Astronomers say a total solar eclipse will make its way across the United States on August 21.
Solar eclipses happen when the moon passes in front of the sun, blocking the sun's light fully or partially. The August eclipse will be total for a broad swath of North America.
Scientists say it is the first total eclipse to be seen across the entire width of the continental United States in 99 years. It will be the first major eclipse visible in the skies above the U.S. since 1970.
The total eclipse will be visible in a 70-mile strip stretching from South Carolina to Oregon. Total darkness - the totality - will last for about three minutes, astronomers say.
"Weather permitting," NASA said, "the entire continent will have the opportunity to view an ecplise as the earth passes in front of the sun, casting a shadow on Earth's surface."
Website GreatAmericanEclipse.com notes that only about 53 percent of Americans live within 400 miles of a site that will see the totality. Those outside the path will have to be content with a partial eclipse.
NASA says it will be 2099 before the next eclipse crosses the continental United States.