Total solar eclipse. Computer artwork of the outer corona of the Sun, seen during a total solar eclipse. The corona is the cloud of low-density plasma (hot ions) that surrounds the Sun, normally obscured by the brightness of the Sun's visible surface. It is revealed during a total solar eclipse, which occurs when the Moon moves in front of the Sun and obscures its surface. 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.