When I say ‘like I’m 5’ I mean it, explain it for my dumb little brain please.
In: 3
(Truly ELI5)
The Harvest Moon is red when the moon rises during sunset.
> Why are sunsets red?
Sunlight bounces off particles in the atmosphere. When the sun is directly above you, sunlight doesn’t have to travel very far through the atmosphere to reach your eye, so the light wavelengths are shorter, making the sky blue; the shortest light wavelength.
When the sun sets, sunlight has to travel through a lot more atmosphere because of the curvature of the Earth. The light has to travel further, stretching the wavelength, making the sunset colours yellow, orange, and red.
> Why is the Harvest Moon red?
When the moon is above you, it’s blue, even if it’s a Harvest Moon. But when the moon specifically rises during a sunset, the blue-ish moonlight turns red (because of the wavelengths), making the moon red.
During the sunset the sky is orange right? Wrong it’s still blue, but the clouds are red. When sunlight travels through Earth’s atmosphere the blue light gets scattered around, this is the colour that we can see in the sky. When that sunlight is cone so is the colour of the sky, which is why we can see the stars and not just black. If only the blue light gets scattered where does red and green go? Well most of the green goes with the blue but a bit of it goes with the red. The red just goes on straight forward. Which is why a sunset looks red, that red light is hitting clouds.
So what does all that mean the harvest moon. Well that light doesn’t disappear it keeps going until it reaches the moon.
In the past I’ve submitted a comment to [a person who asked this question on this sub before](https://www.reddit.com/r/explainlikeimfive/comments/rmp273/eli5_why_does_the_moon_appear_to_be_yellow_at/) that I was quite happy with, and that OP seemed to be helped by it, so I’ll copy that comment here:
This may sound wildly tangential, but do you remember that one SpongeBob SquarePants episode where Mrs. Puff steals SpongeBob’s boat and there’s a gag where SpongeBob gets dragged through a field of giant cheese graters, whittling him down to almost nothing? It turns out, the Earth’s atmosphere more or less does this to light that passes through it.
White light from the Sun contains all colors mixed together. Rays of this light that pass through Earth’s atmosphere get ever so slightly “cheese grated” as they shine through. These “cheese graters” tend to more heavily affect the bluish components of that light more than any other color, though. Thus, the “grated” rays of the light that make it all the way through the atmosphere to you will be slightly blue depleted, making them appear yellowish to you. Or orangish or reddish, depending on how badly the rays got “grated” on their way to you.
Rays that have to travel through more air spend more time being “grated”. So things that shine directly overhead aren’t very strongly affected, since the light path to you takes the minimum distance trip possible through the atmosphere. But stuff closer to the horizon is skimming through the atmosphere at a shallow angle, greatly increasing the amount of air it has to pass through to reach you. So things closer to the horizon get “grated” much more aggressively. This is ultimately what makes sunrises and sunsets so pretty with their brilliant orangish-red hues. And, by the same mechanism, it also makes moonrises and moonsets appear reddish. Thus, harvest moons.
An interesting other case where the moon can appear reddish is in the case of a lunar eclipse (AKA a “blood moon”). The mechanism for this is the same, but there’s a key difference. A harvest moon only appears red to you when it’s low in the sky. The moon is actually always whitish, it’s just the atmosphere between you and the moon distorting that white reflection as described above. But in a lunar eclipse, the moon itself actually turns red. This is because the light at the edge of Earth’s shadow clips past the edge of Earth, thus passing through part of the atmosphere in the process and getting “grated” as it passes through, turning it reddish before it even gets to the Moon to be reflected. The Earth is, in a way, projecting a sunset onto the moon, making the whole thing appear red around the edge of the shadow.
By the way, the blue “shavings” that get grated away in this “grating” process (known as [Rayleigh scattering](https://en.wikipedia.org/wiki/Rayleigh_scattering)) don’t just disappear. They get cast off in random directions. By pure chance, some of these may stray into your eyeballs when you look up at the sky. This is why the sky is blue during the day.
Though, if the moon was high in the sky when it looked off-white, this wouldn’t really apply. At least, not very strongly. A more mundane explanation for a yellowish moon could simply be dust or smoke scattered high in the sky, which can have a similar effect to Rayleigh scattering. This could be caused by wildfires, volcanic eruptions, or even just farmers grinding up plants in their combine harvesters and kicking up a ton of dust from the harvest.
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