On Aug. 21st,
the moon's shadow will shoot across
North America
going more than a thousand miles per hour.
Everyone in this area
will get the chance to see the sun at least
partially covered by the moon.
And along the "path of totality,"
birds will go silent,
the temperature will drop,
the wind will shift direction,
and the stars will come out
as the sun's light is momentarily extinguished.
But eclipses are more than just natural marvels.
Over the centuries, they've spurred major
discoveries about the universe.
The oldest surviving record of an eclipse
comes from Yin,
an ancient capital city in China.
It was carved on a tortoiseshell.
A lot of ancient stories — from the Americas
to Scandinavia to India
— refer to the sun being eaten.
People thought some deity was angry
or these were omens that meant a king would
die.
And some of those kings were nervous enough [that],
they appointed people to study the sky and
take lots of notes
What started out as fearful fascination  turned
into something more ... scientific.
Freaking out about eclipses kick-started astronomy.
One of these early astronomers — Hipparchus —
used an eclipse
to calculate the distance to the moon.
There was a total eclipse in his hometown
but he heard that 600 miles due south
only four -fifths of the sun was obscured.
That was all he needed.
See, Hipparchus was all about this new kind
of math involving lots of angles and triangles.
He calculated that the moon was between this
far away and this far away.
This was hundreds of years before telescopes, and we'd already started to measure the solar system.
By the 17th century, the moon's size and distance
were well-known —
but no one knew why the moon orbited Earth.
Then a real oddball named Isaac Newton figured
out ...
The law of gravity!
But some people needed convincing.
Newton's hype man — Edmund Halley (that guy
they named a comet after)
—said, "OK, Newton's law of gravity determines
the moon's orbit.
So I should be able to use that law to predict
exactly when the moon will go in front of the sun."
He calculated that an eclipse would hit Britain
at 9:05 a.m. on May 3, 1715,
and he printed up these handy posters
that reassured everyone - no, the king isn't
going to die.
Haley got impressively close — the sun went
dark at 9 sharp.
And that eclipse proved Newton wasn't crazy — he
was a genius!
People went on to use Newton's ideas to accurately
predict the orbits of most of the known planets.
But two planets had unexpected wobbles — Uranus
and Mercury.
In France, tireless mathemetician Urbain Le
Verrier used lots of equations to show that
it must be the gravity of unknown planets
pulling Uranus and Mercury off track.
He calculated exactly where the planet pulling
on Uranus would have to be,
and when astronomers went to look — there it was.
They named it Neptune.
Le Verrier was psyched — and he was so sure
they'd find a planet pulling on Mercury, too,
he pre-emptively named it Vulcan.
But here's the problem — Vulcan was supposed
to be right here,
just hidden by the sun's glare.
Vulcan hunters would need an eclipse!
Every time one rolled around,
scientists — including Thomas Edison —
scoured the sky.
But those eclipses never revealed Vulcan.
It just wasn't there.
Mercury's wobbly orbit remained a mystery ...
until ..
Albert Einstein came along.
Einstein had this radical new theory
and it made Vulcan unnecessary.
According to the theory, Mercury was thrown
off course because the sun's bulk was warping
the very fabric of space time.
Einstein's equations predicted the wobbly
orbit perfectly.
But — as always — some people demanded more
proof.
And once again, an eclipse came in useful.
In 1919, the darkened skies
allowed scientists to see stars near the sun
and just as Einstein predicted, the  sun's huge
mass nudged the starlight off course.
Those results made him an instant celebrity.
During a solar eclipse you can see the sun's fiery
atmosphere.
Here's the reddish chromosphere.
In 1868, scientists studying the light of
the chromosphere found evidence
of an unknown element.
They named it helium, after the Greek sun god Helios.
It took another 27 years before someone discovered
helium on Earth.
Where the chromosphere ends, the feathery
corona begins.
It sends blasts of charged particles into
the solar system,
and these space storms can take out electronics
on Earth.
They could even potentially kill interplanetary
astronauts.
So how does the corona work?
Well, there's a lot we still don't know —partly
because we can really only investigate it
when something blocks the overpowering brilliance
of the sun's disk.
We've built advanced instruments to try and
do that,
but the moon still does the job, much much
better.
That's why, even after all these years, scientists
still flock to eclipses,
point their gadgets at the sky,
and make the most of that brief moment
in the moon's shadow to shed new light on
the sun.
Thanks for watching.
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