Why is the rare moon halo perfectly round?

Submitted by Supreme_MangoKat t3_109vr31 in askscience

I understand that moon halos are caused by the light bouncing off tiny ice crystals in the atmosphere, but why does the halo appear perfectly round? Is the circle caused by the Earth bouncing the light back towards the moon? Is the size of the dark center the size of the Earth compared to the moon? Can a Moon Halo's diameter be measured?

I saw my very first moon halo tonight and now I'm fascinated!

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Shadowlance23 t1_j419k70 wrote

I assume you mean this effect: https://en.m.wikipedia.org/wiki/22%C2%B0_halo

It's due to the shape of the ice crystals. They reflect light to you at about a 22 degree angle relative to your point of view. So it's not the light reflecting back, rather it's being bent on its way to your eyes. At angles below 22 degrees, the light doesn't refract so the sky appears darker inside the ring.

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SamQuan236 t1_j41x85j wrote

You are seeing the effects of polycrystalline diffraction. If you were to pass the light through a single crystal, you would see something like a spot or line pattern (kikuchi bands).

Because the orientation of the ice crystals are random in the atmosphere, you get a random orientation during the light scattering. summing up all these many spot patterns from each individual scattering at random rotations gives you a ring.

You could emulate the process with a stencil of dots at whatever pattern you like. Rotate it randomly and then draw in the dots from the stencil. If you do this enough, you will get a ring.

Technically there are many processes going on. Single scattering (which produces the dot pattrn) requires very small amounts of material. As the material (here ice) gets thicker, you will see lines forming (multiple scattering).

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aspheric_cow t1_j42utzm wrote

Ice crystals refract light by ~22 deg. There are ice crystals everywere doing this. But you only see the ones that refract light towards you, which are the ones ~22 deg away from the Moon from your vantage point.

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Puppy-Zwolle t1_j43lc6s wrote

All excellent explanations.

In short. There is no halo to measure. It's 'just' refraction that makes you see a halo. Same as the rainbow. It's a trick of the light. 'Smoke and mirrors' but done by nature.

Here's a video. Not about halos but about rainbows but the explanation is the same.

https://m.youtube.com/watch?v=6PrrPOvEU6w

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superheavydeathmetal t1_j47w4y9 wrote

Because the moon is perfectly* round, and the halo is created from the light being reflected (then refracted, as others have said) from the moon.

If you were able to see a halo around, say Haumea, for instance, the halo would be very oblong.

*The moon is, of course, not perfectly round. But, it is close enough to make the halo appear perfectly round.

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