On Monday, April 8, 2024, millions of people in North America will experience a total solar eclipse — when the Moon completely passes in front of the Sun and temporarily turns day into night. If you're in the path of this year's eclipse, you'll want to make sure you have a good view (and proper eye protection), and if you're not in the path, you may want to consider driving (or flying) to a location that is. That's because another total solar eclipse won't cross the contiguous United States until 2044, twenty years from now — and that's a long wait.
Experiencing a total solar eclipse is a pretty rare event — while they typically occur somewhere on the planet every 18 months, they trace a relatively narrow path across the surface, and only recur in the same place, on average, every 360 to 410 years. If you rarely or never leave the U.S., this is the only chance you'll get to see the cosmic phenomenon for another two decades, and the next one may be on the other side of the country from where you'll be at that time.
Totality — when the sun is completely obscured by the moon — only lasts for a few minutes, if not less, so you'll want to be thoroughly prepared beforehand, so you don't miss it. That means knowing exactly when and where the eclipse will occur, what to expect when it does and, most importantly, knowing how to safely view the eclipse without seriously damaging your eyes.
How does a total solar eclipse work?
There are several types of eclipse which, generally speaking, occur any time an astronomical object (or even a spacecraft) is temporarily obscured by another or its shadow. Lunar eclipses occur when the moon passes through the Earth's shadow and is darkened accordingly. Solar eclipses occur when the moon passes in between our planet and the sun, trailing its shadow across the surface of the Earth. Partial eclipses are when the angle of the moon's orbit means that it won't completely cover the sun, while annular eclipses are when the moon completely passes the sun, but appears too small in the sky to completely obscure it.
Total solar eclipses, however, put on the best show. That's when the moon completely passes across the sun, blocking out nearly all of its light to anyone in its shadow. This shadow is called the path of totality. What makes a total solar eclipse even more special is that the sun is about 400 times further away from us than the moon while also being about 400 times larger than the moon. This cosmic coincidence makes the moon and sun look to be — from the Earth's perspective — the same size. So, when the Moon passes over the sun, it blocks it perfectly, while also allowing us to see some cool aftereffects that wouldn't be possible if the moon were a lot larger.
What will you see during a total eclipse?
Once the Moon's edge starts to cross over the Sun's edge, a partial eclipse will begin. Over the course of a few hours, the Moon will continue to cross in front of the Sun, making the Sun look like a shrinking crescent of light — kind of what the phases of the Moon look like over the course of a month, except sped up. Since the Sun is extremely bright and much of it will be exposed during this phase, it's very important you never look at the Sun without proper eye protection (more on that below).
When the Moon is almost completely in front of the Sun, a phenomenon known as Baily's Beads occurs, which looks like sparkling balls of light on the edges of the Sun-Moon disk. These points of light are actually the Sun showing through the Moon's uneven surface, which includes various valleys, mountains, and craters. These only last for a very short amount of time before complete totality begins.
Once the Sun is completely obscured, you can actually look at it with your eyes, without protection. You may be able to see the sun's chromosphere, which will appear as a thin circle of pink or red around the Moon. You'll also be able to see the Sun's corona — its tumultuous outer atmosphere — which is usually obscured. This burning hydrogen gas will appear as wavy streams of white light emitting in all directions from the disk of the eclipse.
The sky will also be as dark as it would be during dawn or dusk. Because the path of totality is relatively narrow, you may be able to see light from the Earth's surface that is outside of the Moon's shadow.