- 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.
- A diagram shows how a total solar eclipse works and why it darkens the sky during the middle of the day.
A total solar eclipse will darken the afternoon sky from Texas to Maine on Monday.
During the eclipse, the Moon will pass between the Earth and the Sun, completely blocking the sunlight. If you are in the moon’s shadow, the sky will darken for about three to four minutes, depending on your location.
It is the culmination 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 occur 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 these conditions are met, the Earth, Sun and Moon align. This diagram shows what it looks like:
Ana Pélisson
Then, if you’re in the path of totality — which is essentially the center of the moon’s shadow, called an umbra — the moon appears to obscure the sun.
If you are in the penumbra – the outer region of the moon’s shadow – you will see a partial solar eclipse, where the moon appears to partially overlap the sun.
A total solar eclipse occurs somewhere on Earth every 18 months on average. However, it is rare for such a phenomenon to occur in a single location, due to the complex movements of the Earth and the Moon.
The Moon orbits the Earth every 29.5 days, while the 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 Earth and Earth’s shadow off the Moon most of the time.
The Moon’s shadow appears on Earth during a total solar eclipse, seen from the International Space Station. Don Pettit/NASA
There are two points, called nodes, where the Moon’s orbit intersects the Earth’s plane. In the diagram above, the moon is aligned with a node.
The moon aligns with the nodes and sun about twice a year, which is how we get eclipses. A solar eclipse occurs when the Moon is between the Earth and the Sun. A lunar eclipse occurs when the Moon is on the far side of the Earth, farthest from the sun.
What will the total solar eclipse look like?
On the path to totality on Monday, where the Moon’s shadow falls on Earth, the total solar eclipse will have 10 distinct phases, each with different amounts of sunlight visible from the ground.
A series of stitched images show different phases of a solar eclipse. Jonathan Ernst/Reuters
The phenomenon begins with what is called first contact, when the moon begins to pass through the sun. After about an hour, the moon will almost completely obscure the sun and you will begin to see bright light radiating out of the remaining ribbon of sun, known as the “diamond ring.”
Then the moon will completely eclipse the sun, darkening the sky in the middle of the day.
During totality, only the Sun’s outermost atmosphere, called the corona, will be visible and glowing around the moon’s dark disk.
After that, the moon will continue to travel across the sky to form another crescent. The eclipse ends when the moon stops obscuring the sun.
Types of solar eclipses
There are three types of solar eclipses.
Total solar eclipses, like this one, occur when the moon appears to completely cover the sun. If the moon only partially covers the sun, it is a partial eclipse. Many people who are near the path of totality, but not in it, will see a partial eclipse on Monday.
The third type, an annular eclipse, occurs when the Moon is too far from Earth to completely block the sun from our view. The outer edge of the sun remains visible as a bright ring around the moon.
An annular solar eclipse leaves a ring of sunlight visible. NASA
The 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 neighboring United States will occur in 2044.
How to watch the eclipse
People watch a partial solar eclipse in Diyarbakir, Turkey. Sertac Kayar/Reuters
If you plan to view the eclipse, be sure to wear ISO-certified eclipse glasses. They are 1,000 times darker than regular sunglasses. Without them, looking at the sun could damage your eyes.
The only safe time to view the eclipse without glasses is during totality.
Leanna Garfield and Anaele Pelisson contributed to an earlier version of this article.
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