You’ve surely heard by now that the moon will pass between Earth and the sun on August 21, creating a total solar eclipse that will cast a shadow over much of the US. Jimmy Carter was president the last time this happened, so you definitely don’t want to miss it.
The best way to observe this astronomical event is to be somewhere in the path of the totality that will experience total darkness in the middle of the day. If you can’t do this, you have two other options: Buy a pair of solar glasses or make a pinhole.
Solar glasses are super easy to find, but you definitely want to make sure you aren’t getting bogus or counterfeit glasses, because they’ll damage your eyes. Your mother probably told you to never look at the sun because you’ll go blind. You may not go blind, but you’ll definitely regret it.
As for me, I will use a pinhole because it’s fun to make stuff. A pinhole is just a large screen with a tiny hole that projects an image of the sun onto the ground, the wall, whatever. I used one the last time I saw a solar eclipse—way back in 1979—because it was the only option.
So how does a pinhole create an image? This diagram shows the basic idea:
Let me explain with a banana. Light from the sun hits the top of the banana and reflects in all directions. Some of that light enters your eye. Light also hits the bottom of the banana and reflects in all directions—and again, some of it enters your eye. You eye focuses all of this light so that you see the banana.
Now for the pinhole. Again, light hits the top and bottom of the banana, and this light reflects in all directions. Some rays of light pass through the pinhole and strike a screen beyond it. Once the light rays reflect off the screen, it creates an image of the banana.
You might think the pinhole acts a lens. Kind of, but not really. Yes, it forms an image on a screen just like a converging lens (the type you find in a telescope or microscope). The pinhole image offers two advantages over a converging lens. First, anyone can make one. You don’t need a piece of glass (or plastic) and you don’t have to worry about the shape. It’s just a hole. Second, the resulting image doesn’t have a focal point. No matter where you put the screen, you see a clear image. The big disadvantage of a pinhole is light. Since the hole is very small, it only lets a small amount of light through.
Light isn’t a problem with an eclipse, of course, which makes a pinhole the perfect way to observe one. But the pinhole can be used to view things other than a very bright sun. Really, any object (like a banana) would form an image on the other side of the pinhole. However, these images are really quite dim since the object isn’t super bright. You could fix this by making sure the pinhole image is formed in a very dark room—which is exactly the way it used to be accomplished. In fact, the word “camera” comes from “camera obscura” which literally means “dark room”. More than 500 years ago, you could get inside a dark room with a pinhole in it that would have an image projected onto a wall. It would then be possible to paint over that image. Of course much later this room would be replaced by a box with a lens instead of a pinhole and film instead of an artist. However, we still call these things “cameras” which means “room”.
Now back to the eclipse. There are two ways you can use a pinhole to view the eclipse. The first method is pretty simple. Just get a large piece of cardboard and cut a hole in it. Any size, it doesn’t matter. And don’t worry about making it perfectly round or totally square. Tape a piece of aluminum foil over the hole and poke a tiny hole it in. This part is important, because you don’t want to make too big a hole. A smaller hole emits less light, but creates a shaper image. A tack or pin works nicely. Done? Great. Go outside and hold your pinhole perpendicular to the sun. You should see an image on the ground, like this:
See? Easy! I didn’t even make a screen. I just used the sidewalk. You can make a screen with another piece of cardboard if you want. Notice that the image of the sun is quite small. You can make it bigger by moving the pinhole away from the screen, but that makes the image dimmer. Experiment and you’ll find a nice compromise between size and brightness.
If you feel more ambitious, you can a more elaborate pinhole viewer. It’s basically a pinhole camera without film. Start with an empty cardboard box. I found one that is white inside, which makes the image easier to see. If you have a plain brown box, put some white paper in it if you like.
It helps to tape up all the edges to keep light from leaking into the box. Punch a hole in one end so you can peek inside the box to see the image, and do the pinprick-in-foil thing over another hole to let the light in. To use the pinhole, look into the box (cover the opening with your face as much as possible) and point the hole end of the box towards the sun so that your face will be looking away from the sun. Inside the pinhole camera on the back side, you should see an image of the sun.
Here’s what it looks like.
Look carefully and you can see clouds, too. Anything that’s bright will show up in your pinhole box. During the eclipse, you will see the sun as it is partially blocked by the moon. Block out enough extra light and you can see nearby people and other things. Of course, it will be all upside down, because that’s just how pinholes work. Of course, when you’re looking at the sun, it doesn’t matter that it’s upside down. It’s still the sun.