Illustration of the stages before and after a total solar eclipse (centre). The outer corona (white) is seen here, the cloud of low-density plasma (hot ions) that surrounds the Sun but is normally obscured by the brightness of the Sun's visible surface. A solar eclipse 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.