- Hey, smart people, Joe here.

The sun is humongous, 10 times the size of anything else in the solar system.

Uh-oh.

(upbeat music) And 100 times the size of earth.

But earth is part of a lucky cosmic coincidence.

Our moon is 400 times smaller than the sun, but it's almost exactly 400 times closer to us.

This means to us here on earth, they look almost exactly the same size in the sky.

Because of this coincidence, every once in a while, our tiny little moon, even though it's 400 times smaller than the sun, crosses in front of it and blocks its light from reaching earth.

A solar eclipse.

On April 8th, that's gonna happen right here.

The shadow of the moon is gonna pass right over where I'm standing.

You are not gonna wanna miss this.

The US won't see another total solar eclipse for 20 years.

This is a once in a generation chance to see one of the most mind blowing events in the solar system.

Today, I'm gonna tell you everything that you need to know to make the most of this eclipse or any eclipse that you have the chance to see in the future.

We're talking about how to pick the right spot so you can be in the maximum shadow of the moon.

I'm gonna give you some ideas about what to bring, tell you everything there is to see before, during, and after an eclipse, and all the incredible things that you can learn from an eclipse, whether you're a scientist or just someone who wants to sit and feel the awe witnessing an incredible cosmic event.

(upbeat music) First things first.

If you need any persuading that this is worth seeing, well, just consider how special an eclipse is in the first place.

Eclipses happen because the moon's orbit takes it between us and the sun, but the moon orbits between the earth and the sun once a month, every time there's a new moon.

So why don't we get an eclipse every month?

That's because the moon's orbit is tilted a few degrees from earth's orbital plane.

Eclipses only happen when the moon's tilted orbit lines up perfectly with earth's orbital plane between us and the sun, allowing the moon to cast its shadow on our planet.

Between two and five solar eclipses fall somewhere on earth each year, but on average, any given spot on Earth only gets one total eclipse every few hundred years.

Long story short, if you have a chance to see an eclipse, you don't wanna miss it.

The first thing you gotta do is pick the right spot.

You'll want to be in the path of totality.

Totality is the moment during an eclipse when the moon's shadow completely covers the sun, and there's only a small stripe of earth where you can see that.

That's the path of totality on April 8th across this stripe of North America.

Now, outside the stripe, the moon will partially cover the sun, but you won't see totality.

And the closer you are to the center line, the longer totality will last.

There are tons of maps and apps to help you pick a location.

I've dropped links to several down in description so that you can look for a good viewing spot.

The weather though, that's all on you, I can't do anything about that.

I'm just a scientist on YouTube, folks.

Anyway, you'll wanna figure out exactly what times totality will occur at your spot so you don't have to do it on the day of the eclipse when you're freaking out from all the awesomeness.

And to get the most out of the eclipse, consider an app like this solar eclipse timer, which precisely times every key moment you should watch for at your spot.

They're not sponsoring this video or anything, but it's a cool app that really works.

I'll leave you a link down in description, but I wanna pause for a second and just acknowledge the fact that we can calculate this at all is absolutely bonkers.

Eclipses are insanely hard to predict.

Ancient people like the Babylonians, Chinese, and Maya had advanced mathematics and astronomy good enough to construct some of the most accurate calendars in human history.

But none of them can predict exactly when and where solar eclipses would occur.

And that's not surprising because this is one of the messiest problems in orbital physics.

To predict the exact positions of the earth, the sun, and the moon into the future requires factoring in every little tug from every object with gravity in the solar system.

We're not just solving for three bodies in space, it's more than a million body problem.

Together, these wobbling tilted orbits are the literal definition of mathematical chaos.

It wasn't until 1715 that anyone came even remotely close to predicting the time and place of a solar eclipse.

When astronomer Edmund Haley used giant tables full of future positions of celestial bodies to hand calculate the timing of an eclipse over London.

He was only four minutes off.

Must have forgot to carry the three.

These days NASA factors in some 38,000 different overlapping orbital patterns, crunching the data with super computers, letting us plan our eclipse viewing down to the second.

But even with all that fancy computer magic, NASA can only precisely predict eclipses for the next thousand years.

It's a problem that's impossible to solve mathematically.

We just hope to get close.

So remember that as you plan your eclipse day down to the minute or second, the fact that we can do that at all is already an amazing thing.

Look relaxed, natural?

Okay, cool.

Well, next step, once you figured out where you'll be and what time totality is, think about how you wanna see it.

This is where I have to remind you.

Never, ever, ever look directly at the sun.

I just wanna say that one more time.

Never, ever, ever look directly at the sun.

Not if you're wearing sunglasses, not if it's 99% covered.

Definitely not through a telescope or binoculars.

Just don't.

The first thing I'd recommend is getting yourself some cool eclipse glasses.

I got these from a large online retailer named after a South American river, but you can pick them up from of different places in person or online.

Just do your homework and make sure you're getting the real thing.

If you don't have eclipse glasses, though, there are still some really cool ways to see the eclipse.

This one's really simple and easy to make.

It's a pinhole camera projector.

All you have to do is poke a very small smooth hole in a piece of paper and hold it over a light surface under the sun.

Or you can go a little fancier and make yourself a little pinhole camera.

Just take a box like this, trace the bottom out on a white sheet of paper, cut it out, and put it inside the bottom of the box.

On the other end, cut two notches in the lid and cover one of those with aluminum foil.

Poke a tiny hole in there, then stand with your back to the sun, looking through the other hole, and you'll see the entire eclipse projected onto the bottom of that pinhole camera.

So why does this work?

Well, the light from the sun is always shooting out in all directions, but because this hole is so small, only a few rays of light make it through traveling in straight lines.

That means rays from the left side of the sun crossed through the hole and hit the ground on the right side and vice versa.

And what you end up with is a little flip-flopped image of the sun in front of you.

During the eclipse, when the moon moves over the sun, you'll see this circle turn into a crescent and get smaller and smaller.

So this is a safe way to watch the eclipse happen without looking at the sun.

You can see the same effect using anything with tiny holes like a colander, or even the light trickling through the leaves of trees.

You can even overlap fingers like this, leaving tiny holes in between.

Watching an eclipse is awesome even if you don't understand everything that's going on during it.

But if you know a little bit about what's happening and what to expect, there's so much to see, and most of it goes by really fast.

So the last thing you wanna plan ahead is what you're gonna look out for.

On the day of the eclipse, with your glasses on, the first thing you'll see is a tiny little nibble on the western side of the sun.

Again, the solar eclipse timer app can tell you exactly what time this is going to happen wherever you're standing.

As the minutes go by, the color of the sky and the things around you will get a little duller.

The wind may even go still as the atmosphere cools inside the growing shadow.

Around 10 minutes before totality, the sun will look like a thin crescent and everything will darken.

But don't take your eclipse glasses off yet.

You might notice some animals acting weird.

Horses have been known to start shaking their heads and tails.

Dogs might get anxious.

Birds might stop flying or burst into their evening songs.

Humans start making weird ooh and ah sounds.

And in these final seconds before totality, some really cool stuff happens.

Just before the last sliver of sunlight disappears, the valleys and ridges on the moon's surface cut into the crescent, leaving little beads of light that flicker along the edge of the sun just before the moon pinches them out.

In 1836, British astronomer, Francis Baily, first recorded this phenomenon, and today these beads of light are called Baily's Beads after him.

Right before the last of these beads disappears, you'll start to see the sun's atmosphere, the corona.

With the sun's light covered up, wisps of the corona emerge as this faint ring around the moon.

Then the last bead of light pouring past the moon lights up part of the corona, creating what's called the diamond ring.

There's one last thing till the out for as the corona becomes visible, take a look at some light surface like a sidewalk or a white sheet.

If you're lucky, you might see little bands of light and dark squiggling across the surface.

These are shadow bands.

They're super faint.

They kinda look like the shadow of water rippling over sand.

They don't always happen, and the weirdest thing is no one knows for sure why they happen.

The best guess is that turbulence in the air messes with that last narrow sliver of remaining sunlight, kind of like what makes stars twinkle.

And then just like that, the sun is gone.

Welcome to totality.

Picture this, the moon's shadow falls on you and where the sun was, there's just a black disc surrounded by glowing corona.

Everything's about as bright as during a full moon.

It's the middle of the day, yet some bright stars and planets are now visible in the sky.

During totality, you can take your eclipse glasses off and look directly at the covered up sun, and there's some cool stuff that you can only ever see during these few minutes of totality.

Keep an eye out for reddish cloudy shapes surrounding the disc of the moon.

These are called prominences, hot clouds of gas billowing out of the sun along tangled magnetic field lines.

You can actually see some of these in the first photo anyone ever took of an eclipse from 1851.

The coolest thing about these prominences is that they only last for about a day.

So whatever ones you see during totality are completely unique to that moment.

If you can pull your eyes away from the eclipse for a few seconds, take a look around you.

If you can see the horizon, you'll see what looks like a 360 degree dawn.

This is the edge of the moon's shadow, visible in the distance sky, places where the sun is not totally eclipsed.

A sunrise in every direction.

The sky will begin to brighten in the west and darken in the east.

As the moon's shadow races eastward across the earth, the sun will be back soon.

Now, depending on how close you are to the center of the path of totality, you'll experience anywhere from a few seconds to a few minutes of totality.

No matter where you are, it's a good idea to know exactly what time totality ends, or have a solar eclipse timer running so you can get your eclipse glasses on before sun comes back.

Suddenly you'll see a glimmer of light peeking through some valley in the moon's surface on the western edge of the sun.

The first Baily's bead on this side.

It might make another diamond ring.

Over the next few seconds, more Baily's beads will flicker in and out.

You'll have a second chance to look for those shadow bands if you missed them the first time.

Then everything happens in reverse.

Baily's beads fuse into a crescent.

The crescent grows and the sky and the surroundings brighten again.

Before you know it, the eclipse completely ends.

It's an ordinary day again, the sky is bright.

A breeze may pick back up.

Animals go back to normal, except for you, you may never be normal again after seeing that.

A total solar eclipse is a different experience for everyone.

For some of us, it's a chance to witness an incredible cosmic event and just immerse ourselves in amazement.

But for others, it's a chance to do science.

For centuries, people have studied the sun as it disappears behind the moon, and we have learned some incredible things from that.

In 1860, eclipse sketches from around the world captured a sort of tendril snaking out from the sun.

That was probably a coronal mass ejection, a blast of plasma that can cause solar storms here on earth.

And this may have been the first time one was ever recorded.

In 1868, the French physicist Jules Janssen used a spectroscope to look at the sun's corona during a solar eclipse, and he discovered the element helium 30 years before it was found on Earth.

And in 1919, an eclipse helped physicists pull off a major test of Einstein's general theory of relativity.

According to Einstein's theory, everything with mass curve's space time, so massive objects should act a little like a lens, bending the path of any light that passes by them.

And the sun has a lot of mass, so it should bend the path of starlight traveling past it.

So during the 1919 eclipse, scientists looked at stars very close to the darkened sun and compared them with the stars' normal positions in the sky and the sun's gravity bent the light just as Einstein predicted.

You know, we often think of science as this journey from darkness to light, from ignorance to knowledge.

But during a total eclipse, it's the darkness that can be illuminating.

But we won't always have total eclipses on earth.

The moon is slowly drifting farther from earth, so hundreds of millions of years from now, it'll be too small in the sky to fully cover the sun.

After that, there will never be another total eclipse.

So it's incredible that we live on this planet at a time when we can see eclipses at all.

And seeing a total eclipse is the best way I can think of to think about how lucky we are to be right here, right now in this exact moment.

Something to think about.

If you're able to, I hope that you take advantage this year, next year, or whenever you get the chance wherever you are.

Stay curious.