Lunar Corona - Taken by Stefano De Rosa (site) at Villefranche, France a few hours after the full moon on on September 5, '09. The moon was 39░ high. ęStefano De Rosa, shown with permission..
Atmospheric
Optics

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A magnificent corona with three rings visible testifying to scattering by water droplets of very similar sizes.

A corona should be familiar in another context to telescope observers. A star seen through a steady atmosphere and enough magnification shows a small disk surrounded by shimmering rings.

Stars do not show their real disks except with large telescopes and specialised optics. The apparent disk visible through small telescopes when the air is steady is the 'Airy disk' and is produced by the same light diffraction process as the corona. Waves scattered by the edge of the telescope aperture recombine to form the Airy disk interference pattern. Waves scattered by the periphery of a cloud water droplet produce the same pattern, this time called a corona.

The telescope diffracton pattern is minute and needs magnification to see it because the larger the scattering body, the smaller is the angular size of its diffraction wake.