A solar eclipse.
Annular eclipse photographed from Mazatlan, Mexico, May 5, 2012.
A solar eclipse is an eclipse in which Earth passes through the shadow cast by the Moon. Solar eclipses only happen when the Moon is new and when the Moon lies close to the node of its orbit (i.e. when it's roughly in the same plane as Earth's orbit). To see a total solar eclipse the observer has to fall within in the Moon's umbra, the darkest part of the lunar shadow, as it races across our planet. The path of totality is never any wider than 270 kilometers and, since it is swept out at some 3,200 kilometers per hour, the length of totality at given location is never more than 7 minutes 31 seconds, and usually no longer than 3 or 4 minutes. A partial eclipse is seen by observers in the Moon's penumbra, the partial shadow, on either side of the path of totality. An annular eclipse happens when the Moon is near apogee so that its apparent size is less than that of the Sun and a ring, or annulus, of the Sun's disk is still visible even when the Moon and Sun are seen directly in line. The magnitude of a solar eclipse is the fraction of the solar diameter obscured by the Moon at the greatest phase of an eclipse, measured along the common diameter.