Sunday, August 9, 2009

Crepuscular and anti-crepuscular rays

Crepuscular rays are the beams of light we see near sunrise or sunset when clouds or mountaintops are well-positioned to cast shadows or provide gaps that beams of light shine through. (Note that the beam radiating from the above photo at about 1:00 shows a light ray that is then superceded by a shadow.) I have read that there are anti-crepuscular rays too. These are the rays you'd see if you looked in the opposite direction. I haven't noticed any till recently, when I saw these:

These were barely discernable, so below I enhanced the contrast to make them obvious. I made no other changes:

The illusion of separating or converging beams is a great reminder of how perspective affects perception. The rays and shadows are parallel, just as with meteors in a meteor shower, but they give the impression of radiating from a single point.
As much as a look at this crepuscular ray, I can't convince myself into seeing it as a horizontal beam:

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