February - halos You are here: Home Photo gallery Atmospheric optics Diffraction phenomena Corona

A corona (not to be confused with the actual solar corona) is caused by diffraction of light by small cloud droplets, ice crystals, pollen or other particles. Due to the wave-nature of light, light is bend slightly after skimming the edge of a particle. By constructive and destructive interference of this light, diffracted over small angles by millions of small particles, colored rings can show up around the sun or the moon (and even the bright planets). The aureole is the first ring of the corona, consisting of a blue-white disk with the moon or sun at its center, bordered by a red-brown ring. The aureole is very common, when the sun or moon shines through a cloud deck. The actual corona, with sometimes many colored rings, is less common, because the cloud particles have to be of uniform size to allow for constructive interference of the light.

18 photos in gallery
Click images for large photos



















Observing tips

Viewing the corona around the moon is not difficult, but you should avoid looking directly at the sun at any time. For viewing the corona around the sun, you must obscure the sun by an object, like a tree or a building. Even better is to view the sun reflecting in a dark water surface.

Cloud types which may show corona rings well are lenticular clouds (from mountain waves, for example), or certain types of cirrocumulus and altocumulus. The cirrus and cirrocumulus that forms when there is a jet-stream over your location is ideal for corona.

Photography of corona is a little bit difficult. Since the corona rings are so close to the sun (or moon), the right exposure is very critical because the small area which the corona covers may differ up to 3 or 4 f-stops. The sun should be blocked all times, which can best be done by moving into the shadow of a tree (this looks most natural) or another natural structure. If those are not available, or a tree would block too much of the corona, use a traffic sign or a building. Meter the light with the sun area out of the frame, and expose according to the light level between the first and second corona ring.