Why a total solar eclipse is such a big deal

How solar and lunar eclipses work.

Note: This is an update of a video we published in 2015.

Sources:
Getty Images
https://www.amazon.com/Totality-Great-American-Eclipses-2017/dp/0198795696/ref=sr_1_1?ie=UTF8&qid=1500557252&sr=8-1&keywords=totality
NASA: https://svs.gsfc.nasa.gov/cgi-bin/search.cgi?series=383
Eclipse catalog: https://eclipse.gsfc.nasa.gov/SEcat5/catalog.html
Dmitry Chulkov: https://www.youtube.com/watch?v=FrXJfVFbnfU
Bernt Rostad: https://www.flickr.com/photos/brostad/2773255031
mtsrs: https://www.flickr.com/photos/mtsrs/3768574487
CNES/CNRS/NASA: https://svs.gsfc.nasa.gov/11133
Marc Aragnou: https://vimeo.com/108544802
Jesse Olson: https://vimeo.com/57820123
redwing115: https://www.youtube.com/watch?v=yljQ3XsFU_8
Xavier Jubier: https://www.youtube.com/watch?v=E53RbhQjajA
vfr800hu: https://www.youtube.com/watch?v=NlnMc6biFCw
mikewattsuk/bbc: https://www.youtube.com/watch?v=Xt3C5MM7Jkg

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On August 21, 2017, a total solar eclipse is coming to the continental United States for the first time in 38 years, which may make it the most viewed total solar eclipse in history. These events generate so much excitement because the orbital mechanics of the earth-moon-sun system keep the moon's shadow small and mobile. It will touch any given spot on the planet only once in over 300 years on average. For that reason, most people must make a concerted effort if they are to witness this event in their lifetimes. In this video we explain the differences between a solar and lunar eclipse and why some believe that a total eclipse of the sun is the greatest natural phenomenon of them all.

If you're anywhere in the continental United States that day, you can preview exactly what the solar eclipse will look like for your particular zip code, and when it will peak. Check it out: https://www.vox.com/science-and-health/2017/7/25/16019892/solar-eclipse-2017-interactive-map?utm_campaign=vox.social&utm_medium=social&utm_content=voxdotcom&utm_source=youtube

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A solar eclipse happens when the moon’s
shadow falls somewhere on the surface of Earth

And a lunar eclipse is the opposite -- when
the Earth’s shadow falls on the moon

The two sections of the shadow, the dark umbra
and the partially shaded penumbra,

their placement determines which type of eclipse
we can see from Earth.

But not all eclipses are made equal.

The most spectacular, the one for your bucket
list is a total eclipse of the sun.

A total solar eclipse begins as a partial
eclipse.

You’ll notice trees projecting the crescent
sun,

and shadows becoming sharper than normal.

The landscape darkens to a bluish-grey and
you’ll start to feel the temperature drop.

From the west, the moon’s shadow rushes
toward you like a silent storm.

Look up and you’ll see the last sliver of
the sun sparkling like a diamond ring,

before it’s broken into a string of beads
by the moon’s rough terrain.

Now you can see the pearly glow of the sun’s
corona and the pink and red light from the hydrogen

gas of the chromosphere.

Together these make up the sun’s outer atmosphere,
and a total solar eclipse is the only occasion

you have to lay eyes on it.

This is totality and if you get a chance
to see it, you should.

The moon orbits earth every 29.5 days, but
we don’t get eclipses every month.

That’s because the moon’s orbit is not
in line with earth’s orbit.

it’s tilted about 5 degrees.

That doesn’t seem like much but keep in
mind that the scale of the model we’re showing

to you is way off.

If the Earth and moon are this size, the
distance between them should be around 10 ft.

At this distance, 5 degrees is enough to keep
the moon’s shadow off of Earth and the Earth’s

shadow off the moon most months.

So why do we ever get eclipses?

Because there are two points where the moon’s
orbit crosses the sun’s plane, called nodes.

And as the Earth moves along its annual orbit,
those points line up with the sun about twice a year.

As the moon passes between the sun and Earth
at that time, we get a solar eclipse.

When it’s behind Earth at that time, we
get a lunar eclipse.

There are a ton of orbital quirks that make
predicting eclipses really complicated, but

in general we’ll have a few solar and lunar
eclipses of some sort and a few lunar eclipses

of some sort every year.

But you’re more likely to see a total lunar
eclipse in your lifetime than a total solar one.

The totality of a lunar eclipse can last well
over an hour and it’s viewable for anyone

on the night side of earth.

The moon often turns red during a total lunar
eclipse because our planet’s atmosphere

scatters the shorter bluer wavelengths of
light, while the longer, redder wavelengths

pass through.

Or to put it another way, a total lunar eclipse
projects all of the world’s sunsets and

sunrises onto the moon.

Total solar eclipses seem much more rare because
totality lasts just a few minutes, and although

Earth gets a total solar eclipse every 18
months on average, each one is only viewable

by less than half a percent of Earth’s surface.

Eclipse chasers travel all over the world
to put themselves in the path of the shadow.

In a total solar eclipse, the moon precisely
covers the sun from the vantage point of some

place on Earth.

This is possible because by coincidence, the
sun and the moon appear to be about the same

size in our sky.

While the sun is 400 times bigger than the
moon, it’s also about 400 times farther away.

But this alignment isn’t constant.

the moon has an elliptical orbit.

Its size varies about 12% throughout a month.

When it’s closer to us, we can get total
solar eclipses, but less than 30% of solar

eclipses are total.

More often, we get partial eclipses, where
the alignment is a bit off, or annular eclipses,

where the moon is too far away to fully block
the sun, leaving a ring of sunlight around

the moon.

In the far future, earth will only get annular
and partial solar eclipses because our moon

is moving further away.

We know that because Buzz Aldrin and Neil
Armstrong left mirrors on the moon in 1969.

Astronomers bounce lasers off those mirrors
to measure the moon’s distance.

And that’s how they found out that the moon
is moving away from Earth by more than 3 cm

per year.

So in a billion years or so, whatever creatures
live here will witness Earth’s very last

total solar eclipse.

“We can see on the Radio One screen, a fantastic
total solar eclipse taken from the pictures

above the clouds.”

“This is just fantastic.”

A lot of early civilizations feared eclipses.

They were often seen as an attack on the sun
or moon by the forces of darkness.

But now that we understand our place in space,
eclipses are an occasion for awe,

and for gratitude.

All over the galaxy rocks are casting shadows
on other rocks.

But only here, as far as we know, is there
someone to notice them.