Why next week's solar eclipse has scientists so excited

In less than a week, the midday sky across a narrow swath of Canada, the United States and Mexico will darken as the moon glides in front of the sun.

A total solar eclipseis more than a pretty sight to behold: it's a phenomenon that gives scientists the opportunity to study our nearest star, the sun.

Total solar eclipses happen, on average, once every 18 months. So while they are not rare in and of themselves, it is rare for any one spot on Earth to experience one.

According to Fred Espenak, a retired NASA astronomer and the agency's lead eclipse expert, the rarity is that during a total solar eclipse, only about one per cent of Earth's surface experiences totality when the sun is completely obscured by the moon. That means that on average, a givenspot on Earth will experience this event about once every 375 years.

But there are many other reasons why total solar eclipses are such a marvel.

Why is this eclipse so special?

If eclipses themselves aren't all that rare, what's all the hubbub?

"People keep asking: 'Why is this eclipse cool or unusual?'" said Elaina Hyde, director of the Allan I. Carswell Observatory at Toronto's York University. "Well, not only does it have a relatively long total eclipse, it actually travels the path of totality across land, a large swath of North America."

Earth is more than 70 per cent ocean, which means that these events often pass over water. But this time, Hyde said,"it's not stuck in the oceanand there are more people that will get a chance to see it, just because that little, tiny, tiny, tiny swath actually crosses a large portion of Mexico or the U.S., andCanada."

Even when eclipses do occur over land, they don't often fall across widely populated areas. But somehow the stars (or the moon and sun?) aligned to have not just one, but two total solar eclipses pass over North America in recent years this eclipse followsonein the United States in 2017.

The last total solar eclipse to pass through Canada's most populous city,Toronto,was in 1925. This year, it's Montreal's turn.

Another reason this eclipse is specialis that it is somewhat of a long-duration solar eclipse.

For example, the longest duration for the 2017 total solar eclipse was two minutes and 40 seconds. For next week's solar eclipse, the longest duration will be four minutes and 28seconds.

A history of discoveries

Solar eclipses are nothing new to humanity. However, early on in our history, some civilizations reacted tototal solar eclipses with fear. Some believed it was a sign that the gods or a god was unhappy; others worriedthey were bad omens.

But soon, ancient astronomers began to understand what they truly were. Some time around 460 BC, the Greek historian Herodotus wrote that ThalesofMiletus predicted when a total eclipse would take place.(It's believed this was the onlyeclipse he accurately predicted.)The ancient Babylonians also began to predict the eventssomewhere between350 BCand50 BC. They did this based ona cycle called saros, whichwe still use today.

Helium was discovered thanks to a total solar eclipse in India in 1868.French astrophysicist Jules Janssenused a special type of prism, called a spectroscope, that breaks down visible light into its components. He noticed an emission line that didn't match up with sodium, which is what others believed it to be. At the same time,astronomerJoseph Norman Lockyer discovered a similar line, though not during an eclipse.However, the pair were ridiculed for their discovery, and it would take another 30 years to confirm that they had observed helium, the second-most abundant element in the universe.

And then there was Einstein.

"One of the predictions of Einstein's theory of general relativity is that masses, large masses, would actually bend light. But it's really hard to test that, unless you have a really large mass," said Sarah Gallagher, director of the Institute for Earth andSpace Exploration at Western University and president of the Canadian Astronomical Society.

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While we now know that Einstein's theory of general relativity is robust having been put to the test time and time again initially, it wasn't very well received.

On May 29, 1919, British physicist Arthur Eddington decided to test the idea contained in Einstein's theory: that masses could bend light. He travelled to the island of Principe off the coast of West Africa to observe a total solar eclipse, photographingand measuringthe positions of various stars before and after. While Einstein's calculations weren't perfect, he was correct: the sun did indeed bend the light of nearby stars.

Today, space telescopes like the Hubble and the James Webb Space Telescopeshow this in a far more dramatic fashion, with the light of distant galaxies bending aroundareas of large masseswhat we call gravitational "lensing".

One of the best things about a total solar eclipse is being able to see the sun's corona, or its outermost atmosphere. The sun's brightness normally stops us from seeingit, so the prime opportunity arises every time there is a total solar eclipse.

How you can get involved

Though we have certainly learned a lot over the centuries about our sun, we are still puzzled by some of its mechanics and effects, so scientists continue to study it.

But you don't have to be a scientist to contribute to solar research.

NASA is running several citizen science projects to which you can contribute.

One is called Eclipse Soundscapes, which invites people to record the sounds around them. Why? Because during totality, birds and insects can become understandably confused when day suddenly becomes night. It's a continuation of a 1935 study, which collected 498 observations from naturalists, game wardens and the general public.

Another is called SunSketcher,which will help scientists get a better understanding of the shape of the sun. That's rightscientists don't really know our star's shape. While it looks round, due to its rotation, scientists think it's more oblate, or bulgy, similar to Jupiter.

Andfinally, scientists along the path of the eclipse will be watching the sun's corona closely, as it is very dynamic and changes over time. A group from the Southwest Research Institute in Texas will follow up on their 2017 observations with the help ofa NASA plane, which will fly along the path of totality from Texas to Maine to study the elusive corona.

From scientists to sightseers, next week's total solar eclipse will have many heads tilted upward, enjoying the spectacular show.

As Gallagher said, "Every eclipse is special."