The sight of the Moon eclipsing the Sun
has fascinated
generations of onlookers.
Our ancestors came up with
a variety of explanations –
some more accurate than others...
But over the centuries it turned out
there was a lot more to learn from
solar eclipses than just how they worked.
The spectacle of a total solar eclipse
has led scientists
to new discoveries about the Sun,
the Solar System
and the physics of the Universe itself…
When humans first started
studying the heavens,
we didn’t know much about
what our Solar System really looked like.
But solar eclipses
did give us some hints:
for example that the Sun must
be further away from us than the Moon.
Ancient Greek astronomer Aristarchus
wanted to know how much further…
And decided to figure it out
with simple geometry.
Aristarchus deduced that when the Moon
is exactly half-lit,
the positions of the Sun,
Moon and Earth
must form a right-angled triangle.
So he measured the angle between
the Sun and the half-Moon
and used that to calculate
the relative distances to the two objects.
Since the angle is large,
the Sun must be
many times further away
from us than the Moon is.
And since they appear to be
the same size in the sky,
as we can see during an eclipse,
the Sun must also be
many times bigger than the Moon.
Aristarchus was even one of the first
to suggest that the Earth
orbits the Sun
and not the other way around.
Though, no-one took
that idea very seriously...
Fast-forward about 2,000 years
and Victorian scientists
were taking solar eclipses
very seriously indeed.
Many intellectuals of that time
were avid eclipse chasers –
hoping to learn more about the Sun
and the strange halo of light
visible around it during an eclipse.
The founding editor of Nature
wrote about the strange halo
– named the corona –
in the first edition of the journal,
speculating on what it might be.
Further study of eclipses proved that
the corona was part of the Sun,
and not coming from the Moon
as some had suggested.
We now know that the corona
is made from extremely hot plasma,
shaped by the Sun’s magnetic field.
It's only visible
to the naked eye
when the Moon blocks out
the much brighter light
from the Sun itself.
Around the same time
as they were studying the corona,
scientists spotted an unknown line
in the spectrum of sunlight
during an eclipse.
This line turned out
to be a new element – helium –
now known to be
the second most common element
in both the Sun
and in the Universe itself.
By 1919 a lot more
was known about the Sun.
So in May of that year,
Arthur Eddington used
a total eclipse to study something
completely different:
how gravity bends light.
He photographed a cluster of stars
visible near the edge
of the eclipsed Sun.
But the stars looked
like they were in a
different position than usual.
This was because the light
from the distant stars
had to pass so close to our Sun,
that the Sun’s mass
actually bent the light -
an effect called gravitational lensing.
It was the first experimental proof
of Albert Einstein’s
general theory of relativity.
And it made Einstein
an overnight celebrity.
In the almost 100 years since,
people have continued to study the Sun,
the Solar System
and the theory of general relativity.
Today most of this research
no longer requires waiting
for a total solar eclipse.
Scientific instruments on Earth
and in space can study the Universe
in amazing detail.
But some scientists still travel
to the site of total eclipses –
and even chase the Sun’s shadow
in super-fast jets -
just to see what further secrets
these spectacular events might reveal.