An Eclipse Shadow Experiment


My favorite thing about eclipses is how all the shadows through trees become little crescents instead of round blurring spots.

This is related to why shadows from things farther away are fuzzier occurs, which is because the sun is a disk instead of a point. The fuzzy part corresponds to the area where you could see part of the sun. The fuzzy zone gets bigger farther from the object for the same reason that things look smaller in the distance.

In an eclipse, however, the sun casts blurry crescents instead of blurry circles. This is because each circle was a (doubly-mirrored) image of the sun in the sky, and during an eclipse, the sun looks like a fat crescent. When the light goes through a tiny hole, you get a clear but faint image (as in a pinhole camera). When the light goes through a larger hole, you get a brighter but fuzzier image. It's really lots of faint images on top of each other, but not lined up quite right. When you have a really big hole (or the open sky), there's images everywhere and they all balance out and it just looks like sunlight.

On the eclipse a few days ago, I tried an experiment. I got a decorative "crystal" (i.e. ball of glass with lots of facets) that casts rainbows all over (reflection and refraction are both involved). Then I took it out in the eclipse light and got my mom to photograph the rainbows (I was holding the crystal at the time). My hypothesis is that the rainbows should be formed by a smeared crescent (the shape of the sun) instead of a smeared circle.

I think that's what happened, but it's hard to tell. Especially since I have no idea what angle the crescent should be at. I think the right rainbow in the second image has the crescent arms pointing right, with the bottom one longer. However, I would appreciate opinions from other people.

Note: The rainbow image has significant colour enhancement/repair.

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