Clean Air

Images by Elmar Schmidt 12000m (39,000ft) over Southern Germany on June 20, 2011 on his return from Namibia to research lunar eclipse darkness. ©Elmar Schmidt, shown with permission

The image is noteworthy for what is not visible.

The sun is eclipsed by the wing of the Air Namibia Airbus. Its presence is only weakly apparent by rimes of light reflected and/or diffracted by the curved wing surface.

We almost never see this sight from lower down in our dusty, water droplet and aerosol laden troposphere. The sun is usually surrounded by an extremely bright aureole.


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Here the sun is not eclipsed by the wing and its light dazzles the camera CCD and scatters from window scratches.
The aureole is normally a few degrees  in diameter, its apparent size depending on the size and concentration of atmospheric dust and aerosol.    

The scattering particles diffract incident light to direct it forward but slightly deviated to give the glow around the sun.    Air molecules themselves scatter light (hence our blue sky) and thus an aureole is never totally absent.

Diffraction by nearly monosized particles would give a sharply defined aureole surrounded by coloured rings – a corona.

High concentrations of stratospheric dust or sulfate aerosol from volcanic eruption produce a strong and large aureole known as a Bishops Ring, a milky glow fading at the edge to straw and reddish colours.