Anticipation is building for the total solar eclipse on April 8. On that day, parts of Mexico, the United States and Canada will be plunged into twilight-like darkness as the moon passes between the sun and the Earth. Solar eclipses of any kind are rare, but total solar eclipses—in which the moon completely blocks the face of the sun—are even more uncommon. Between 2001 and 2100, just 68 total eclipses have occurred or will occur worldwide, representing a little more than 30 percent of all...