Why does the moon turn red during a total lunar eclipse?

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Why does the moon turn red during a total lunar eclipse?

By Jeanna Bryner 16 November 2021

The fiery glow of a lunar eclipse is caused by the same phenomenon that makes the sky blue.

This composite image shows the moon during a lunar eclipse in Tokyo on Jan. 31, 2018. The large crimson moon was viewable in many corners of the globe. (Image credit: KAZUHIRO NOGI/AFP via Getty Images)

During a lunar eclipse, the face of the moon will turn a shade of red.

The fiery glow is most dramatic during a total lunar eclipse, but even during partial and penumbral lunar eclipses, our lone satellite is cast an auburn color. 

When the moon tiptoes into the outer portion of Earth's shadow, becoming totally bathed in the darkest part of that shadow, why isn't the result a "lights out" for the sky? Why instead does the moon become engulfed in a light-orange to blood-red glow? 

Perhaps counterintuitively, the phenomenon that explains why the sky is blue is the same one that turns our moon red during a lunar eclipse. It's called Rayleigh scattering, or the preferential scattering of certain wavelengths of light off of teensy particles — those about one-tenth the wavelength of the light or smaller. During the day, the sun's light waves — which are made up of a swath of colors corresponding to their individual wavelengths — get filtered through our atmosphere, where the tiny nitrogen and oxygen gas molecules let the longer wavelengths such as reds, oranges and yellows, pass through straight to the ground (missing our line of sight). But the shorter wavelengths — such as violets and blues — get absorbed and then scattered every which way, giving them more chances to hit our eyes.  

A lunar eclipse happens when the sun, moon and Earth are lined up in that exact order. (Image credit: Shutterstock)

Here's how that same light-scattering phenomenon makes our moon look red. During a total lunar eclipse, the sun, Earth and moon are perfectly lined up in that order. When the Earth is directly in front of the sun, it blocks the sun's rays from lighting up the moon. At that moment, you would see a ring of light around the moon, because even though our planet is way bigger than the sun, our home star's light bends around the edges of Earth. This light gets reflected onto the moon.

"The darkened terrestrial disk is ringed by every sunrise and every sunset in the world, all at once," according to NASA. However, the sun's light must pass through Earth's atmosphere before it hits the moon. Along the way, the shorter-wavelength blue light gets scattered about before reaching the moon, leaving the reds and oranges unscathed to bathe the moon's surface. And voila, a red moon. 

Sunsets often appear red because of the angle of the sun. At such a low position in the sky, the sun sends the longer-wavelength red light right to your line of sight.  (Image credit: Shutterstock)

Sunrises and sunsets take on reddish hues for this reason: "When the sun is higher in the sky during the day, the red light passe[s] straight through to the ground. While blue light gets scattered and therefore is more likely to be in your line of sight," astronomer Shannon Schmoll, director of Abrams Planetarium at Michigan State University, told Live Science in an email. "At sunset, since the sun is low in the sky, the red light passes through into your line of sight, while the blue get[s] scattered away from it."

The moon will change various shades during different stages of a total lunar eclipse, morphing from an initial grayish to orange and amber. Atmospheric conditions can also affect the brightness of the colors. For instance, extra particles in the atmosphere, such as ash from a large wildfire or a recent volcanic eruption, may cause the moon to appear a darker shade of red, according to NASA. 

The moon doesn't always hide completely behind Earth's shadow. During partial lunar eclipses, the sun, Earth and moon are slightly off in their alignment, and so our planet's shadow engulfs just part of the moon.

A novice skywatcher might not even notice the third type of lunar eclipse, the penumbral kind, in which the moon sits in Earth's penumbra, or its faint outer shadow.

To see this atmospheric science in action, look no further than the longest partial lunar eclipse of the century, which is due to grace the skies between Nov. 18 and. 19. The fiery moon will be visible in all 50 U.S. states.

The next two lunar eclipses will be total eclipses and will occur on May 16, 2022 (visible in the Americas, Europe and Africa), followed by one on Nov. 8, 2022 (visible in Asia, Australia, the Pacific and the Americas), according to NASA.

Editor's Note: This article was updated on Nov. 16, 2021.

Original article on Live Science.

Jeanna Bryner

Live Science Editor-in-Chief

Jeanna is the editor-in-chief of Live Science. Previously, she was an assistant editor at Scholastic's Science World magazine. Jeanna has an English degree from Salisbury University, a master's degree in biogeochemistry and environmental sciences from the University of Maryland, and a graduate science journalism degree from New York University. She has worked as a biologist in Florida, where she monitored wetlands and did field surveys for endangered species. She also received an ocean sciences journalism fellowship from Woods Hole Oceanographic Institution.

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