What is the circle outside the moon?

Halo is an optical phenomenon in nature. It is a colored aperture formed by the refraction of ice crystals when the light of the sun or the moon passes through high and thin white clouds (cirrus, cirrostratus or cirrocumulus), and the color sequence is ultraviolet red. The aperture around the sun is called solar halo, and the aperture around the moon is called lunar halo. "Lunar Halo" is formed by light passing through the cirrostratus at high altitude and being refracted by cold crystals, so that the seven-color composite light is dispersed around the moon into infrared purple halo or arc. Lunar halo is an aperture formed by the refraction of moonlight by ice crystals (solids formed by freezing tiny water particles) at high altitude. Both solar and lunar halos are refracted by ice crystals, just like the principle of rainbow. The appearance of solar halo or lunar halo often indicates that the weather will change to a certain extent. The appearance of lunar halo often indicates that the weather will change to some extent. Generally speaking, a solar halo means it may rain, while more lunar halos mean it will be windy. So there is a folk proverb: "The sun is dizzy in the middle of the night and the moon is dizzy at noon." In ancient times, the lunar halo was called the hairy moon! "