Sky-watchers across the United States are gearing up for the best cosmic spectacle in nearly a century, when a total solar eclipse will race over the entire country for the first time since 1918. On August 21, tens of millions of lucky people will be able to watch the moon completely cover the sun and turn day into night for a few fleeting minutes.

The main event will be visible from a relatively narrow path, starting in Oregon and ending in South Carolina. In between, the total eclipse will cross multiple cities in 12 states, prompting plans for countless watch parties, cosmic-themed tours, and scientific observations. (Also see “100 Years of Eclipse-Chasing Revealed in Quirky Pictures.”)

While many people will be traveling to be sure they can see the moon fully blot out the sun, viewers in other parts of the U.S., as well as the rest of North America and parts of Central and South America, will get to enjoy a partial eclipse, when the moon appears to take a bite out the sun.

Here’s everything you need to know to be part of this incredible sky show.

WHAT EXACTLY IS A TOTAL SOLAR ECLIPSE?

A total eclipse of the sun happens when the moon completely blocks the visible solar disk, casting a shadow on Earth. To see a total eclipse, you need to be in the darkest part of this shadow, known as the umbra. People in the lighter part of the shadow, or the prenumbra, will see a partial eclipse.

This in turn means that eclipses can only happen when the moon is precisely aligned between Earth and the sun. Such an arrangement does not occur every month because the moon’s orbit is tilted compared to Earth’s, so that the lunar disk and the solar disk don’t always cross paths.

What’s more, the moon’s orbit is slightly elongated, and the distance between the lunar orb and Earth changes over time. When the moon is farther away, its apparent size isn’t large enough to completely cover the sun, and viewers will see what’s known as an annular eclipse, when a “ring of fire” surrounds the dark lunar disk.

WHY IS THIS ECLIPSE SPECIAL?

Because of the orbital dynamics involved, the moon’s shadow falls on different parts of Earth during each total solar eclipse. Sometimes eclipses are only visible from remote locations or from out at sea, making it difficult for very many people to be in the path of totality.

The path for last total solar eclipse, in March 2016, crossed parts of Indonesia but was otherwise visible only from the waters of the Pacific Ocean. And the continental U.S. hasn’t seen a total solar eclipse since February 1979, when one crossed the Pacific Northwest.

By contrast, the August 21 eclipse will cross the U.S. from coast to coast, with totality visible from several major cities and other locations that are easily accessible to millions of people. The last time this happened was in June 1918, when the path of totality crossed the country from Washington State to Florida.

In addition to attracting record numbers of viewers, easy access to the August event will be a boon to scientists who use eclipses to study the sun’s upper atmosphere, or corona. This mysterious region extends for millions of miles into space but is so faint that it is usually lost in the glare of the star’s disk. During an eclipse, the bright star is covered up, allowing astronomers on the ground to explore the corona.

WHERE SHOULD I BE FOR THE BEST SKY SHOW?

In August, the total eclipse will be visible from a 70-mile-wide corridor stretching about 2,500 miles diagonally from west to east. Depending on where along this pathway you are, the moon will cover the entire sun for up to 2 minutes and 40 seconds.

The path begins in Oregon and crosses through portions of Idaho, Wyoming, Nebraska, Kansas, Missouri, Illinois, Kentucky, Tennessee, Georgia, and North Carolina before ending in South Carolina.

WILL WATCHING A SOLAR ECLIPSE HURT MY EYES?

When it comes to viewing a solar eclipse, safety is paramount. Looking at the sun anytime without eye protection may cause permanent damage.

The best way to watch an eclipse unfold is to use special glasses with lenses made from a solar filter shaded thousands of times darker than any personal sunglasses. Wearing these filtered glasses, you can safely watch the partial eclipse before and after totality, as the moon’s disk moves across the sun. (Read about a project to provide eclipse glasses to communities around the world.)

Once the moon completely covers the sun, it is safe to look at the eclipse with your naked eyes. As day briefly turns to night, stars and planets can become visible in the sky, temperatures noticeably drop, and your surroundings may become silent as animals react to the sudden darkness.

People outside the path of totality can use filtered glasses to watch the partial eclipse. And anyone can watch indirectly using a pinhole camera, which projects an image of the sun onto a flat surface for safe viewing.

WHAT SHOULD I DO IF I CAN’T TRAVEL?

If you can’t get into the path of totality this August, people around the world will be able to join in via live-streaming services such as those offered by NASA and astronomy education group Slooh.

And if you miss this eclipse entirely, you’ll only have to wait until July 2019, when the next total solar eclipse crosses Chile and Argentina. However, the next total solar eclipse to cross the United States won’t happen until April 2024.

Clear skies!

Andrew Fazekas, the Night Sky Guy, is the author of Star Trek: The Official Guide to Our Universe and host of NG Live! Mankind to Marspresentations.