# Coronas

A circle of light quite distinct from the 22° halo can sometimes be seen around the sun or moon if there are thin clouds composed of water droplets or ice crystals of nearly uniform size. This circle is smaller, on the order of 10°, and is caused by diffraction of light by the small particles. The appearance is often that of alternating blue-green and red circles. The diffraction by a tiny opaque barrier shows the same pattern as that of an aperture of the same size and shape in an otherwise opaque screen (Babinet's principle).

The moon corona image above was taken on March 19, 2005 on a night when thin clouds were moving swiftly across the sky. The colors are subtle, but you can see a blue tint to the corona close to the moon and a reddening further out. This indicates diffraction in tiny water droplets. The shorter wavelength blue light produces diffraction maxima closer to the sun.

 Corona from fogged eyeglasses
 What size droplet would it take to diffract light at 22°?
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# Glories

A rainbow band around the shadow of your aircraft seen on a cloud below is an example of a glory. For more distant clouds, the plane's shadow would be smaller, but the glory would be the same angular size if the water droplets were the same size.

 The phenomenon is one of diffraction, with smaller droplets giving larger glories. The nature of the diffraction is similar to aperture diffraction, but is diffraction from the water droplets in the cloud. A small opaque circular scatterer gives a diffraction pattern like that of an aperture of the same size. Mie scattering is thought to be the mechanism controlling the intensity of light scattered back to the viewer. However, mathematical modeling of the details is quite complex.

This photo of a glory was taken by Frank Starmer and is used by permission with all rights reserved to Frank. It is remarkable in that the second order diffraction is visible and a hint of third order.

Below are links to posted examples of glories:

 NASA, glory over South Africa Glory from Earth Matters Glory by Bryan Mundt Glory from helicopter, Mauna Loa, Wiki
 Glory on a nearby cloud
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# Glory on Close Cloud

 This glory was observed on an approach to Anchorage through clouds which were sometimes very close, sometimes distant. At this point the aircraft shadow is larger than the glory, but as the clouds became more distant, the shadow was smaller while the glory stayed the same size. Remarkable telescoping of the shadow size occurred within the glory.
 Examples of glories on close and distant clouds
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# Iridescent Clouds

The corona around the sun is often hard to see because it is close to the sun and masked by the extreme brilliance of the sun. When the same diffraction effects occur in clouds some distance from the sun which are composed of tiny uniform droplets, colorful bands or "iridescence" can be seen. The example below right shows the orientation of these iridescent clouds to the Sun, which is at the edge of the building.

Below are links to posted examples of iridescent clouds:

 Iridescent alto-cumulus, Wiki Iridescent cloud edge, Wiki Iridescent cloud, Finland Iridescent cloud Ober Gatlinburg
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# Heiligenschein

Examining dew heiligenschein a bit more closely, following the discussion of Lynch and Livingston, tiny dewdrops act as essentially spherical lenses, focusing the light at a distance of about 0.7 x the drop radius behind the drop. If that concentrated light strikes a leaf or blade of grass, part of it is retroreflected through the droplet, proceeding backward toward our eyes. The retroreflection produces the appearance of a bright diffuse glow around the viewer's head. It is spread over a few degrees by the fact that the droplet "lenses" have large amount of aberration and don't send their light back at exactly 180° .

Below are links to posted examples of heiligenschein:

 Heiligenschein from grass Heiligenschein around a balloon Heiligenschein Buz Aldrin on Moon Heilgenschein in wheat field
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Reference
Lynch and Livingston
Sec 4.12

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# Specter of the Brocken

Below are links to posted examples of the Specter of the Brocken:

 English Lake District Spectre of the Brocken Spectre of the Brocken, Canary Islands Skiddaw Lesser Man, English Lake District
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