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

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)

The only total lunar eclipse of the year will light up the sky this Wednesday (May 26), when the full moon (a supermoon due to the satellite's nearness to Earth) passes through Earth's shadow. During the so-called Super Flower Blood Moon lunar eclipse, the face of the moon will turn a brick-red hue.

The fiery glow is the most dramatic of the three types of lunar eclipses (the other two are called partial and penumbral). 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?

Related: Total lunar eclipse Wednesday will make supermoon turn blood red

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, going 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.

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.

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Jeanna Bryner
Jeanna is the managing editor for LiveScience, a sister site to SPACE.com. Before becoming managing editor, Jeanna served as a reporter for LiveScience and SPACE.com for about three years. Previously she was an assistant editor at 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 science journalism degree from New York University. To find out what her latest project is, you can follow Jeanna on Google+.