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If you live in the US and missed the last total solar eclipse in 2017, good news! You're about to get another chance. There will be a total solar eclipse passing through Texas and the Midwest states on April 8. Remember that in a solar eclipse, the moon's shadow falls on the Earth. If you're in this shadow, it's going to look really weird. But also awesome.
Even if you're not in the path of totality, you can still see something. All of the continental states will get at least a partial eclipse. (Check out the map here at NASA's eclipse page.) And do I need to tell you this? Never look at the sun without special glasses, even when it's mostly blocked by the moon. You may still be able to get some safe solar viewers before the big event.
But there's another way to view the solar eclipse without glasses: using a pinhole projector. It's super simple to make and easy to use. All you need is something flat like a piece of cardboard. Then you poke a hole in it with a pin. That's pretty much it. When light from the sun passes through the hole, it will project an image onto some flat surface (like a sidewalk).
If you did this on a normal day you'd see a circular dot of light. You might think that's because the hole is round. But during the eclipse you will see a crescent shape caused by the moon passing in front of the sun. It's both awesome and safe for your eyes.
Actually, you don't even need to make a pinhole viewer—they already exist all around us. If you stand under a tree, the small spaces between the leaves will act as pinholes to project a bunch of little crescent images. Here's a picture I took during the 2017 eclipse:
Fun With Pinholes
Just for fun, here's a question for you. Most pinholes are round (because pins have cylindrical shafts). But what if you replaced the circular hole with a square one? What shape would a round sun project onto the ground? Would it be a circle? Would it be a square? Or maybe it would be a squircle! What about a triangular hole? What would happen then?
I actually have a card from PUNCH (Polarimeter to Unify the Corona and Heliosphere) that demonstrates this with three holes—circular, triangular and square. Check it out.
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I'll show you what happens, but first try to guess. And while you're thinking about that, let's talk about pinholes. Consider the absolute simplest case: a tiny circular hole with a tiny red light. (I chose red just because it shows up well.) Here's what happens:
You can see that since the hole is small, there's only one way for the light to pass through. This means it will hit the screen in a straight line from the source and through the hole. Now, what if we add a second light source? Let's make this one green.
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Now we get two spots on the screen, since the green light can also pass through the hole only one way. But notice that the spots on the screen are upside down relative to the lights. I could add a bunch more lights in between that would create a series of spots across the screen.
Now, let's take that idea to the extreme and replace the lights with a continuous object like a cat, where light reflecting off each point on the cat would pass through the tiny hole at a different angle. Light from the top of the cat would make a spot at the bottom of the screen. Light from the bottom of the cat would hit the top—and likewise for everything in between. Altogether, we'd get an entire image of the cat, upside down, on the screen.
Yes, a pinhole can create an image similar to the way a camera does. There's one big difference, though: The cat image will be very dim. Because the hole is so tiny, it lets only a small amount of light through. A camera gets around that by using a larger aperture, with a lens to focus the image on the “screen” (the film or sensor). But you actually could use this simple setup to make a pinhole camera—it's a real thing. I even made a pinhole video camera once.
What About Bigger Holes?
But what about holes that are not tiny and not round, like on the PUNCH card? When the hole is larger than a pinprick, it gets complicated. But we can approximate the effect by using lots of little pinholes close together. Let's start with two tiny holes. Here's a diagram:
Now there are two paths for the light from the source to make it to the screen. That means there are two spots. Adding the green light, both lights make two spots.
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Now, let's make a bunch of tiny holes, arranged in a grid to approximate a square hole. Of course, the more holes we use, the closer it will be to an actual square hole. I'm using 25 holes, arranged in a 5-millimeter-wide grid, so each light will make 25 spots on the screen. (I didn't want to draw that many lines, so I did it in Python.) Check it out:
You can see that each light source casts a square image onto the screen. That's sort of what you'd expect—a square hole makes a square image. But let's stop and think about what we're seeing. This is actually an upside down image of the object—which, instead of a cat, is a pair of lights 6 centimeters apart. And the two parts of the image are made of squares.
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To really understand this, let's move the screen, starting with it just 1 millimeter away from the hole and ending 30 centimeters away. Here's what I get in my model:
Notice that when the screen is right next to the hole, we get an image of the hole—a single square. Then, as we move away from the hole, we get an image of the object, the light source—a pair of red and green lights. And it's an image made up of squares! If the object was a big round sun instead of a pair of point-light sources, we'd get a big round image made of tiny squares.
Don't believe me? How about another example? What if we use a circular light—like the ring of light around a total eclipse—passing through a triangular hole? Here's what it looks like as you pull the screen back.
Again, if the screen is next to the hole, you get the shape of the hole (a triangle). If it's farther away, you get the shape of the light source (a circle). So the shape of both the light source and the hole do indeed matter. When the screen is close to the hole, it's a triangle made up of circles. When it's far from the hole, it's a circle made of triangles. That's just cool.
Using the PUNCH Card
Let's try this with the PUNCH card. Here's the “screen” (which is actually just a wall) close to the hole.
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You can see that the three holes make images in the shape of the hole (triangle, circle, square). What about when you move the card farther from the wall?
Now, it's just three circles. They are circles because that's the image of the sun, and the sun is round. But if you did this during the eclipse, you would see three crescents as the moon moves in front of the sun. See? Even if you can't make it to a location with a total eclipse, you can still have a ton of fun with a pinhole.
