Astronomy EssentialsSpace

A total lunar eclipse looks red. Why?

Lunar eclipse: Red full moon in a black background with white dots.
View at EarthSky Community Photos. | Shaun Tarpley in League City, Texas, captured this beautiful shot of the last total lunar eclipse on May 15, 2022, and wrote: “This image was taken from my backyard. The iOptron Skyguider Pro allowed me to take this 13-second image at roughly 700mm to bring out the detail in the moon and sky.” Thank you, Shaun!

Coming up … Total lunar eclipse of November 8, 2022

During a lunar eclipse, you’ll see the Earth’s shadow creeping across the moon’s face. The shadow appears dark, 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.

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 transitioning moons on a black background 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.

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.

Large red sphere in the center of 6 other spheres on a black background.
View at EarthSky Community Photos. | Michael Hoag in Eastern Kansas, created this stunning composite of the lunar eclipse on May 15, 2022, and wrote: “A composite of several photos. Nice seeing conditions for the eclipse. (The) front moved through dropping the humidity and clearing the skies.” Thank you, Michael!

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 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 Earth, moon, and sun with Earth shading the moon.
In a lunar eclipse, the sun, Earth and moon line up, with the Earth in the middle. The moon passes through Earth’s shadow. Image via NASA.

Bottom line: Coming up … The total lunar eclipse of November 8, 2022. At maximum eclipse, the moon will look red. But why? Earth’s atmosphere is the key.

Coming up … Total lunar eclipse of November 8, 2022

May 2022 Lunar eclipse photos, via EarthSky’s community

Post your eclipse photo to EarthSky Community Photos

November 5, 2022
Astronomy Essentials

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Deborah Byrd

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