Total solar eclipse. Computer-enhanced image of the Sun's corona seen at the moment of totality during a total solar eclipse. The corona is a cloud of low-density plasma (hot ions) surrounding the Sun. Normally it is not visible, being obscured by the brightness of the Sun's visible surface, but it is revealed during a total solar eclipse, which occurs when the Moon blocks the light from the rest of the Sun. The moon's surface can be seen due to light reflecting off the surface of the earth, a phenomenon known as earthshine. This moment of totality lasts for only a few minutes. Total solar eclipses usually occur less than once a year, and can only be seen from a small area of the Earth's surface.