The passage of the Moon through the Earth’s shadow, resulting in a darkening which may range from barely detectable in a penumbral eclipse to very dark when the Moon enters the Earth’s umbra. Lunar eclipses can occur only at full Moon when the Moon is close to the node of its orbit around the Earth. When the Moon is slightly to the north or south of the node, it does not pass centrally through the umbra, and may undergo only a partial eclipse. During umbral eclipses the Moon appears darkened to varying degrees (see danjon scale), determined principally by the clarity of the Earth’s atmosphere. Refraction of sunlight through the atmosphere on to the eclipsed Moon produces a coppery or reddish coloration in some events, while darker eclipses may appear steel-grey. Coloured fringes may be seen around the edges of the umbra during the partial phases. A lunar eclipse is visible over the entire hemisphere of the Earth from which the Moon is above the horizon. The maximum duration of totality for a lunar eclipse, when the Moon passes centrally through the umbra, is 1 h 47 m.

http://www.mreclipse.com/Special/LEprimer.html Lunar eclipse predictions, reports, and information.