what I mean?” says Rhonda Coleman, who has seen six solar eclipses and lives in Oregon. “It seems to fill the sky, but your photograph will only be a memory.”

When the totality happens, the sky goes dark. Stars come out. You can see the corona — the sun’s wispy outer atmosphere.

“The disc of the sun is a black, black, black — like the blackest hole you can ever imagine, ringed with these beautiful wispy white coronal streamers,” Coleman says. “With the naked eye you can see, they're called prominences. It's the flames on the sun. ... There's also a very interesting 360-degree sunset around the rim of the Earth”

While you’re dazzled by the corona, consider this: The sun’s atmosphere is actually hotter than its surface, but no one really knows why. (In 2018, NASA will launch the Parker Probe Plus, a spacecraft that will come within 4 million miles of the surface of the sun, to help sort it out.)

Will it change my life?

Many people report feeling a profound sense of awe during and after a total solar eclipse. “You suddenly feel as though you can see the clockwork of the solar system,” Wright says.

He continued:

We kind of know — in the back of our minds — that we live in a giant ball and it revolves around a hot ball of gas, and we’re floating in space. But you don’t really believe it until you see something like a total solar eclipse, where everything is all lined up and you go whoaaa. Other planets pop out. You got instant nighttime. And you can see Mercury and Venus usually. And sometimes Mars and Jupiter. ... It looks like the pictures from the textbook. It’s not entirely a science thing anymore. ... It’s mostly a thing where you have a better appreciation of where you are in the solar system.

Your experience may vary.

Outside the totality, what will the rest of the country see?

All of the continental United States will see a partial eclipse. Even people way north in Bangor, Maine, will see 54.22 percent of the sun covered at the peak of the eclipse.

Do normal laws apply during totality, or am I allowed to do weird, wolflike things without the fear of repercussions?

You can do weird, wolflike things any day of the year! It’s 2017.

Will my pets freak out?

Probably not. I posed this question to Bill Kramer, a 16-time eclipse viewer who runs eclipse-chasers.com, an online community for, well, you know.

Kramer says pets get confused and will think it’s nighttime.

“Some dogs bark at the eclipse,” he says. “Some dogs detect the emotion of the moment, or anxiety beforehand, and react accordingly. Never heard of one reacting like some do to fireworks or gunshots. The eclipse is a silent thing, except for the ambient sounds and cheers. ... Cats, on the other hand, are cats.”

I need to see this! Where? Where?!

To make an absolutely precise map of the eclipse, you need to account for the geography of the moon, the geography of the Earth, and the angle of the sun’s light hitting Earth.

NASA makes this easy: All of this information was used to create these maps of the total eclipse path as it passes through each state.

For instance, here’s Missouri. Check out the rest of the state-by-state maps here.

And here’s the overview:

Check out an interactive version of the map here. (Click on any spot in the US to get a time for when the eclipse will peak there.)

Can I still plan a trip to see the totality?

Yes. But hotels and campsites in many places along the totality are already booked. Many eclipse chasers have had RVs and hotels booked for years. And travel on the day of the eclipse may get dicey.

Government officials in Oregon are telling people to prepare to shelter in place, since the traffic may come to a standstill. (“Don't burn your deck down in a barbeque, don't cut yourself,” one official told the Oregonian. “Really prepare yourself and be vigilant.” They warned that cellphone networks might be strained.)

But there’s still time to plan your trip.

I had some success opening a search map on Airbnb on one screen and the map of the eclipse path on another.

Remember: The total eclipse is passing through plenty of rural stretches of America, too. If you want to beat the crowds, with some planning, you can find a place to stay or camp.

Many websites suggest you check out the major cities where the total eclipse is passing through. But you’ll get just as good of a view in Sparta, Tennessee (population 5,075), as in Nashville (population 684,000).

Brian, can I come with you?

No.

Will bad weather ruin it?

Always a risk.

If you’re feeling really anxious about picking out the “perfect” spot to view, you’ll want to find a place with little cloud cover. An eclipse during a cloud-covered day is still cool (it will get very, very dark), but you won’t be able to see the dark mask of the moon in front of the sun.

The weather, as you know, can be hard to predict. Here, the National Oceanic and Atmospheric Administration has crunched its data on the average cloud cover that typically occurs on August 21. The darker the dot, the greater the chance of clouds. “The chance for clearer skies appears greatest across the Intermountain West,” NOAA explains.

Will I go blind?

Not if you’re careful! On a normal day, staring straight into the sun can harm your eyes. Eclipse day is no different.

NASA warns: “It is never safe to look directly at the sun's rays — even if the sun is partly obscured.” The intense light from the sun can damage your retina and cause “permanent scotoma or ‘blind spot’ in the central vision,” according to the Lancet. Even when the sun is 99 percent obscured, it can still cause damage.

During the partial eclipse phases, and even through the eclipse’s “Baily’s beads” and “diamond ring” phases — when you can see the last bits of sun peeking through the craters of the moon — you need eye protection. You can only take the protective glasses off when the moon has completely covered the sun during totality.

Regular sunglasses won’t block enough light. You’ll need glasses that filter all but 0.003 percent of visible light and block out most ultraviolet and infrared as well. “Such filters usually have a thin layer of aluminum, chromium or silver deposited on their surfaces that attenuates ultraviolet, visible, and infrared energy,” NASA’s eye safety page explains.

You could grab a pair of the darkest available (No. 14) welder’s glasses. But this is even easier: Pick up a dirt-cheap pair of disposable eclipse glasses. The American Astronomical Society points out there are five manufacturers that meet international standards for eclipse eye protection. They are:

You can buy a pair for less than $1. Or you may be able to pick up a pair for free at your local library. The Space Science Institute, an education nonprofit, is distributing 2 million pairs of specs to 4,800 libraries across the country. Find out if your local branch has them here.

What does a total solar eclipse look like from space?

In 1999, a French astronaut on the Mir space station snapped this photo of a total solar eclipse over Europe.

Do any other planets have solar eclipses?

Kramer, the veteran eclipse chaser, did the math and found that only two other moons in the solar system are the right size to produce a total solar eclipse: Pandora and Epimetheus, which both orbit Saturn — though Saturn doesn’t have a solid surface from which to view these eclipses.

So Earth is still your best bet.

I can’t believe I just read 3,000 words on solar eclipses. Can I just see some cool photos now?

Yes.

(If you have questions not answered in this post, let me know! Brian@vox.com.)