Viewing a Solar Eclipse the Natural Way

There was a partial eclipse of the sun when I was living in California in the early 90’s, and I noticed that during it the sidewalk was covered with crescent-shaped spots of light. Palm trees were making natural pin-hole cameras: the long leaves high overhead had a lot of small gaps between them, and they were so far from the ground they counted as “small” by the standards of geometric optics.

Pin-hole cameras work by restricting the path light from any point on the surface of the sun can take to reach the viewing screen, which can be any flat surface, although a bit of white paper works best.

Pinhole camera geometry

Pinhole camera geometry

The sun spans about 1/4 of a degree of sky. To get a good image of it the pinhole needs to have a considerably smaller angular diameter than that when seen from the screen. So if the screen is 1 m away from the pinhole card, the pinhole has to be to be quite a bit less than 0.8 cm in diameter. So a hole that is a millimetre or so will work fine, and let a reasonable amount of light through.

I used a heavy card notebook back and punched a hole in it with drill bit I had lying around. A sharp knife would do just as well. Careful you don’t cut yourself!

Heavy card from back of notebook with 1/8″ hole in it.

The important thing is that the card doesn’t let any light through. The amount of light getting through a 2 mm hole means that the darker the shadow, the better.

In action, it is useful to have a small notebook or the like to cast the image onto. The notebook makes it easy to get the angle right. Projecting on the ground is OK, but unless the sun is near the zenith the angle will make the image elliptical, not round.

But what about the natural way?

All those round spots of light are images of the sun cast on the sidewalk by gaps between the leaves high above. They aren’t round because the gaps are round: the shape of the pin-hole doesn’t matter if it is far enough away. What matters is the shape of the sun, which is round. When the moon gets in the way, those spots of light will have a bite taken out of them.

It’s interesting to think that this evidence of geometric optics was around for tens of thousands of years before anyone noticed it, and in the three hundred years since we’ve learned everything from biochemistry to plate tectonics. It’s almost as if looking really carefully at the world can teach us about how it works. It certainly isn’t the case that we are smarter or in any way better people than our ancestors. We are just lucky to be alive now, when generations of people have been free to look closely at the world and report what they see, as opposed to what the ideological hegemony of the day insists they must be seeing.

The quality of images varies depending on how big the leaf-hole is and how far away it is. Northern trees don’t work quite as well as palms, but are good enough for going on with. If the hole is too big, the image will be bright, but blurry:

If the hole is too small, the image will be dim and likely crossed by branches or other obstructions:

But with a little trial and error, an image can be found that’s just right:

Happy viewing, and remember: never look at the sun with your naked eyes! You won’t feel any pain while doing it, but even a few seconds of viewing the sun directly will damage your eyes, and like a sunburn the damage will develop a while later, and you will go blind.

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