This is a community service announcement.

If you're captured by a tribe of cannibals on August 21 of this year, please remember to raise your hands to the heavens and threaten to block out the sun.

Get the timing right, and the sky will blacken.

The stars will emerge, and you'll be declared a god.

Oh, it will only work if that cannibal tribe is located somewhere in a narrow band across the continental United States.

Yep, the US is going to witness its first total solar eclipse in 40 years.

15 00:00:41,000 --> 00:00:44,180 Solar eclipses are rare and special events.

Today I want to talk about why they're so cool, and what you need to know in order to see this one.

First, let's go over the two eclipse types.

A lunar eclipse is when the Earth's shadow falls on the moon.

A solar eclipse is when the moon's shadow falls on the Earth.

The moon has to be on the opposite side of the Earth compared to the sun to catch the Earth's shadow and so lunar eclipses are always during full moons.

And the moon has to be between the sun and the Earth for its shadow to hit us.

So solar eclipses are always during new moons.

But there's a new moon every month.

Why the 40 year wait for an American solar eclipse?

Well, two things-- alignment and shadow size.

The moon's orbit about the Earth and the Earth's orbit about the sun, the ecliptic plane, are misaligned by about 5 degrees.

Most months, the moon's shadow misses the Earth.

Only when the moon crosses the ecliptic plane during a new or full moon can an eclipse occur.

That alignment happens about twice a year and usually results in one solar eclipse and one lunar eclipse.

Sometimes you even get three eclipses.

The solar eclipse on August 21st will be preceded by a partial lunar eclipse on August 7th.

It'll be partial because the Earth won't completely covered the sun from the moon's perspective.

Anyone who can see the moon will see that lunar eclipse.

However, to see the solar eclipse, you need to be in the narrow path of the moon's shadow.

This time that path crosses the United States.

The fun starts at 9:04 AM Pacific time, when the edge of the moon's shadow first reaches the West Coast.

So that's the penumbra of the moon's shadow, where the sun isn't completely blocked.

Let's say you are standing on an ocean cliff in Depot Bay, Oregon, because that's where the umbra, or the total eclipse, will first make landfall.

At first, you won't notice much change in the glorious sunny day, fingers crossed for the weather.

But something about the light seems clearer, the shadows sharper.

Take a walk through the pine woods behind you, and you might notice the dappling of light between the shadows has changed.

Usually those dapples are circular.

Now there's a bite taken out of them.

In fact, the spaces between the leaves act like pinhole cameras, projecting the sun's image to the ground.

In those dapples, you are witnessing the beginning of the eclipse.

This is where you might want to set up your own pinhole camera to watch the oncoming eclipse.

There are plenty of resources online for how to make one.

But whatever you do, don't look directly at the sun, at least not yet.

Unless you have highly specialized glasses, that is.

So no joke here-- you need these, and sunglasses won't cut it.

You need aluminized mylar or number 14 welder's glass, something approved specifically for viewing the sun.

The easiest way is to search for eclipse glasses online.

Over the course of the next hour, the moon eats further and further into the sun.

You start to realize the two objects are the same size on the sky.

Now this is really pure chance.

The moon is much smaller, but much closer than the sun.

And it just so happens that they have roughly the same angular size on the sky.

This August, the moon will span 0.538 degrees, just big enough to completely obscure our 0.527 degrees sun.

Now it isn't always so perfect.

The moon's orbit is elliptical, and so sometimes it eclipses the sun when it's a bit further away from the Earth.

It appears smaller to us and so doesn't completely block the sun.

Such eclipses are called annular eclipses, because they leave a ring of bright sunlight around the moon's disk.

I mean, they're OK, I guess.

But August will give us a total solar eclipse, and those are incomparable.

But enjoy it while you can.

Every year, the moon steals a little bit of Earth's rotational kinetic energy and drifts about 3.8 centimeters away from us.

And so in the future, it won't be able to block out the entire sun anymore.

In fact, in only half a billion years, the Earth will have its final total solar eclipse.

As partiality deepens, the shade by the trees becomes full of deep crescents.

Now there's something definitely off about the light.

It's eerie.

It's like everything has a high contrast Instagram filter.

We're nearing totality.

With your glasses on, you see the sun is a mere sliver, and it's shrinking.

The sky's blue deepens, a breeze stirs, and the temperature drops.

Thin shadows ripple unpredictably on plain surfaces.

These are called shadow bands, and they've perplexed scientists for a long time.

They may be an atmospheric effect, perhaps from the same turbulence that makes stars twinkle.

But now it's the last thin sliver of the sun that's twinkling.

The stars, by the way, start to come out.

The sun's final crescent contracts to a last spark on one side of the moon, like a diamond ring.

And then, it's gone.

Now you can take your glasses off.

It's the one time you can stare safely straight towards the sun.

And a lot happens at once.

For probably the first time in your life, you see the chromosphere, red from a specific electron transition in the hydrogen of the sun's upper atmosphere.

Below it, you see Baily's beads, sparkling around the rim of the moon.

Really, they're extra sunlight peeking through lunar valleys.

They fade quickly.

What shows up next is the ghostly light of the corona, the sun's outermost atmosphere.

The corona is always there, but its faint glow is normally overwhelmed by the glare of the sun's surface.

Right next to the black sun, you'll see the bright star Regulus and its constellation, Leo.

A little further away, you'll see the planets Venus, Mars, Jupiter, and the elusive Mercury.

Mercury orbits so close to the sun that it's hard to catch in a dark enough sky.

Now, Mercury will shine almost as brightly as Mars.

Further out still, you'll see Orion.

Totality will reach Depot Bay at 10:16 AM Pacific time and last just under two minutes.

That central umbral shadow is about 110 kilometers or 70 miles across, and it will race eastward at about 2,000 miles per hour.

Stand on a mountain along the path, and you'll see the umber racing towards you.

From Oregon, the eclipse races through Idaho, Montana, Wyoming, Nebraska, Kansas, Iowa, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina, before finally slipping into the Atlantic at Cape Romain, South Carolina, at 2:49 PM Eastern time, an hour and a half after it first made landfall.

As totality fades, Leo, Orion, the planets and the corona disappear in midday twilight.

Bailey's beads and the red chromosphere flash along the edge of the moon, the diamond ring emerging on the Western side.

The shadow bands scatter, and the bright blue sky returns like nothing ever happened.

So what do you need to do to make sure you catch the great American eclipse?

Well, Google the path of totality and find a good spot along that narrow strip stretching from Oregon to South Carolina.

That includes looking up typical August weather for candidate spots.

You need clear skies.

Have a backup plan in case of clouds, and of course, be checking the weather forecasts several days earlier.

Also, remember that everyone else will be chasing the eclipse, too, so book hotels early.

But honestly, it's already too late for some places.

Alternatively, find somewhere way off the beaten track.

Most importantly, sort out serious eye protection, like I already discussed.

Look, I can describe a solar eclipse to you, but you really have to see one for yourself.

Astronomical events of this caliber can change your life.

I saw my first solar eclipse on Flinders Island off the south coast of Australia when I was a kid, and it really enhanced my love of astronomy.

Who knows?

Without it, I may never have ended up doing a show like "Space Time."