Eclipse Shadows

Atmospheric optics adage - When the sky has something interesting, check also in the opposite direction.

Martha Deacon did that during the solar eclipse of 20th May '12. Shadows were strangely shaped with fingers apparently displaying long curved nails.

©Martha Deacon,  shown with permission.

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The sun is a disk rather than a point light source. Point source shadows are always sharp, sun-disk shadows change form with distance to the surface where they display. Close-up the shadow is the object's silhouette. Further away the dark shadow (umbra) develops a lighter edge (penumbra). Further still and no dark umbra remains. We have merely a penumbral blur. Place the eye anywhere in this penumbra and part of the sun will be visible.

Top left is a simulation of a penumbral shadow. We usually do not notice them.

Replace the solar disk by the thin crescent of an eclipse. Shadows become unfamiliar. In some directions the sun acts more like a point source and shadows become that much sharper and darker in some directions. The finger in the simulation is still recognisable.

The crescent can be regarded as a arc of point sources. Each casts its own shadow. The overall shadow acquires crescent-like features. At left the finger tip has become a crescent shaped shadow and the hand claw-like.

The hands in the images have taken on crescent-like qualities.