A total lunar eclipse looks red. Why?

September 5, 2025

26

View at EarthSky Community Photos. | Patrick Prokop in Savannah, Georgia, created this composite image of the different phases of a lunar eclipse. Wonderful! Thank you, Patrick.

Coming up … Total lunar eclipse of September 7

During a lunar eclipse, you’ll see the Earth’s shadow creeping across the moon’s face. The shadow appears dark, shaped like a bite out of a cookie, until the shadow completely covers the moon. Then, during the breathtaking time of totality, the shadow on the moon’s face appears red, rusty orange or copper-colored. Why?

Why a lunar eclipse looks red

The reason stems from the air we breathe. During a total lunar eclipse, the Earth lies directly between the sun and the moon. Earth casts its shadow on the moon as a result. If Earth didn’t have an atmosphere, then, when the moon is entirely within Earth’s shadow, the moon would appear black, perhaps even invisible.

However, something much more subtle and beautiful actually happens, thanks to Earth’s atmosphere.

Earth’s atmosphere extends about 50 miles (80 km) above Earth’s surface. During a total lunar eclipse, with the moon submerged in Earth’s shadow, there’s a circular ring around Earth, the ring of our atmosphere. The sun’s rays pass through this ring.

Sunlight contains a range of frequencies

White sunlight consists of a range of different colors, or frequencies. As sunlight passes through our atmosphere, the green to violet portion of the light (electromagnetic) spectrum is, essentially, filtered out. This same effect, by the way, is why our sky is blue during the day. Meanwhile, the reddish portion of the spectrum is least affected.

What’s more, when this reddish light first enters our atmosphere, it’s bent (refracted) toward the Earth’s surface. And it’s bent again when it exits on the other side of Earth. This double bending sends the reddish light onto the moon during a total lunar eclipse. It also explains why sunrises and sunsets look red.

Line of 26 moons, changing from orange to white, in dark sky over a tall white obelisk monument. — View at EarthSky Community Photos. | Sergio Garcia Rill captured these lunar eclipse images on May 15-16, 2022, over the San Jacinto Monument in La Porte, Texas. He wrote: “I took individual images at 850mm of the phases of the moon. And later I resized them (downsized), and re-arranged and overlaid with an HDR processed image of the monument, using Photoshop.” Thank you, Sergio!

The brightness and color of a lunar eclipse

Depending on the conditions of our atmosphere at the time of the eclipse (dust, humidity, smoke, temperature and so on can all make a difference), the surviving light illuminates the moon with a color that ranges from copper-colored to deep red.

A moon in total eclipse never appears as bright as a full moon, but how dark it gets varies. The totally eclipsed moon was barely visible in December 1992, not long after the eruption of Mount Pinatubo in the Philippines, due to so much dust in Earth’s atmosphere.

Large pink-orange full moon surrounded by smaller moons in different phases of the eclipse. — View at EarthSky Community Photos. | Kathy Hunter caught these views of the lunar eclipse on March 14, 2025, from West Virginia. Kathy wrote: “My first composite!” Thank you, Kathy.

Photo of red moon, an eclipsed moon, with a crescent-shaped illuminated portion on one side. — View at EarthSky Community Photos. | Cecille Kennedy in Depoe Bay, Oregon, wrote: “The forecast was rainy, and the clouds were thick. We didn’t see the moonrise. Hours later, there was a clearing on the other side and a few stars became visible. I went outside to see the most beautiful blood red moon playing hide and seek with the clouds. I managed to take a few shots before dark clouds covered the night, and the rains came.” Thank you, Cecille!

All total lunar eclipses do not look alike

Can anyone know in advance how red or dark the moon will appear during a total lunar eclipse? Not really. Before an eclipse takes place, you’ll hear people speculate about it. That uncertainty is part of the fun of eclipses, so enjoy! And watch for the red moon during a lunar eclipse.

Three images of the moon that look mostly orange, except for the moons on the side, that have a bluish tone at the top. — View at EarthSky Community Photos. | Petr Horálek captured these full moons from the Cerro Tololo Observatory in Chile. Petr wrote: “I made it happen (with no sleep yet) to finalize today’s lunar eclipse triplet, as the eclipse was truly beautiful over the CTIO Cerro Tololo observatory, Chile. Colors in the Earth’s shadow were vivid, including the turquoise effect at the start and even end of the eclipse (where primarily the ozone layer causes a bluish tint, referring to Richard Keen’s explanation from 2007). The effect was easily capturable on camera, but also nicely visible to binoculars.” Amazing, thank you! Image via Petr Horálek/ CTIO (Cerro Tololo Observatory)/ AURA/ NFS/ NOIRLab.

What about that blue band?

Another color to watch for at the beginning and end of totality is a blue band of light along the limb (edge) of the moon. This blue band is light passing through our ozone layer – which absorbs red light – that allows blue light to come through. The blue band is frequently caught in photos but may be hard to see visually.

Diagram of sun, Earth, and moon lined up with Earth shading the moon. — In a lunar eclipse, the sun, Earth and moon line up, with the Earth in the middle. Image via NASA.

Bottom line: Coming up … the total lunar eclipse of September 7, 2025. At maximum eclipse, the moon will look red. But why? Earth’s atmosphere is the key.

March 13-14 total lunar eclipse: Astonishing images via EarthSky’s community

Post your eclipse photo to EarthSky Community Photos

Deborah Byrd

View Articles

About the Author:

Our Editor-in-Chief Deborah Byrd works to keep all the astronomy balls in the air between EarthSky’s website, YouTube page and social media platforms. She’s the primary editor of our popular daily newsletter and a frequent host of EarthSky livestreams. Deborah created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Prior to that, she had worked for the University of Texas McDonald Observatory since 1976, and created and produced their Star Date radio series. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. In 2020, she won the Education Prize from the American Astronomical Society, the largest organization of professional astronomers in North America. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. “Being an EarthSky editor is like hosting a big global party for cool nature-lovers,” she says.

#total #lunar #eclipse #red