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What Happens During a Solar Eclipse? Get Ready for the Great American Eclipse on August 21, 2017

North America is preparing itself for the August Eclipse, so we thought it would be a good time to explain how eclipses work and what’s going on as the Moon briefly destroys the Sun.
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Team: Fraser Cain – @fcain / frasercain@gmail.com
Karla Thompson – @karlaii / https://www.youtube.com/channel/UCEItkORQYd4Wf0TpgYI_1fw
Chad Weber – weber.chad@gmail.com
Chloe Cain – @chloegwen2001

On Monday, August 21, 2017, the Moon is going to destroy the Sun. Briefly. But don’t worry, the Sun will return.

For the first time since 1991, the United States will experience a total solar eclipse. In fact, everyone in North America will get a chance to see some version of this epic event – 500 million people will see the Sun dim to some extent.

But for the fortunate people on an epic path from Lincoln City on the Oregon Coast, to Charleston, South Carolina, they’ll see one of the most dramatic and awe inspiring events of the sky.

If you’re one of the people along that path, and you’ve got the proper eye protection – make sure you’re wearing proper eye protection – you’ll see a bite appear on the side of the Sun. Over the course of over an hour, you’ll see more and more of the Sun disappear, blocked by the Moon.

At the halfway point, the last remnants of the Sun will be completely covered, casting the area in darkness. The Sun’s mysterious corona will appear as a ghostly halo around the Moon, and the stars will come out. For a few brief minutes, you’ll experience totality.

And that’s when the eldar Lovecraftian gods will come pouring forth from this eldritch rift in spacetime, to consume our planet in madness. Or, you know, maybe not, the ancient scrolls are unclear.

What is clear is that the Sun will return, slowly but surely. Then there’ll be traffic jams as a huge population returns home, spiritually awakened by this momentous event.

Will you be there? I will.

But what’s going on? What are the underlying geometries that create such a spectacular moment for us to witness?

Eclipses are caused because of the orbital alignments of the Earth, Sun and Moon. The Earth travels around the Sun, and the Moon travels around the Earth. When the Moon is in the Earth’s shadow, that’s a lunar eclipse, and when the Moon’s shadow falls on the Earth, that’s a solar eclipse.

The Earth orbits the Sun at an average distance of about 150 million kilometers, and the Moon orbits the Earth at a distance of about 385,000 kilometers. But in one of the craziest coincidences, the size of the Moon and the size of the Sun are almost exactly the same from our perspective.

The Sun is 400 times bigger than the Moon, but it also happens to be 400 times farther away.

The Moon is slowly drifting away from the Earth, so in a few billion years, this will no longer be the case. The Moon will no longer be big enough in the sky to perfectly cover the Sun.

We don’t get eclipses every month because the orbits of the Earth and Moon are slightly inclined from each other. Sometimes the Moon passes above the Sun, other times it passes below.

From the Earth’s perspective, you’ll see the Moon chomp away and block the Sun. But seen from space, you’ll see a better idea of what’s going on. The Moon casts a shadow down onto the surface of the Earth.

For the 2017 eclipse, the shadow appears out in the Pacific ocean, and then races across the surface of the planet. For anyone actually caught in the shadow, they’re able to see the complete eclipse, where the Moon fully blocks the Sun.

For everyone else on either side of the shadow, you see a partial eclipse, where only a chunk of the Sun is blocked by the Moon.

Every year, there are usually a couple of total solar eclipses, visible from somewhere on the planet, and there are larger chunks that can see a partial eclipse.

The Earth follows an elliptical path around the Sun, and the Moon makes its own elliptical path around the Earth. So the size of both the Sun and the Moon can be slightly different sizes in the sky. When the Sun is at its largest, and the Moon is at its smallest, that’s an annular eclipse. The Moon doesn’t completely cover the Sun and create the special event of a total eclipse.

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