There’s only one way to see the stars while
the sun is out.
And that’s during a total solar eclipse.
You have to be at right place at the right
time.
Under a clear sky.
Standing somewhere along the narrow path where
the moon aligns perfectly between the sun
and the Earth.
When the moon passes in front of the sun's
disc, it darkens the sky just enough for distant
stars to become visible.
There have been many photos of total solar
eclipses.
But this one is special.
It helped prove a radical idea.
That redefined gravity.
And turned Albert Einstein into a celebrity.
Because the stars in this photo aren’t where
they’re supposed to be.
Isaac Newton laid the foundation for understanding
the physical universe in the Principia, published
in 1687.
In it, he defined gravity as a force of attraction
that draws massive objects – like stars
and planets – toward each other, and keeps
them in orbit.
And for more than 200 years, gravity was defined
this way: as an attracting force.
But Albert Einstein saw gravity as something
completely different.
According to his theory of general relativity,
which he published in 1915, gravity isn’t
a force between objects in space.
It’s the influence of objects on the shape
of space itself.
According to Einstein, massive objects like
the sun bend the space around them.
So when a smaller object moves in a straight
line along this space, it gets diverted because
of the curve caused by the mass of the larger
object.
That puts one object in an orbit around the
other.
And if Einstein was right then the same curve
would divert the path of light as well.
Meaning if you observed distant stars through
a telescope on Earth while the sun is in front
of them, their light, deflected by the sun’s
gravity, would make them appear slightly out
of position.
It was a revolutionary idea.
But there was a big conflict keeping Einstein
from testing it.
The world was at war.
Einstein lived Germany at the time.
But his work landed in the hands of a British
astrophysicist: Arthur Eddington.
Even though they were on opposing sides of
the war, Eddington, along with astronomer
Frank Dyson, set out to test the theory.
They would photograph a total solar eclipse.
They needed to compare the position of a cluster
of stars in the night sky with a photograph
of the same stars during an eclipse.
If the stars’ apparent positions had shifted,
it would prove that starlight was traveling
through space curved by the sun’s gravity.
The May 1919 eclipse was the ideal one for
this experiment.
The sun would be in front of a very dense
cluster of stars, the Hyades.
And that meant multiple bright stars would
be visible during the eclipse.
Planning began in 1917, and a couple of years
later, two expeditions departed England.
One led by Eddington went to the island of
Principe in West Africa, and the other headed
to Sobral, Brazil.
Two locations that were in the path of the
eclipse and had favorable climates.
Each group traveled with powerful photographic
telescopes that could record detailed photos
of space onto glass plates.
Photographing the eclipse that May required
transporting, and then carefully assembling
them, in the field.
With the plates tilted 45 degrees on one of
the telescopes to include as many stars as
possible.
And this was the result.
This is one of the few successful plates from
the 1919 expeditions.
It came from Brazil.
It shows the eclipse during totality, the
sun’s corona bursting forth, and the rarely
seen solar prominence.
Most importantly, bright stars of the Hyades.
Back in England, Eddington compared the position
of the stars from the eclipse plate with another
of the night sky, using a machine that can
take measurements within photos at the microscopic
level.
The comparison revealed that the stars had
shifted during the eclipse by roughly the
amount that Einstein predicted.
According to Newton’s calculations, starlight
should bend near the sun too.
But if Einstein was right, that deviation
would be twice what Newton predicted.
Eddington's result showed that the deflection
of the stars came closer to Einstein's calculation
than Newton's.
It wasn’t a perfect match, but it was close
enough to validate the theory of general relativity,
and completely shift our understanding of
the universe.
The success of the experiment was first announced
in The Times of London on November 7th, 1919.
Almost a year to the day after the end of
World War I.
An Englishman had gone to great lengths to
prove the ideas of a German, and the news
that space is warped by the planets and stars
excited the world.
Einstein, who before this moment was only
known in the physics world, essentially became
a celebrity overnight.
He remained an international pop culture icon
for the rest of his life.
And a favorite subject of press photographers.
Observing eclipses continued to be one way
of testing general relativity for decades
to come.
With more sophisticated equipment repeatedly
confirming the accuracy of Einstein's theory.
General relativity allowed physicists to begin
to understand advanced concepts about the
universe – like black holes.
Which ultimately led to this: the first photograph
of a black hole, taken in 2019.
A century after Eddington first proved Einstein’s
theory with a photo – and completely changed
our definition of gravity.
