August 21st is going to be an insane day across the United States as millions of people gather along a thin line that stretches from coast to coast to watch the moon pass directly in front of the sun in a total solar eclipse

Only those along that line will see totality — when the moon blocks the sun perfectly — but it is worth making the effort to get to it. I know, because I've seen five of them.

The Earth is the only planet in our solar system with a moon that is exactly the right size and distance from us, so that when it passes in front of the sun, it covers the sun exactly. Other planets have moons that pass in front of the sun as well, but they are either too small or too far away from their planet to cover the sun completely.

For example, in 2013, the Curiosity rover on Mars caught Phobos, one the two moons orbiting the red planet, passing in front of the sun, revealing the potato-like shape of the little moon, but it only covered part of the solar disc

phobos eclipse

In 2013 NASA's Curiosity rover captured these three images of Phobos, the tiny moon of Mars, passing in front of the sun in a partial eclipse. (NASA)

Our much larger moon is still 400 times smaller in diameter than the sun, but the sun happens to be 400 times farther away, so from where we stand, they both appear to be exactly the same size in the sky. This cosmic coincidence gives us the spectacle of a total eclipse, and this summer will be the first one to track right across the United States in almost a hundred years.

While many people in Canada will see part of the sun covered during the event, only those along the centre line, which is a little over 100 km wide, will get the full effect. And that effect is spectacular.

Pictures of totality usually show the sun as a black disk with an ethereal halo around it called the corona. This is the atmosphere of the sun that reaches out millions of kilometres into space and is actually hotter than the surface of the sun itself — still somewhat of a scientific mystery.

It is only during the few minutes of totality when this corona is visible to the naked eye (and the only time you can look at the sun directly without protective glasses.) But there is much more to the eclipse experience than the moon covering the sun. The entire environment changes in a matter of minutes in a way that cannot be captured in a photo or video.

SOLAR-ECLIPSE/

A member of Astronomical League of the Philippines in Manila uses solar sunglasses to watch a partial solar eclipse in March, 2016. (Erik De Castro/Reuters)

Ideally, you'll see a solar eclipse on a clear sunny day with no clouds in the sky. During the first hour, as the moon begins to eat away at the sun, it can only be viewed safely through filtered glasses (not sunglasses), telescopes with solar filters, or projection devices. Not much changes around you, but as more and more of the sun is covered, you begin to notice that colours are beginning to fade and a slight greyness pervades everything.

The temperature starts to noticeably drop, and shadows have crisper edges. Any reflections of sunlight, such as the surface of rippling water or shiny surfaces, take on a diamond-like appearance. Under trees, sunlight that makes it through the leaves make little crescents of light on the ground as the gaps between the leaves act as pinhole cameras projecting images of the sun.

Finally, as the last bit of the sun's brilliant surface disappears behind the moon, the light all around dims down as fast as the lights in a theatre before a movie begins. People begin screaming with delight as the moon's shadow, traveling anywhere from 2300 to 4700 km/h, passes overhead.

Clearly, something cosmic is happening in the heavens and we mere mortals can do nothing to stop it.

At that moment, glasses come off, filters are removed from instruments and everyone looks up to take in the rare sight of the sun as a perfectly round ring of light. The corona is a brilliant, but not blinding, pure white halo resembling rabbit fur, with long filaments extending out into nothingness. Along the inside of the ring, blood red tongues lick outwards, which are huge eruptions larger than the entire Earth blasting off the surface of the sun.

SOLAR-ECLIPSE/LAREUNION

A combination of pictures shows the beginning to the end (top L to bottom R) of an annular solar eclipse seen from a beach of the French Indian Ocean island of La Reunion, September 1, 2016. (Gilles Adt/Reuters)

If that is not enough of a spectacle, four planets will also become visible during this eclipse, two on either side of the sun. Venus and Mars on one side, with Mercury and Jupiter on the other.  

Eclipses are often referred to as "day turning to night," but that is not quite what happens. While the sky does turn dark, it does not go black as night. Instead, the area close to the sun turns a deep cobalt blue, which becomes lighter blue lower down, and then sunset gold all the way around the horizon. It is not like any sky you have ever seen.

After two and a half minutes (longer or shorter depending on location) of this spectacular sight, a piercing beam of light appears on one side of the sun as the moon continues its steady march across the sky. Everyone instinctively turns away because it is simply too bright to look at. The lights of the day come back up just as quickly as they went down, glasses and filters go back on, and the main event is over.

A solar eclipse is a brief event but definitely worth seeing. You have to be on that centre line at exactly the right time to see it, so choose a spot on the map and head there. Traffic will be crazy, accommodation will be non-existent, and crowds will be thick, so plan ahead. You will not regret the effort it takes to get there.