Pinholes and eclipses

My friend Julie Laskaris showed me a picture of sunlight passing through leaves during a partial solar eclipse, something like this:

(Image from here.)

I remember seeing the same phenomenon during an eclipse I saw in Berkeley in 1991.

The crescent shapes occur because each little gap in the leaves acts like a pinhole camera, projecting an upside-down image of the partially-blocked Sun. If you’re going to see the eclipse in a couple of weeks, look out for this effect. You can even produce it for yourself by holding up your hands so that your fingers make small gaps that the sunlight can pass through.

I was looking around for explanations of this phenomenon, and I came across this NASA page. One thing I was very interested to learn from this page is that Aristotle noted this phenomenon:

In the fourth century BC, Aristotle was puzzled. “Why is it that when the sun passes through quadrilaterals, as for instance in wickerwork, it does not produce a figure rectangular in shape but circular?” he wrote. “Why is it that an eclipse of the sun, if one looks at it through a sieve or through leaves, such as a plane-tree or other broadleaved tree, or if one joins the fingers of one hand over the fingers of the other, the rays are crescent-shaped where they reach the earth?

I’ve read a lot of Aristotle’s writing on astronomical topics, and I’ve even inflicted him on my students from time to time, but I’d never encountered this. It turns out that it’s in the book Problems, which I’d never heard of and which some people seem to think might not be Aristotle at all.

Julie is a classicist, so it was quite fitting that her showing me this picture led me to learn something about Aristotle.

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Ted Bunn

I am an associate professor of physics at the University of Richmond. In addition to teaching a variety of undergraduate physics courses, I work on a variety of research projects in cosmology, the study of the origin, structure, and evolution of the Universe. University of Richmond undergraduates are involved in all aspects of this research. If you want to know more about my research, ask me!