A bright ring around the Sun or Moon formed by ice crystals is an atmospheric halo.
Halos occur when light passes through or reflects from ice crystals in high, cold clouds such as cirrus and cirrostratus. The crystals act as tiny prisms or mirrors, redirecting light into recognizable arcs and rings. The common 22-degree halo appears as a circular ring at an angular distance of about 22 degrees from the light source.
Sun halos are often visible as pale white rings, while moon halos may be easier to notice because the dark sky provides stronger contrast. Their appearance can signal high cloud cover ahead of a weather system, but a halo alone does not reliably predict rain.
A halo differs from a corona, which is a much smaller colored ring produced mainly by diffraction around the Sun or Moon. Rainbows form from liquid water droplets and are usually seen opposite the Sun.