A corona is a series of alternating dark and bright rings of light surrounding the moon or less often, the sun. The name derives from the Latin corona = crown. It occurs when the moon is seen through a thin stratiform cloud that is comprised of tiny water droplets. Corona results from the diffraction of light around the droplets.

If cloud droplets are of uniform size, a corona is colored with blue-violet on the inside and red on the outside. These color rings may repeat themselves as fainting further from the moon. Corona is only a few degrees in radius, hence, is far smaller than a halo. A full corona shows an inner bluish-white disk with a brownish-red outer edge called aureole, and an outer set of rings ranging in colour from violet on the inside to red on the outside. Uniform drop size gives the purest colours, but when there is a wide range of sizes only the aureole may be visible. The radius of a corona is inverseley proportional to the size of the cloud droplets.