Lunar eclipse

A lunar eclipse happens when Earth lies between the Sun and the full Moon, and the Moon passes through Earth's shadow. The shadow has a dark inner region, the umbra, and a lighter outer region, the penumbra. Although a full Moon occurs roughly once each month, an eclipse does not: the Moon's orbit is tilted by about five degrees relative to the plane of Earth's orbit around the Sun. Most full Moons therefore pass above or below the shadow; an eclipse occurs when the alignment is close enough for the Moon to enter it.

In a total lunar eclipse, the entire Moon enters Earth's umbra. In a partial lunar eclipse, only part of the lunar disc enters the umbra, so the dark shadow appears to cover a portion of the Moon. In a penumbral eclipse, the Moon passes through Earth's faint outer shadow without entering the umbra; the resulting dimming may be subtle. The category depends on the Moon's path through the two parts of the shadow, not on a change to the Moon itself.

During totality, the Moon is not usually completely black. Some sunlight passes through Earth's atmosphere and is refracted into the umbra. As the light travels through the atmosphere, shorter blue and violet wavelengths scatter more readily than longer red and orange wavelengths. The filtered light reaching the Moon can make it appear coppery or red. The appearance varies with conditions in Earth's atmosphere, including dust and clouds; 'blood moon' is a popular description rather than a separate astronomical type.

A lunar eclipse can be seen from the portion of Earth where it is nighttime and the Moon is above the horizon, weather permitting. Unlike viewing the Sun during a solar eclipse, looking at a lunar eclipse with the unaided eye is safe; no special eye protection is required. An event may unfold over several hours as the Moon enters and leaves Earth's shadow, while the period of totality, when it occurs, is shorter.