What Is a Penumbral Eclipse?

Penumbral Lunar Eclipse

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Tonight, February 10, 2017 will bring the year’s first lunar eclipse. It’s a penumbral eclipse. What exactly does this mean?

First, the facts:  This penumbral lunar eclipse is this evening, Friday, February 10, 2017. It will begin at 5:34 p.m. with best viewing at 7:44 p.m. This is a very subtle kind of eclipse which may appear like a darker-than-usual Moon. It’s easiest to view from the eastern portions of North America, as it will be too light in the western time zones.

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, penumbral 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. 

lunar-eclipse-diagram.png

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. 

If you’re interested, I will talk more about this eclipse and other Moon facts tonight during our live astronomy show with Slooh. Watch it here.

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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Tonight's eclipse

Hi, OK, so I live in Eastern WA, what time is best to view the eclipse ?

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