On average, solar eclipses happen a handful of times every year. But, total solar eclipses — when the moon completely blocks out the sun — occur only every 18 months or so. The so-called “Great American Total Solar Eclipse” (the folks at NASA evidently prefer to spend time on more important things like searching for aliens than on hip marketing buzzwords) will cross the United States on Monday, August 21, 2017. For just a few minutes on that date, the sun will pass behind the moon, creating a beautiful “twilight” effect with large streaks of light dancing in silhouette around the moon. It will be the first total solar eclipse in the history of the country to pass solely over the continental U.S.