What Is a Penumbral Eclipse? | Almanac.com

What Is a Penumbral Eclipse?

Bob Berman
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Ever heard of a β€œpenumbral eclipse” of the Moon? What is this? Is it an eclipse? Sort of. Let’s get to the question …

What is a Penumbral Eclipse?

To be honest, penumbral lunar eclipses are not that exciting if you’re just looking at the Moon. The Full Moon really doesn’t change its appearance during a penumbral eclipse as it does during a total eclipse of the Moon.

This is a very subtle kind of eclipse which may appear like a darker-than-usual Moon. Sometimes there’s a very slight gray shading on one part of the Moon, but almost nobody notices it.  

The diagram below shows different types of lunar eclipses. It all depends on the path taken by the Moon as it passes through Earth’s shadow. If the Moon passes through the outer circle but does not reach the inner circle, it is a penumbral eclipse. See credit.


Still, the penumbral concept is pretty interesting.  Turns out, everything casts two different shadows. 

  • If you look at your own shadow on the sidewalk you’ll see a main part where the Sun is completely blocked out. But there’s also a less dark blurry fringe surrounding your shadow.  That’s your penumbral shadow. If an ant ventured into this penumbral section it would see the sun partially but not fully blocked.
  • Our planet casts a black umbral shadow into space. Anything venturing into it is completely robbed of sunlight.  Earth’s umbral shadow gets smaller and smaller the farther it goes.  It tapers like a chopstick and disappears entirely a million miles from us in the anti-sunward direction.

But Earth’s penumbral shadow behaves differently. It gets larger as it goes farther away from us.  So it’s very easy for a nearby celestial object like the Moon, if its not lined up exactly, to venture into our penumbral but miss our umbral shadow. 

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