Last night, I saw a ring around the moon. A ring around the moon is also called a lunar halo. What causes this beautiful sight? It is not magic, or fairies dancing in a circle. Moon halos are caused by ice crystals in thin cirrus or cirrostratus clouds, high in the atmosphere.
According to the Farmers’ Almanac, weather lore says a lunar halo is the precursor of unsettled weather, especially during the winter months. This is often true, as cirrus and cirrostratus clouds usually precede rain and storm systems.
According to EarthSky, the halo effect is a result of refraction (splitting of light) and reflection from thousands of tiny ice crystals. As light passes through these hexagon-shaped ice crystals, it is bent at a 22 degree angle, creating a halo 22 degrees in radius (or 44 degrees in diameter). The crystals have to be oriented and positioned with respect to your eye, in order for the halo to appear.
A prism effect of light passing through these six-sided ice crystals separates the light into its various colors, resulting in a halo tinged with very pale rainbow colors with red on the inside and blue on the outside. A lunar halo is similar to a rainbow produced by sunlight and rain falling between your eyes and the sun.
As the poet said, “lovely snowflakes, they fall nowhere else,” so too, the sight of the ring around the moon is unique to your eyes, here and now.
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