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Why The Moon Turns Red During A Total Lunar Eclipse

- A blood moon lunar eclipse wasn't always something

to look forward to.

When the moon turned red thousands

of years ago, the ancient Mayans

and Mesopotamians feared that something monstrous

and evil was eating the moon.

They would shout at the night sky

to try and fend off the ravenous beasts

and since the average lunar eclipse lasts

around 100 minutes, and the moon returns

to normal afterward, they were probably convinced

that their whooping and howling actually worked.

We know now that the moon doesn't need our protection,

but why does it turn red in the first place?

(peaceful music)

Whenever you look up at a full moon,

you're seeing sunlight that's reflected

off the lunar surface,

so if something were to block that sunlight

say, the earth, then in theory,

the moon should disappear from view

but during a total lunar eclipse

when the moon passes through the earth's shadow,

we get a red moon, not a vanishing one.

So what's going on?

To figure it out, let's take a quick trip

to the lunar surface.

This is a NASA simulation of what the earth looks like

during a total lunar eclipse.

Notice the red ring around our planet.

Everywhere you see that ring is either a sunrise

or a sunset, and while it's true that no direct

sunlight is reaching the lunar surface

at this moment, earth's atmosphere is bending

the red wavelengths of light around the planet,

so that redness you see during a blood moon eclipse

is a combination of light from every sunrise

and sunset on earth, all happening at once.

So the moon appears red for the same reason

that sunrises and sunsets on earth are red

because of a phenomenon called Rayleigh Scattering.

Named after the British physicist, John William Strutt,

also known as Lord Rayleigh, who discovered it

in the late 19th century.

It describes how different colors of sunlight interact

with the earth's atmosphere.

Look at the sky during daytime, for example.

It appears blue because air molecules

in earth's atmosphere scatter blue light more easily

than red, but during sunrise and sunset,

the light travels through more of earth's atmosphere

before reaching your eye which has two consequences.

First, it means more overall sunlight is scattered,

making the sun appear dimmer.

That's why you can easily gaze upon the sun at sunset

compared to at high noon.

And secondly, more scattering means more blue light is

scattered away, leaving the redder wavelengths behind.

Similarly, the ring around earth

during a total lunar eclipse is red

because the sunlight travels through a long stretch

of earth's atmosphere, from one end

of the planet to the other.

So rather than fear a blood moon like the ancient Mayans

and Mesopotamians, why not think of it as a romantic moment?

After all, it's the only time when you can see the sunrise

and sunset simultaneously.

(peaceful music)