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What Causes a Blood Moon?

All blood-moons are red, but only in the literal sense. They appear red, that’s why we call them blood-moons in the first place. 

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The Moon actually doesn’t become red, however. It’s only because of the earth’s atmosphere’s influence on the sun’s light that we see it red.

When white light from the sun shines on the moon, its surface reflects all the light, making it appear white. During a lunar eclipse, no direct light from the sun reaches the moon.

As we know, a lunar eclipse occurs when the earth comes in between the moon and the sun. In this orientation, the earth casts a shadow on the moon. And depending on the actual position of the moon behind the earth, there is a possibility that the moon is completely within the shadow of the earth. In this scenario, the moon should be invisible in the night sky, because no light from the sun directly reaches it [1].

But then why does it appear red?

The earth’s atmosphere extends more than 80 km above the earth’s surface. When sunlight reaches earth, it is an obvious obstacle in the path to the moon. However, sunlight “leaks” around the atmosphere and reaches the moon.

The light from the sun undergoes scattering [2] and the colors of lower wavelength (blue, green, etc.) are “filtered” out. What remains is the colors of higher wavelengths, i.e., orange and red. This red light illuminates the moon and since its surface reflects all the light it receives, the moon appears red to us. We call this a blood-moon.

[1] There is a scale that is used for the brightness of the moon during a lunar eclipse. It’s called the Danjon scale. This is significant for solar cycles.
[2] The phenomenon of scattering mentioned here is Rayleigh scattering. This is the same phenomenon which make the sky seem blue.

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