The ring around the moon is the glow, or halo, which surrounds any light visible in intense darkness. Technically the ring is known as the moon's albedo.
It is faint because of the roughness of the moon's surface. This reduces the light reflected by the moon to slightly less than 0.1% of the amount it receives from the sun and other stars.
The result is that the moon's reflected light as seen on earth amounts to only half a millionth of direct sunlight. Its faint, mysterious and gauzy halo was considered by ancient peoples to be like the illumination round the heads of their gods.