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 eclipse is 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 given spot 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.

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 ocean and 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., and Canada."

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 follows one in 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 special is 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 28 seconds. 

Solar eclipses are nothing new to humanity. However, early on in our history, some civilizations reacted to total solar eclipses with fear. Some believed it was a sign that the gods or a god was unhappy; others worried they 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 Thales of Miletus predicted when a total eclipse would take place. (It's believed this was the only eclipse he accurately predicted.) The ancient Babylonians also began to predict the events somewhere between 350 BC and 50 BC. They did this based on a cycle called saros, which we still use today.