When and how does a lunar eclipse occur?
An eclipse of the moon can only happen at full moon, when the sun, Earth and moon line up in space, with Earth in the middle. At such times, Earth’s shadow falls on the moon, creating a lunar eclipse. When this happens – and it happens two to four times every year – everyone on Earth’s night side can see the eclipse.
There are three kinds of lunar eclipses: total, partial and penumbral. According to eclipse expert Fred Espenak, about 35% of all eclipses are penumbral. Another 30% are partial eclipses, where it appears as if a dark bite has been taken out of the moon. And the final 35% go all the way to becoming total eclipses of the moon, a glorious event.
In a total eclipse of the moon, the inner part of Earth’s shadow, called the umbra, falls on the moon’s face. At mid-eclipse, the entire moon is in shadow, which may appear blood red at times.
In a partial lunar eclipse, the umbra takes a bite out of only a fraction of the moon. The dark bite grows larger, and then recedes, never reaching the total phase.
In a penumbral lunar eclipse, only the more diffused outer shadow of Earth falls on the moon’s face. This third kind of lunar eclipse is much more subtle, and much more difficult to observe, than either a total or partial eclipse of the moon. There is never a dark bite taken out of the moon, as in a partial eclipse. The eclipse never progresses to reach the dramatic minutes of totality. At best, at mid-eclipse, very observant people will notice a dark shading on the moon’s face while others will look and notice nothing at all.