Astronomy Essentials

What is a penumbral eclipse of the moon?

Full moon in penumbral eclipse; there is a shading on the top right side of the moon.
View at EarthSky Community Photos. | Nils Ribi in Sun Valley, Idaho, caught the November 30, 2020, penumbral lunar eclipse. He wrote: “The penumbral eclipse of the full moon, November 30, 2020, at 2:43 a.m., the time of greatest eclipse, in Sun Valley, Idaho. It was nice to see that the eclipse was not that faint here.” Thank you, Nils!

The next penumbral lunar eclipse: March 24-25, 2024.

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.

Diagram with Earth between sun and moon showing moon passing through Earth's shadow.
In a lunar eclipse, Earth’s shadow falls on the moon. And if the moon passes through the dark central shadow of Earth – the umbra – a partial or total lunar eclipse takes place. Then, if the moon only passes through the outer part of the shadow – the penumbra – a subtle penumbral eclipse occurs. Diagram via Fred Espenak/ Lunar Eclipses for Beginners. Used with permission.

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The three types of lunar eclipses

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

Two full moons side by side with the one on the right slightly shaded.
View larger. | Left, an ordinary full moon with no eclipse. Right, full moon in penumbral eclipse on November 20, 2002. Master eclipse photographer Fred Espenak took this photo when the moon was 88.9% immersed in Earth’s penumbral shadow. There’s no dark bite taken out of the moon. A penumbral eclipse creates only a dark shading on the moon’s face. Image via Fred Espenak. Used with permission.

Some eclipse photos

Orange-red full moon.
This is what a total eclipse looks like. This is the total eclipse of October 27, 2004. Image via Fred Espenak.
Composite image showing 4 stages of the partial lunar eclipse. The moon gets darker from top left to bottom right.
View at EarthSky Community Photos. | Lorraine Boyd in Delmar, New York, wrote: “Even though we had cloudy skies, there were breaks and I was able to capture the full Beaver Moon partial lunar eclipse. It was a beautiful sight to see.” It was, wasn’t it? Thank you, Lorraine!
Moon with a lightly darker area at top left.
View at EarthSky Community Photos. | Here is the penumbral eclipse of July 4-5, 2020. As you can see, it’s not very noticeable. Greg Redfern in central Virginia commented: “Taken at maximum eclipse for the penumbral lunar eclipse. May be some shading in the upper left quadrant.” Thank you, Greg.

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.

Next penumbral lunar eclipse: March 24-25, 2024

January 1, 2024
Astronomy Essentials

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