For about two minutes on the afternoon of August 21, the sky over Nashville will turn almost as dark as night. Ditto Casper, Wyoming; Columbia, South Carolina, and the southern suburbs of St. Louis. They will be darkened by one of nature’s most spectacular events: a total solar eclipse. It may be the most-watched eclipse in history.

The eclipse takes place as the new Moon passes directly between Earth and the Sun. By an astronomical coincidence, the Moon and Sun appear almost exactly the same size in Earth’s sky, allowing the Moon to completely cover the Sun. The sky will turn dark, and the Sun’s hot but faint outer atmosphere, the corona, will shine like silver tendrils around the intervening lunar disk.

A composite image shows fine detail in the Sun’s corona during a 2013 eclipse. [Jay Pasachoff/Allen Davis/Vojtech Rusin/Miloslav Druckmüller] This eclipse is the first to cross the United States since 1979. It will begin over the Pacific Ocean, when the Moon’s dark shadow first touches Earth. The shadow will reach the Oregon coastline at 10:16 a.m. PDT, and race southeastward before exiting over South Carolina at 2:49 p.m. EDT. The shadow will cross 11 states.

Totality will reach its maximum length of about 2 minutes, 40 seconds, over southern Missouri. The path of the eclipse will reach a maximum width of about 70 miles, with the eclipse lasting longest along the centerline of that path. Nashville is the largest city within the eclipse path, although parts of Kansas City and St. Louis will lie inside the shadow as well.

According to the American Astronomical Society, 12.2 million people live along the path of totality, 50 million live within 100 miles of that path, and 88 million are within 200 miles — a few hours’ drive. As a result, AAS and other organizations anticipate a hefty turnout for the total eclipse.

While the rest of the United States will miss out on the total phase of the eclipse, the entire country will experience a partial eclipse, where the Moon covers a part of the solar disk. From cities close to the eclipse path (such as Portland, Oregon, where the Moon will cover 99 percent of the Sun, or Atlanta, with 97 percent coverage), the sky will turn noticeably darker and the temperature will drop a bit. Even so, skywatchers will need eye protection to look at the eclipse.

This map shows the path of the eclipse across the United States. [NASA/Scientific Visualization Studio]