In
atmospheric optics, a
corona is a series of colored rings that appears around a luminous source, typically the
sun or
moon, resulting from the scattering of light by particles of relatively uniform size, typically in the range of 20 µm in diameter. If the particles are too big or not uniform enough, or if the layer of particles is too thick the corona will degenerate into a hazy white disc. In the case of the sun or moon, these particles are usually water droplets or ice needles in thin
altostratus,
altocumulus, and sometimes
cirrostratus clouds, although a corona can also be generated on a small scale using terrestrial light sources and any of a wide variety of particulate matter.
The classic corona is rather small, with an angular diameter of less than 5°, as compared to the much larger phenomenon known as a halo. The corona typically consists of a bright aureole, bluish in the center and brownish on the edges, surrounded by a series of outer rings ranging from blue on the inside to red on the outer edge, often passing through green and yellow in the middle. It is these outer rings that are most properly termed the "corona", although term is often appled the entire phenomenon.
Coronae are easiest to see around the moon, because the moon is not too bright to look at, but occur just as commonly around the sun, where they are much brighter and potentially more spectacular. A simple way to observe a corona is to block out the sun with your hand or other object, leaving part of the corona in view. Otherwise, special goggles or other viewing apparatus may be used. The problem is essentially the same as trying to view a solar eclipse.
To view an image of a corona, see http://www.atoptics.co.uk/droplets/corona.htm