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. So at such times, Earth’s shadow falls on the moon, creating a lunar eclipse. Lunar eclipses happen a minimum of two times to a maximum of five times a year. As a matter of fact, there are three kinds of lunar eclipses: total, partial and penumbral.
In a total eclipse of the moon, the inner part of Earth’s shadow, called the umbra, falls on the moon’s face. Then at mid-eclipse, the entire moon is in shadow, which may appear blood red.
Next, there is a partial lunar eclipse, where 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.
Finally, there’s a penumbral lunar eclipse, when only the more diffuse outer shadow of Earth – the penumbra – falls on the moon’s face. In fact, 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. That’s because there is never a dark bite taken out of the moon, as in a partial eclipse. So the eclipse never progresses to reach the dramatic minutes of totality. And at best, at mid-eclipse, very observant people will notice a dark shading on the moon’s face. Others will look and notice nothing at all.
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 beautiful natural event.
What to expect from a penumbral eclipse
Some eclipse photos
Bottom line: There are three kinds of lunar eclipses: total, partial and penumbral. A penumbral eclipse is very subtle. At no time does a dark bite appear to be taken out of the moon. Instead, at mid-eclipse, observant people will notice a shading on the moon’s face.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. In 2020, she was the Education Prize from the American Astronomical Society, the largest organization of professional astronomers in North America. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.
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