On August 21, people across the United States will be able to witness a total solar eclipse.
As the moon passes in front of the sun and casts a shadow over the Earth.
Although total solar eclipses occur regularly across the planet, they very rarely recur
in the same location.
As the path of the moon's shadow, or umbra, only covers a narrow portion of the Earth.
Nevertheless, scientists throughout history have used these celestial events to help us
understand more about our planet, the solar system and the universe beyond.
The total eclipse of the moon back in the 16th century was used to make the first measurement
of the width of the Atlantic ocean.
Settlers from Britain had landed in Virginia in the United States and the question was,
where were they?
It was possible to work out the latitude because you can work out from the height of the sun
at midday exactly how far north of the equator you are.
But how far across the globe they've gone?
How far their longitude was?
How do you do that?
Well a total eclipse of the moon, or any eclipse of the moon, is visible over the whole of
the night time side of the Earth.
So in London they measured the time when the eclipse started or finished according to our
clocks and then in Virginia they did likewise with their clocks.
And then they came back and told the people in London what time it happened according
to them from which they worked out that it was a five hours time difference.
Using this, they could then use their knowledge of longitude and time to calculate the distance.
But scientists have also used these events to better understand the dynamics of our solar
system.
In particular Edmond Halley.
Who back calculated the dates of the eclipses in the past.
Because eclipses have been recorded throughout history whenever they happened.
And he found that according to his calculations, the further he went back there was a mismatch
between the date when they were recorded and the date when his calculations said they should
have happened.
His calculations just assumed that the moon was going around the Earth in a regular trajectory
that never changes.
And he deduced that this regular change throughout history, going back in history, was because
the moon's orbit is changing.
Technically, the moon is gradually getting further away from the Earth.
I think it's at about the rate that your nails grow.
So that is a remarkable example of science making use of this natural phenomenon.
It also means that after a certain point total solar eclipses will no longer exist as the
moon will be too far away to completely block out the sun's light.
But in 1919, a solar eclipse provided the opportunity to test out a revolutionary new
theory - the test of general relativity.
Where according to Einstein's theory light is bent by gravitational forces and the sun,
which is the biggest source of gravity in the solar system, should therefore bend the
trajectory of light rays.
The theory suggested that the sun's gravity could bend the light of nearby stars around
its mass.
So that stars behind the sun would be visible to us on Earth.
Although Newton's established theory also allowed for this, Einstein's modified theory
of gravity suggested that it would occur at twice the amount.
The only problems was that the intensity of the sun's light, would outshine any starlight
travelling close by, making it impossible to test under normal conditions.
It was only in the brief period of a total solar eclipse, about seven minutes, that gravitationally
shifted stars might be detected.
No pressure then.
But British astronomer Arthur Eddington took up this challenge and sailed to West Africa
where he could get the best view of the eclipse.
Eddington took photographs of the total eclipse of the stars in the background because you
can see the stars during totality.
And indeed the measurements confirmed Einstein's theory.
And this what he photographed.
Including in his paper that: "a deflection of light takes place in the neighbourhood
of the sun and that it is of the amount demanded by Einstein's generalised theory of relativity".
And it's all down to some measurements made in the seven minute window.
Scientists are still using total solar eclipses today to learn about how the sun works.
In particular to study its corona.
A very tenuous, very hot gas, which only about one millionth of the intensity in light terms
of the glowing ball of the sun.
The shapes and the lines of the corona are showing what the nature of the sun's magnetic
field is at that time.
During a total eclipse, when the central sun is obscured, then this tenuous corona is visible.
And so, on August 21, NASA will be flying two jets to chase the path of totality across
North America.
Providing seven minutes to capture infrared measurements of the sun's corona.
As well as taking opportunity to generate the first ever thermal map of mercury.
And so after centuries of eclipse events, it seems kind of humbling that scientists
are still using them to inform our understanding of the solar system.