Total Solar Eclipse
Photograph by Nat'l Optical Astronomy Observatories
The sun's outermost region, called the corona, shines like a halo around the moon during a total solar eclipse. Such eclipses occur when a new moon passes in front of the sun. They don't happen often—only about once a year—since the tilted orbits of the sun, moon, and Earth make their alignment rare. Total solar eclipses are of special interest to astronomers because it is the only time the sun's corona can be seen from the Earth's surface.