During a lunar eclipse, the moon passes through the Earth’s shadow and is not or only partially illuminated by the sun. A lunar eclipse only occurs at full moon.
Because the moon’s orbit is inclined to the ecliptic plane (about 5°), this does not occur at every full moon, but only when the moon is close to one of the two intersection points of the lunar orbit and the ecliptic plane, which occurs approximately at every sixth full moon (after about 177 days).
As lunar eclipse not only such events are referred to, where the moon is covered from the Earth´s umbra partially or completely, but also those inconspicuous events where the penumbra is runned through.
In contrast to a solar eclipse a lunar eclipse can be seen from anywhere in the night side of the Earth and looks everywhere the same. So we can observe a lunar eclipse much more frequently from a fixed location on Earth than a solar eclipse. Globally seen solar eclipses happen more often than flashy lunar eclipses.
There are four types of lunar eclipses:
- Total Lunar Eclipse: During the lunar eclipse the moon completely enters into the umbra of the Earth. Long-wavelength (red) light from the sun, which is able to penetrate the Earth’s atmosphere and can be broken into its umbra, is the cause that the moon is faintly visible as a so-called Blood Moon.
- Partial Lunar Eclipse: Only a part of the moon plunges into the umbra of the Earth, the rest is still in Penumbra. The edge of the umbra is mapped circularon the moon (also at the beginning and the end of a total lunar eclipse).
- Total Penumbral Eclipse: The moon fully immerses in the penumbra of the Earth, but not in the umbra. The total penumbral eclipse is extremely rare because the moon diameter is approximately equal to the width of the penumbral ring. In these rare cases in which the moon moves right through the penumbra, it can occur that the penumbra is slightly smaller than the moon. Then this is a not quite total penumbral eclipse, already a partial eclipse umbra or both. The last total penumbral eclipse took place on 14 March 2006, the next of this kind is expected on August 29, 2053. The moon seems to be noticeably darker at the point, which is closest to the umbra.
- Partial Penumbral Eclipse: The moon plunges only partially into the penumbra. And is imperceptibly darkened. With the naked eye it is only possible to observe the darkening on the site of the umbra if the magnitude is bigger than 0.7. This kind of eclipse can be observed with equal frequency as the partial lunar eclipse because of the approximate equality of moon diameter and ring widthof the penumbra.