Why does the moon turn red during an eclipse?


Credit: Anne Dirkse

This week’s eclipse got me wondering about something – we all learned in school that a lunar eclipse happens when the earth comes between the sun and the moon, putting the moon in the earth’s shadow. But nobody ever explained the red glow that the moon takes on during the eclipse. Why does that happen? It’s actually the same reason that the sky is blue and sunsets are orange- Rayleigh scattering. What’s Rayleigh scattering, you ask? Read on to learn more.

Rayleigh scattering

Rayleigh scattering is a phenomenon where electromagnetic radiation (like light) runs into something (like air molecules) and gets “scattered” in a thousand different directions. But it doesn’t just break apart like a grapefruit hitting a softball bat. Instead, the beam of light uses some of its energy to turn each air molecule into a sort of mini light bulb, which then shines light in all directions… So how does that work?

I'm On A Boat

Yeah… this stopped being funny, like, at least 5 years ago.

Going back again to things we learned in high school, we all known that light is a wave. So, to keep things simple, we’ll use an analogy with a boat on water. Imagine for a second that you’re on a boat in the middle of a perfectly calm sea. Out of nowhere, a massive wave comes and lifts your boat up 10 feet in the air, then drops it back down to its previous position. The massive wave has gone by, but your boat is still left rocking back and forth from the trauma. As it rocks back and forth in the water, it creates much smaller waves which flow away from it in all directions.

This is basically how Rayleigh scattering works, only on a much smaller scale and with electromagnetism. A light wave (like the big wave in our water example) comes and disturbs an air molecule (our boat), leaving it moving around. Most of the wave rushes on by in the same direction it was already going, but now the molecule is left gyrating around, creating much smaller light waves that emanate out in every direction.

Making colors

OK. So Rayleigh scattering redirects light or something. What does that have to do with the color of the sky and moon? Everything actually. Because of some very complicated physics, light waves with lower lengths (like blues) are scattered much more easily than light waves with longer lengths (like reds).

When a beam of light enters the atmosphere, it begins to scatter as it runs into air molecules on its way down to earth. All of the different colors of light scatter, but the blues scatter much more readily than the greens, yellows, and reds. This is why the sky is usually blue.

OK, but why are sunsets orange and red, and what does this have to do with the moon?

Let’s do a bit of a thought experiment. Let’s imagine a beam of light that comes from the sun in a straight line, enters the earth’s atmosphere, and leaves the atmosphere on the other side without ever running into the ground. For the sake of example (I have no idea if these distances are right), we’ll imagine our light beam is entering the atmosphere around Los Angeles, getting to its lowest point around Dallas, and leaving the atmosphere around Atlanta. Also, let’s pretend it’s 5 PM in LA, so it’s already starting to get dark out on the East Coast.

Light Diagram

Our friendly beam of light enters the atmosphere above LA, where it immediately starts running into air molecules and scattering. The blue light scatters much more easily than the other colors, so a lot of the blue light gets scattered towards the ground, turning the sky over LA a perfect shade of blue.

Our beam gets to Dallas, where the sun is starting to set. By the time it’s gotten here, it’s lost a lot of its blue light, which has been scattered to the ground all through California, Arizona, and New Mexico (as well as sent back out to space). So, the remaining portion of our beam of light contains much more orange and red than it does blue. As the remaining portions of the light scatter over Dallas, they turn the sky a brilliant orange/red – a beautiful sunset.

Finally, our beam gets above Atlanta, where it’s already getting dark. The last bits of twilight are clinging to the sky, but for the most part, it’s night outside. Our beam has been very effectively filtered by the atmosphere at this point – the blue light is almost entirely gone, and what remains is just a few hints of red that are still sneaking through. Hardly anything is being scattered towards the ground, and the sun has already dipped below the horizon, so the sky remains black.

Oh yeah, this question was about the moon…

So, what does all this have to do with an eclipse? Well, now we can finally come back to it. Let’s take the thought experiment that we just did, and let’s put the moon directly behind the earth, where it will be in the path of our little beam of light. Our light turns the sky in LA blue, runs out of blue and turns the sky in Dallas orange/red, becomes an even deeper shade of red, exits the atmosphere around Atlanta… and then runs smack into the moon! And that’s why lunar eclipses glow red – the light that’s hitting them has passed through the earth’s atmosphere, where all the blues and other colors have been scattered out so only the red remains.

But wait, there’s more!

So, there’s only one other little complication that I should mention. Light doesn’t really pass straight through the earth’s atmosphere. Instead, it “refracts” or gets bent, as it passes through, much like sunlight passing into a magnifying glass and getting bent so it all focuses on one tiny little area. Thus, when the earth is between the sun and the moon, sunlight coming straight from the sun runs into the earth before it could ever get to the moon. The red beams of light that end up hitting the moon are actually curving through the earth’s atmosphere and getting focused behind it, sort of like a magnifying glass. Cool!

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