Why the Blood Moon Eclipse Turns Red

he moon turns blood red in this 3:30 a.m. ET view of the total lunar eclipse on April 15, 2014 as seen by a telescope at the University of Arizona's Mt. Lemmon SkyCenter at Steward Observatory atop Mt. Lemmon, Arizona.
The moon turns blood red in this 3:30 a.m. ET view of the total lunar eclipse on April 15, 2014 as seen by a telescope at the University of Arizona's Mt. Lemmon SkyCenter at Steward Observatory atop Mt. Lemmon, Arizona. (Image credit: Mt. Lemmon SkyCenter/University of Arizona)

On Sunday (Sept. 27), skywatchers across the United States will be in for a special treat: The full "supermoon" (a full moon when the moon is closest to Earth in its orbit) will go into eclipse. A lunar eclipse occurs when the moon passes into the shadow of the Earth.

There are several "flavors" of eclipse. A penumbral eclipse happens when the moon just grazes the thinnest part of Earth's shadow. If the moon partly goes into the shadow, observers see a partial eclipse. A total lunar eclipse is the most spectacular of the three: The moon goes fully into the shadow and appears either red or brown.

Sometimes an eclipse moon is called a "blood moon" because of this rusty color. But why does the moon turn red, and not simply darken to black when it goes into the shadow? As NASA explains, it's because the Earth's atmosphere extends beyond the planet, and sunlight passes through it, still reaching the moon. [Supermoon Lunar Eclipse 2015: Complete Viewing Guide]

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"During a total lunar eclipse, white sunlight hitting the atmosphere on the sides of the Earth gets absorbed and then radiated out (scattered). Blue-colored light is most affected," NASA officials wrote online. "That is, the atmosphere filters out (scatters away) most of the blue-colored light. What's left over is the orange- and red-colored light."

The light through Earth's atmosphere then falls onto the moon. NASA notes that the red light seen during a lunar eclipse is much dimmer than a typical moon's light. That happens because the red light is reflected back to Earth, and it is much dimmer than the white light the sun usually shines onto the moon's surface.

The moon turns different shades of red, orange or gold with each eclipse. That's because the shade of the light reaching the moon depends on what is in Earth's atmosphere (the amount of water and particles), as well as the atmosphere's temperature and humidity, NASA wrote. For example, a recent volcanic eruption could send more particulates into the atmosphere, further darkening the moon during an eclipse.

Editor's Note: If you snap an amazing picture of the Sept. 27 total lunar eclipse and want to be featured in a story or gallery, you can send photos, comments, and your name and location to managing editor Tariq Malik at spacephotos@space.com.

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Elizabeth Howell
Staff Writer, Spaceflight

Elizabeth Howell (she/her), Ph.D., is a staff writer in the spaceflight channel since 2022 covering diversity, education and gaming as well. She was contributing writer for Space.com for 10 years before joining full-time. Elizabeth's reporting includes multiple exclusives with the White House and Office of the Vice-President of the United States, an exclusive conversation with aspiring space tourist (and NSYNC bassist) Lance Bass, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, "Why Am I Taller?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and a Bachelor of History from Canada's Athabasca University. Elizabeth is also a post-secondary instructor in communications and science at several institutions since 2015; her experience includes developing and teaching an astronomy course at Canada's Algonquin College (with Indigenous content as well) to more than 1,000 students since 2020. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon: https://qoto.org/@howellspace