Why is the Moon Red?

Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

It may be scary or really cool, depending on how you take it, but a red Moon does not spell doom. Find out why it's really red whether it's near the horizon or overhead.

A Red Moon

It might be slightly alarming. You go out at night and look up at the sky and bam! The Moon is this crazy deep red color. What's going on? Is the world coming to an end? Do you need your vision checked? Did aliens spill a bunch of paint on the Moon? Nope, everything is probably ok, actually.

Find out why as we explore the reasons why the Moon may be red.

The Atmosphere

One big reason for why the Moon may be red tonight is Earth's atmosphere, which is a gaseous layer surrounding our planet. As you probably already know, our atmosphere helps to filter out a lot of stuff. For instance, you may have heard that it helps filter out U.V. radiation, the radiation that causes skin cancer when we tan. Well, U.V. radiation is actually a type of light. And just like this kind of light can be filtered out, so can the only form of electromagnetic radiation (light) we can see with our naked eyes, that of visible light.

Our atmosphere scatters and filters out short wavelengths of visible light, such as blue. However, longer wavelengths of light, like the color red, can penetrate the atmosphere with relative ease. The thicker the atmosphere, the more the shorter wavelengths of light are scattered. So if you came out that night and saw the Moon on the horizon and it was really red, you now know why. The atmosphere is thicker at the horizon than directly above you. Thus, when the Moon is directly overhead, the light it reflects down to Earth from the sun has relatively little atmosphere to pass through. This means more wavelengths of light reach your eyes and the Moon appears pretty much white.

Nearer the horizon, the light the Moon reflects has to pass through a thicker layer of our planet's atmosphere. This means relatively more red wavelengths of light reach our eyes in comparison to other shorter wavelengths of light. This means that the Moon appears red to our eyes even though, in outer space, it's still as white as ever.

Note how in this image, as the moon nears the horizon it becomes redder and redder in color as the light it reflects has to pass through ever more of Earths atmosphere.


But maybe you're screaming really loudly now: 'NO! The Moon was far above the horizon and it was still red! How could that be based on what you just said?' Fear not, there is an explanation. An alternative reason for why the Moon was red in this case, especially if you saw that it was red while it was far above the horizon, may have to do with air pollution. Particles in the air from pollution caused by industrial emissions or forest fires essentially create an extra layer to the atmosphere. These particles, like the normal atmosphere, scatter light in the same way as the Earth's atmosphere. This means that redder wavelengths of light get through better than bluer ones do. Ergo, the Moon still appears red even when high overhead.

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