On August 21, millions of people across the United States will be able to watch the 2017 solar eclipse

We're just days away from the total solar eclipse of Aug. 21 and it's a good time for a refresher course on how to safely observe the event. Your parents probably told you to NEVER look directly at the sun with your naked eye. In fact, you've probably been told that by lots of reputable sources (including our own Space.com). But according to NASA and four other science and medical organizations, it's OK to look at a total solar eclipse with the naked eye — but only when the face of the sun is totally obscured by the moon.

A total solar eclipse happens when the central disk of the sun is completely covered by the moon. Many people have probably seen a partial solar eclipse, in which the disk of the moon appears to take a bite out of the sun's disk, but never fully obscures it. But total solar eclipses are a much rarer sight. And on Monday, a total solar eclipse will cross the continental U.S. from coast to coast.