The entry of one celestial body into the shadow of another; strictly speaking, an eclipse of the Sun is an occultation, not an eclipse. In any one year, the maximum total of solar and lunar eclipses visible from Earth is seven. The minimum number is two, both of which must be solar. Eclipses frequently come in pairs, with a lunar eclipse preceded or followed within a fortnight or so by a solar eclipse at the opposite node of the Moon’s orbit. As seen from any one place, lunar eclipses are twice as common as solar eclipses.

Solar eclipses occur when the new Moon lies close to the node of its orbit and at the same longitude as the Sun. The Moon obscures at least part of the Sun’s disk as seen from a comparatively limited ground track where the cone of shadow cast by the Moon falls on the Earth’s surface. Along the centre of this track a total eclipse may be seen; eclipses occurring around lunar apogee may be annular eclipses. A partial eclipse is seen either side of the central path of totality or annularity, which sweeps eastwards at around 3200 km/h. The maximum diameter of the Moon’s umbral cone at the Earth is 270 km, but the actual width of the shadow path on the Earth’s surface can become much greater than this at high latitudes, where the umbral cone strikes the surface at an oblique angle. At a total eclipse, the Moon moves across the Sun’s disk for an hour or so, until the Sun is completely covered and its corona becomes visible. By coincidence, the Sun and Moon have much the same apparent angular diameter (about ½°). Total solar eclipses are rare at any one place on the Earth, so astronomers usually have to travel long distances to see them. A solar eclipse can last up to 3 hours from first contact to fourth contact; totality has a theoretical maximum duration of 7 m 32 s, but is usually much shorter. A lunar eclipse is visible from Earth wherever the Moon is above the observer’s horizon, and occurs as the full Moon passes through the Earth’s shadow. A total lunar eclipse occurs when the Moon passes through the dark central umbra of the Earth’s shadow. Partial lunar eclipses are also seen, as are penumbral eclipses, which are often barely noticeable. The Earth’s shadow is much broader than the Moon itself, so that lunar eclipses may last for up to 4 hours from first contact to fourth contact; totality lasts up to 1 h 47 m.

Planetary satellites are also eclipsed by the shadows of their primaries; those of the Galilean satellites of Jupiter are readily observable.

eclipse:

(a) In a total lunar eclipse the Moon, viewed from point A, is faintly illuminated by sunlight refracted through the Earth’s atmosphere. (b) An observer at A sees the Sun’s disk completely obscured by the Moon in a total solar eclipse, but the solar corona becomes visible. (c) In an annular eclipse of the Sun, an observer at A sees the Moon framed by the outline of the Sun. In (b) and (c), observers within the Sun’s penumbra see a partial solar eclipse.

http://eclipse.gsfc.nasa.gov/eclipse.html NASA eclipse website.