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Why an eclipse paints the moon red

When the moon passes through Earth’s shadow, it paints the town red. David Ropeik explains in “Gee-Whiz Science.”
During a total lunar eclipse, the moon has a red glow ... but why?
During a total lunar eclipse, the moon has a red glow ... but why?
/ Source: contributor

When Earth passes between the sun and the moon, you’d figure that the moon would disappear, right? Wrong. Even seasoned lunar eclipse watchers might be surprised to learn how bright the moon is supposed to be.

When the moon is fully covered by Earth’s shadow, it will:

A. Fade to black

B. Shine on, shine on

C. Turn into gooey melted brie

D. None of the above

Moonbeams are no more than reflected sunbeams. But even without direct sunlight, when the Earth intercedes between our two bright celestial neighbors and blocks the sun from shining directly on the moon, the moon still shines, kind of, during an eclipse. The answer is D.

The reason for redness
So why can you still see the moon during a total eclipse? It’s because some sunlight is still hitting it, and for that you can thank our atmosphere. Particles in the atmosphere cause the light rays coming from the sun to bounce around. Some are refracted, or bent. They get redirected through the atmosphere and out around behind Earth and onto the moon, which is blocked only from direct sunlight.

Thus, the moon is still visible in the sky. However, the refracted rays of sunlight doing the illuminating turn the moon a strange reddish. Or copper. Maybe rust.

That’s because of all the bouncing around those rays had to go through on their way through the atmosphere. The more atmosphere that sunlight travels through, the more the blue and green parts of the spectrum are scattered. That’s why sunrises and sunsets are yellow and pink and red. The low early or late sun, hitting the atmosphere at a shallow angle, has to fight through more atmospheric particles on its way to your eye, and the reddish wavelengths get through better.

The same thing happens to sunlight refracted onto the moon during an eclipse. The sunlight hits the atmosphere on the sides of Earth at a shallow angle and is carried through a lot of atmosphere until it’s redirected out onto the moon “hiding” from direct sunlight. The red end of the spectrum is all that can get through that much interference. So the moon in total eclipse appears as an eerie, glowing copper ball in the sky.

How does it rate?
It may sound odd, but lunar eclipses are actually rated by their brightness. On the Danjon scale (Andre Danjon, French astronomer) a moon you can barely see is a zero. A bright copper-red or orange moon during total eclipse rates a 4.

Total lunar eclipses tend to be brighter when the atmosphere is relatively clear of volcanic dust. A dirty atmosphere blocks more sunlight and dims the eclipse. So does the weather, and it’s not just whether the weather is cloudy overhead. The weather around the globe counts, too. The cloudier the global atmosphere, the less sunlight makes it around to the moon, and the dimmer a show we get.

Unlike a quickie solar eclipse, which flashes past in a maximum of 7 1/2 minutes, the lunar variety happens at a more leisurely pace. The shadow made by Earth as it blocks the sun is 6,480 miles wide, out where the moon crosses through it.

The moon itself is only 2,160 miles wide, and it’s traveling about 2,300 mph. So it’s roughly three hours from the time the moon first touches the shadow, known as the umbra, until the last part of the moon passes out of it. The middle third of the journey is the part of the eclipse that’s “total.”

Don’t give yourself a neck cramp trying to spot the precise moment of the event’s start or finish: Sunlight hitting the rest of the moon will keep the leading edge of shadow from showing up distinctly.

For the best view of a lunar eclipse, try binoculars. They bring you in closer, but preserve a wide enough field of view to see the whole show. And don’t worry about looking at it directly with the naked eye. That’s only a problem with the radiation streaming from a partial solar eclipse.

David Ropeik is a longtime science journalist and currently serves as Director of Risk Communication at the Harvard Center for Risk Analysis. This is an updated version of a column that first appeared in January 2000.