- A total solar eclipse will be visible from Texas to Maine on Monday.
- This cosmic event occurs when the Earth, sun, and moon align perfectly.
- One diagram shows how a total solar eclipse works, and why it darkens the sky in the middle of the day.
A total solar eclipse will turn afternoon skies dark from Texas to Maine on Monday.
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During the eclipse, the moon will cross between the Earth and the sun, completely blocking out the sun's light. If you're in the moon's shadow, the sky will go dark for about three to four minutes, depending on your location.
It's the climax of a cosmic dance between our planet, the moon, and the sun.
What causes a total solar eclipse
During a total solar eclipse, three key conditions happen at the same time: The moon is in the "new moon" phase; the moon crosses the plane of the Earth's orbit; and the moon is at its closest point to Earth in its orbit.
When those conditions are just right, the Earth, sun, and moon line up. This diagram shows how that looks:
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Then, if you're in the path of totality — which is basically the center of the moon's shadow, called the umbra — the moon appears to obscure the sun.
If you're in the penumbra — the outer region of the moon's shadow —you'll see a partial solar eclipse, where the moon appears to partially overlap the sun.
A total solar eclipse happens somewhere on Earth about every 18 months on average. It's rare for one to occur in any single place, though, because of the complex movements of the Earth and moon.
The moon orbits Earth every 29.5 days, while Earth has its own orbit around the sun. The moon's orbit is tilted about five degrees, which is large enough to keep its shadow off the Earth and the Earth's shadow off the moon most of the time.
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There are two points — called nodes — where the moon's orbit crosses the Earth's plane. In the diagram above, the moon is lined up on a node.
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The moon aligns with the nodes and the sun about twice per year, which is how we get eclipses. A solar eclipse happens when the moon is between the Earth and sun. A lunar eclipse happens when the moon is on the other side of the Earth, farthest from the sun.
What the total solar eclipse will look like
In the path of totality on Monday, where the moon's umbra falls over Earth, the total solar eclipse will have 10 distinct phases, each with different amounts of the sun visible from the ground.
The phenomenon kicks off with what's called first contact, when the moon starts to pass across the sun. After about an hour, the moon will almost completely mask the sun, and you'll start to see a bright light radiate out of the sliver of remaining sun, known as the "diamond ring."
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Then the moon will fully eclipse the sun, turning the sky dark in the middle of the day.
During totality only the sun's outermost atmosphere, called the corona, will be visible glowing around the dark disc of the moon.
After that, the moon will continue to travel across the sky to form another crescent. The eclipse ends when the moon ceases to cover the sun.
Types of solar eclipses
There are three types of solar eclipses.
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Total solar eclipses, like this one, occur when the moon appears to completely cover the sun. If the moon only somewhat covers the sun, that's a partial eclipse. Many people who are near the path of totality, but not in it, on Monday will see a partial eclipse.
The third type, an annular eclipse, occurs when the moon is too far from Earth to fully block out the sun from our perspective. The outer edge of the sun remains visible as a bright ring around the moon.
A total solar eclipse is considered the most spectacular. Globally, only about a third of all solar eclipses are total.
The next total solar eclipse in the contiguous US will be in 2044.
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How to watch the eclipse
If you plan to watch the eclipse, make sure you are wearing ISO-certified eclipse glasses. These are 1,000 times darker than regular sunglasses. Without them, staring at the sun could damage your eyes.
The only safe time to look at the eclipse without glasses is during totality.
Leanna Garfield and Anaele Pelisson contributed to an earlier version of this post.
