A lunar eclipse occurs when the shadow of the Earth passes across the surface of the Moon. In other words, at some point on the Moon’s surface, the Earth’s disc would appear to pass in front of the Sun.
Unlike solar eclipses, when a lunar eclipse occurs it is visible from anywhere on Earth where the Moon is visible. This means that, although there are actually fewer lunar eclipses than solar eclipses, they seem to occur more often because they can be observed from a much larger area on the Earth’s surface when they do occur.
Total and Partial Lunar Eclipses
A total lunar eclipse takes place when the Moon passes through the umbra of the Earth’s shadow, such that no part of the Sun’s disc would be visible to an observer anywhere on the Moon’s surface.
A partial lunar eclipse occurs when just a portion of the Moon’s surface is within the umbra or penumbra of the Earth’s shadow. A total lunar eclipse also has a partial phase as the Moon enters into the Earth’s shadow, passing first through the shadow’s penumbra. The penumbra is the region of the Earth’s shadow where, to an observer on the Moon’s surface, some portion of the Sun’s disc would still be visible behind the Earth.
Even the part of the Moon within the umbra of the Earth’s shadow will not in total darkness, due to light refracted through the Earth’s atmosphere. As blue light is scatted more readily by the atmosphere, most of the light reaching the Moon’s surface will be in the red part of the spectrum. For this reason, a total lunar eclipse is often known as a ‘blood moon’.