On Aug. 21, North America will experience its most accessible total eclipse of the sun in decades. For those lucky enough to be in the right place at the right time, the event promises to be otherworldly.

From Imax films to video games to virtual reality, today’s average North American has access to a visual banquet like no previous generation in the history of human civilization.

Yet, come Aug. 21, Mother Nature is set to top it all with a spectacle that dwarfs any Hollywood blockbuster.

For the first time in decades, the long dark finger of the moon’s shadow is set to sweep across the continental United States, plunging entire cities and towns into an eerie midday darkness. And for a few precious minutes it will be possible to look up and be reminded that we are but spectators in a grand celestial ballet that has been under way since the solar system was born.

A total eclipse of the sun is one of life’s genuine experiences. It is a fleeting and rare phenomenon that connects Earth and sky in a manner that is both hard to fully take in while it’s under way and impossible to forget when it’s over.

Dedicated eclipse chasers understand this and many routinely travel halfway around the globe for a glimpse of a total eclipse that typically lasts only a few minutes. But not since February 1979 has the experience been so tantalizingly close to home. In this case, the track of the total eclipse does not enter Canada, but many Canadian cities will see more than three-quarters of the sun obscured by the moon during its peak. Meanwhile, thousands are making plans to hop across the border for the main event.

Cosmic Coincidence

The basic mechanism underlying a solar eclipse is easy to grasp. The moon, as it orbits around Earth, passes in front of the sun every now and again. Whenever that happens, the sun’s light is temporarily blocked and the result is a solar eclipse. If the sun is completely covered, the eclipse is total.

A tilt in the moon’s orbit means that such events can only happen a couple of times per year and often the alignment is not exact, so many solar eclipses are only partial, and seen only in remote parts of the world.

Partial solar eclipses occur over a wide area and are common enough that most adults remember having seen one or at least remember one happening where they live. Total eclipses, in contrast, can only be seen along a track, known as the “path of totality” that is different for each eclipse but typically about 100 kilometres wide at mid-latitudes. That’s enough to make total eclipses rare in any given location. But dig a little deeper and you’ll find a coincidence that ranks among the most astounding of heavenly head spinners.

The sun is a star, a big blazing ball of plasma that takes centre stage in our solar system.

The moon, on the other hand, is a small rocky orb that happens to be Earth’s only natural satellite.The coincidence is that the two appear to be almost exactly the same size when viewed from Earth’s surface. While the sun is about 400 times larger than the moon, it also just happens to be about 400 times farther away. There’s no apparent reason why this should be so. On paper, it’s a mathematical fluke, but in the sky it’s pure magic. And this is what makes a total solar eclipse such a breathtaking event to witness.

A Star Revealed

Because the moon is just large enough to cover the sun but not larger, a total eclipse offers a rare chance to see the sun’s hot-but-tenuous outer atmosphere, called the corona, where high-energy particles, guided by the sun’s powerful magnetic field, peel off into space. Closer in is the chromosphere, a ruby-red layer of hot gas that lies directly above the sun’s roiling surface. Less predictable but more dramatic are solar prominences, towering spires and loops of hot gas that sometimes reach upward from the fiery maelstrom below and can be spotted hanging from the edge of the sun as though in suspended animation.

Day to day, these phenomenon are hiding in plain sight, concealed by the sun’s blinding glare and the brilliant blue sky. Only during a total eclipse is the sun’s true character as a dynamic star so exquisitely revealed.

The visual highlight of a total eclipse is the appearance of the sun’s corona, an extended zone of ionized gas that is normally hidden by the glow of the daytime sky. The corona looks ghostly white as it emanates from behind the dark disc of the moon. Its detailed appearance is shaped by the sun’s magnetic field. Observers may also catch a glimpse of red where tongues of hot gas called prominences can sometimes be seen extending thousands of kilometres from the sun’s surface. NASA

Eclipse chasers have trained themselves to look for other phenomena that grow stranger and more abundant as totality nears.

The partial phase of the eclipse begins when the edge of the moon first shows up in silhouette as a slight notch in the sun. This can only be observed safely with appropriate filters that can block the sun’s eye-damaging ultraviolet and infrared rays.

For the next hour-and-a-half, the notch grows into a bite that seems to devour more and more of the sun until it looks like a shiny crescent. By the time the sun is more than 90 per cent covered, the landscape is noticeably darker while shadows cast on the ground look sharper. In the final moments before totality, birds react as though caught off guard by an early nightfall. The narrowing arc of the sun is suddenly broken into a string of dots – an effect known as Baily’s Beads – as the tallest mountain peaks on the edge of the moon begin reaching across the last remaining sliver of light.

Then the darkest part of the moon’s shadow, known as the umbra, sweeps in. Up above, bright stars and planets appear in the deep twilight blue and the sun is gone, replaced by what looks like a perfectly circular black hole punched in the sky, surrounded by the feathery white aura of the solar corona.

Chasing Shadows

Canadians hoping to catch a piece of the moon’s shadow on Aug. 21 face an unusual abundance of choices. Rather than clustering on small islands in the Pacific or heading to remote villages on another continent, they can situate themselves along a more than 4,000- kilometre-long track that stretches from Oregon to South Carolina.

The path of totality is crisscrossed with highways, and dotted with hundreds of towns. It includes Casper, Wyo., Lincoln, Neb., and Nashville as well as parts of St. Louis and Kansas City.

On the Pacific coast, the total eclipse will be a morning event. By the time it reaches the Atlantic, it will be nearly 3 p.m. local time. Carbondale, Ill., is nearest the point where totality is longest, clocking in at just more than 2 minutes and 41 seconds.

But duration is secondary to weather considerations. For that reason, many eclipse chasers have already committed to watching the event somewhere on the dry central plain rather than on the humid and frequently cloudy regions east of the Mississippi.

Of course, when the big day arrives, any place along the path has a chance at being beautifully clear or tragically rainy. In the end, luck is the final arbiter that determines who will see the eclipse and who will miss it, despite best efforts.

This high-stakes game will not be an issue in Canada where the eclipse is only partial and will unfold over a roughly two-and-half-hour window in most cities. Many local astronomy clubs and science centres are planning events that will allow people to watch the partially eclipsed sun safely.

Day of Reckoning

Only slightly less amazing than the fact that eclipses happen at all, is the realization that ancient mathematicians learned how to predict them.

This is no small feat. Eclipses follow a complex series of overlapping cycles that repeat every 6,585-and-one-third days. In practice, it means that if you witness a total eclipse there will be another one similar to it about 18 years later. But that eclipse will happen somewhere else. The direct predecessor of this summer’s total eclipse was a memorable one that sliced across Europe, the Middle East and India in August 1999. At the time, it was deemed the most-watched eclipse in history. This one may top it.

The modern era of the science dates back to another American eclipse, on June 16, 1806, that was observed by Spanish astronomer Jose Joaquin de Ferrer in Kinderhook, N.Y. It is de Ferrer who is credited with coining the term “corona,” but the eclipse is better known as the one that was reportedly used by Tecumseh, the Shawnee chief, and his brother, to rally tribes across the Midwest to push back against the encroachment of European settlers.

By 1878, Maria Mitchell, America’s first woman professor of astronomy and mathematics was instigating another kind of revolution when she led an all-female group of Vassar College students to the wilds of Colorado to observe a total eclipse.

Other historic eclipses allowed astronomers to discover the element helium and prove that gravity bends light, a key prediction of Einstein’s theory of relativity. And until the arrival of the space-age, a total eclipse remained the only way to study the sun’s atmosphere in detail and its relationship to effects on Earth, including the Northern Lights.

Planning Ahead

Scientists are planning to make the most of this year’s eclipse too. One effort includes flying two NASA research jets within the fast moving shadow. This will stretch out the duration of the total eclipse for seven minutes, allowing for more detailed study of rapidly changing structures within the sun’s corona.

For many astronomers and educators, the eclipse is the perfect occasion for public science outreach. Thanks to the Internet, it has never been easier to link up with people and institutions that are planning events and webcasts around the eclipse.

For those who can’t make it to the path of totality, all the attention on the Aug. 21 event may serve as an incentive to plan for the next opportunity. That will come on July 2, 2019, when a total eclipse will swing across the South Pacific and cross over Chile and Argentina. Of greater interest to Canadians will be the eclipse of April 8, 2024, whose path will include several U.S. states before brushing across bits of southern Ontario, Quebec’s Eastern Townships and the Maritimes. Less than seven years away, it represents the next chance for many Canadians to stand in the moon’s shadow without leaving home.

But when the cosmos beckons, it’s rarely wise to wait.