On Aug. 21, 2017, American skywatchers will be treated to a rare and spectacular celestial show — the first total solar eclipse visible from the continental United States in nearly four decades. Next year's "Great American Total Solar Eclipse" will darken skies all the way from Oregon to South Carolina, along a stretch of land about 70 miles (113 kilometers) wide. People who descend upon this "path of totality" for the big event are in for an unforgettable experience, said eclipse expert Jay Pasachoff, an astronomer at Williams College in Massachusetts. "It's a tremendous opportunity," Pasachoff told Space.com. "It's a chance to see the universe change around you." A total solar eclipse last darkened soil on the U.S. mainland on Feb. 26, 1979. But August 2017 will mark the first time in 99 years that such an event is "readily available to people from coast to coast," Pasachoff said. A rare event The fact that total solar eclipses occur at all is a quirk of cosmic geometry. The moon