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Moon Halos.

Moon Halos. On nights with diffuse high clouds and a bright moon, a halo can appear about the moon. This results from light refracting (through an angle of 22 degrees) through hexagonal ice crystals in the atmosphere. The mathematics is as follows:.

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Moon Halos.

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  1. Moon Halos. On nights with diffuse high clouds and a bright moon, a halo can appear about the moon. This results from light refracting (through an angle of 22 degrees) through hexagonal ice crystals in the atmosphere. The mathematics is as follows:

  2. Hexagonal ice crystals look something like this.

  3. A side-on view of an ice crystal

  4. When light strikes the boundary between two different substances (air and ice) it refracts. Light ray Air-Ice boundary

  5. Snell’s Law is the physical relation governing refraction. It states that the angle of incidence and the angle of refraction are related by the formula The constant k depends on the nature of the two substances. For air-ice, it is about 1.31.

  6. Light enters the crystal, and then leaves it, making two refractions. θ1 θ2  θ3 θ4

  7. Here, the angle α represents the total angle the light bends. θ1 θ2  θ3 θ4

  8. The necessary relations needed to determine α are on the next slide.

  9. θ1 θ2  θ3 θ4

  10. We solve the relations on the next slide.

  11. “Technically speaking”, there are about a zillion ice crystals in the atmosphere all oriented randomly. The angle will take many different values.

  12. This means that the light will be scattered through many different angles, though there is a catch.

  13. If there are many different angles of incidence that yield approximately the same α, light will seem to build up at that angle.

  14. Light builds up most where the slope is smallest. This is 21.83929990 degrees.

  15. This is the classic 22˚ moon halo. Moonlight Diffuse clouds Apparent Halo 22˚

  16. Light inside the halo is refracted out in a cone that misses the observer, making this region less bright to the observer. Moonlight Diffuse clouds Apparent Halo

  17. Light outside the halo is refracted out in a cone that misses the observer, making this region less bright to the observer. Moonlight Diffuse clouds Apparent Halo

  18. Light on the halo is refracted out in a cone that the observer sees, making this region brighter to the observer. Moonlight Diffuse clouds Apparent Halo

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