What is a Penumbral Eclipse?

Penumbral Lunar Eclipse


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This Friday, September 16, the Moon turns full.  And it’s a penumbral eclipse. And what does penumbral even mean?

First, the facts:  The Harvest Moon rises this Friday.  It’s dubbed with this name because its early moonrise (right after sunset) over successive days provides extra light to give more time for folks to gather their crops at harvest time. That itself is kind of cool.

It’s also a penumbral eclipse. The eclipse happens on Friday after lunch (EDT), when the Moon won’t even be out.  You’d have to see it from Europe or Asia or on a live feed from these places here.

This is the third penumbral eclipse this year so we’ve had a strange glut of them.

From many internet sites, almanacs, and other media outlets, you’ll just hear the word “eclipse.”  Is it an eclipse?  Sort of.  Let’s get to the question …

What is a Penumbral Eclipse?

To be honest, pnumbral 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.  Sometimes there’s a very slight gray shading on one part of the Moon, but almost nobody notices it. 


The above diagram 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. 

And that’s what happened three times this year—and will occur again on Friday. I’ll talk more about this eclipse and other Moon facts this Friday during our live Moon show with Slooh. Watch it here.

~ By  Bob Berman

About This Blog

Welcome to “This Week’s Amazing Sky,” the Almanac’s blog on stargazing and astronomy. Wondering which bright objects you’re seeing in the night sky? Want to learn about a breathtaking sight coming up? Bob Berman, longtime and famous astronomer for The Old Farmer’s Almanac, will help bring alive the wonders of our universe. From the beautiful stars and planets to magical auroras and eclipses, we’ll cover everything under the Sun (and Moon)!


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