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The name of this luminous ring is the lunar halo: an optical phenomenon that happens when the moon’s light passes through tiny ice crystals suspended in the atmosphere. The result is a ring of light with an area up to 44 times larger than that of the Earth’s satellite on full moon days.
The phenomenon takes place in the troposphere, at about 17 kilometers of altitude, thanks to the ice crystals that form cirrus clouds. When the lunar light passes through this layer of clouds, there is a refraction, that is, it can change direction. That’s what forms the halo in the sky — something similar happens when you dip a straw into a glass and the image above and below the liquid are mismatched.
The circular shape of the halo is the result of the hexagonal structure of the crystals.
The rainbow is another example of a visual spectacle caused by the interaction between the light that enters the planet and elements in the atmosphere — the difference, in this case, is that the light source is the Sun and the refraction medium is suspended droplets.
Although not as colorful, the lunar halo is often used in popular meteorology: there are those who guarantee that a ring around the Moon is a sign of rain.
CONSULTANCY Samuel Rocha de Oliveira, from the Department of Mathematics and Statistics at Unicamp