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That sounds very much like a 22° halo. It's not an astrophysical phenomenon; it results from the refraction of light by ice crystals in Earth's atmosphere. If this is what it is, it should be a fuzzy but regular circle centered on the Moon, with a radius of about 22 degrees. If you hold your fist out at arm's length with the thumb extended, the angular ...


I'm guessing what you see is the moonlight being scattered by the hexagonal ice crystals in cirrus or cirrostratus clouds, which lie at very high altitudes, 5-6 km and above. The light is scattered by roughly 22º, and because of the slight wavelength dependence, the halo actually has rainbow-like colors, although often they are so faint that you just ...


It may shave off 1 or 2 magnitudes, but it depends on many factors: nature of the object (star, nebula, galaxy), altitude (higher altitude has less light scatter), transparency, etc. There's no One Single Answer To Rule Them All. Light pollution from artificial sources has a greater impact in most cases.

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