A lot of people gathered yesterday,  Aug. 21, 2017, as skies darkened from Oregon to South Carolina in the first total solar eclipse visible from coast to coast across the United States in 99 years.

---

 A total solar eclipse occurs when the disk of the moon appears to completely cover the disk of the sun in the sky. The fact that total solar eclipses occur at all is a quirk of cosmic geometry.

The moon orbits an average of 239,000 miles (385,000 kilometers) from Earth — just the right distance to seem the same size in the sky as the much-larger sun. However, these heavenly bodies line up only about once every 18 months.

See pics below