Why does moon in total eclipse look red?

If Earth didn’t have an atmosphere, the moon would be dark – perhaps even invisible – when entirely eclipsed within Earth’s shadow.

Large orange-red eclipsed moon.

Photo via Fred Espenak.

Coming up…Total lunar eclipse of January 20-21, 2019

During a lunar eclipse, you’ll see the Earth’s shadow creeping across the moon’s face. The shadow will appear dark, like a bite taken 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 often suddenly changes. Instead of dark, it appears red. Why?

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

Thanks to Earth’s atmosphere, what actually happens is much more subtle and beautiful.

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

Sunlight is composed of a range of frequencies. As sunlight passes through our atmosphere, the green to violet portion of the light spectrum is, essentially, filtered out. This same effect, by the way, is what makes our sky blue during the day. Meanwhile, the reddish portion of the spectrum is least affected.

What’s more, when this reddish light first entered the atmosphere, it was bent (refracted) toward the Earth’s surface. 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.

Dark planet Earth with a pink rim on the left. Orange moon on the right.

The amazing photo montage at left combines an image of Earth, taken by the Apollo astronauts, and one by the Japanese lunar probe Kaguya during a lunar eclipse in 2009. A person standing on the moon during a total lunar eclipse would witness the Earth eclipsing the sun. The pink ring around the Earth is our atmosphere aglow with the combined light of all the sunrises and sunsets on the planet. At right, the fully eclipsed moon takes on their color. Photos via JAXA (left), Jim Fakatselis. Image and caption via AstroBob.

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

In December 1992, not long after the eruption of Mount Pinatubo in the Philippines, there was so much dust in Earth’s atmosphere that the totally eclipsed moon could barely be seen.

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

Diagram of Earth lined up with sun and moon.

A lunar eclipse takes place when the sun, Earth and moon line up in space. The moon passes through Earth’s shadow. Image via Wikimedia Commons.

Bottom line: The moon can look red during a total lunar eclipse because of sunlight that’s filtered and refracted by Earth’s atmosphere.

Visit EarthSky’s Best Places to Stargaze to find an eclipse-viewing location

Post your eclipse photo to EarthSky Community Photos

Best New Year’s gift ever! EarthSky moon calendar for 2019

Deborah Byrd