Life's Little Mysteries

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

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This composite image shows a blood moon lunar eclipse as seen in London and the Acacus mountains in the Libyan desert.
This composite image shows a blood moon lunar eclipse as seen in London and the Acacus mountains in the Libyan desert. (Image credit: Richard McManus/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. In addition, perfection is a must: A total lunar eclipse happens only when the sun, Earth and moon are perfectly lined up. 

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? 

Here's why: Picture yourself standing on the moon (lots of dust and craters at your feet), looking down on Earth during the spectacular night-sky event. When the Earth is directly in front of the sun — blocking the sun's rays from lighting up the moon — you'd see a fiery rim encircling the planet. 

"The darkened terrestrial disk is ringed by every sunrise and every sunset in the world, all at once," according to NASA. 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.

But not before it travels through our atmosphere, which filters out the shorter-wavelength blue light, leaving the reds and oranges unscathed to bathe the moon's surface. And voila, a red moon. 

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.

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Wednesday's total lunar eclipse is expected to be visible in Australia, parts of the western United States, western South America and Southeast Asia, according to timeanddate.com. Other areas of the world, including the entire U.S., will be able to see at least some stages of the lunar eclipse, including its partial and penumbral phases.

As for the other types of lunar eclipses, the next three penumbral eclipses will occur on May 5–6, 2023, March 24–25, 2024 and Feb. 20–21, 2027, according to timeanddate.com. The next total lunar eclipse, expected to be visible from parts of Asia, Australia, much of North America, South American, the Atlantic, Indian and Pacific Oceans, and Antarctica, according to timeanddate.com, will occur on May 15–16, 2022.

Editor's Note: This article was first published in 2016 and updated for the Super Blue Blood Moon lunar eclipse of 2018 and again in 2021.

Original article on Live Science.

Jeanna Bryner
Managing editor, Scientific American

Jeanna Bryner is managing editor of Scientific American. Previously she was editor in chief of Live Science and, prior to that, an editor at Scholastic's Science World magazine. Bryner 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, including the gorgeous Florida Scrub Jay. She also received an ocean sciences journalism fellowship from the Woods Hole Oceanographic Institution. She is a firm believer that science is for everyone and that just about everything can be viewed through the lens of science.