I became an impromptu science teacher today

I too got swept by Eclipse Fever on the last days and when Sunday came I decided that we needed to do something cool to see the Eclipse. We started with the idea of building a cereal-box pinhole projector, but I knew that the image that you could see on those was pretty small, you barely can make out the shape of the sun.

So I decided to go big. I knew I would need a tall box, but I didn’t know how big. I had the basic understanding on how the pinhole projector works:

The light comes from the left side in the diagram, and as it goes through the pinhole, it gets projected inside the box. The box has to be made as dark as possible to make sure the light doesn’t bounce around too much and you can see just the primary image, and not all the reflections. For that reason, this setup is also called a “Camera Obscura”.

The important thing is that this is a linear projection, which means that there is no distortion, as most lenses produce. The image will be projected on the paper with exactly the same proportions as it had on the other side. That is, the angle of the light entering the box was the same as the angle of the light reaching the projection plane. This was when I realized how to calculate how tall my “cereal box” had to be in order to see a large projection of the sun.

My first goal was to project the sun as large as a sheet of paper, which in its shortest side measures 21.5cm, this lead me to the following diagram:

If the goal was to have the size of the sun in the projection (b) to be 215mm, I would need to know two things. The first is what is the angle (t) and finally what is the height (h) of the triangle, which is the measure that I was looking for.

It turns out that the measure of the size of celestial bodies in the sky is commonly measured in the angle it takes on the sky, and for the sun, that is 0.5 degrees. This is a very small angle, I knew h would be a large number, but I didn’t know how large.

It took me a while to recover my elementary geometry skills, but in the end, the trick is turning this isosceles triangle into two right triangles, then just plug in the formula: h = (b/2)/tan(t/2), which replacing in the numbers gets me to h = 0.1075/tan(0.25), which gives me the scary number of 24.63 meters. Well, that was a bit bigger than I expected, let’s just say I didn’t have a cereal box that big hanging around.

A cereal box that big would serve a *lot* of breakfasts

So I had to accept to scale down a little bit, but I wasn’t ready to go back to the cereal box, and then I had the light-bulb moment: PVC pipes would provide me with a tall, but manageable camera obscura. I managed to find a hardware store nearby that had pipes, fitting, a saw and some black paint.

The pipe was ~ 3 meters long and ~ 75 mm wide (it was actually 10 feet long 3 inches wide, but who uses imperial units), so the first step was to make it more manageable. I sawed it in 4 different parts, such that I could disassemble it for transport.

This was way harder to saw than anticipated, the pipe had a surprisingly thick wall.

The next step was to paint the interior of the pipes with a flat black primer, in order to reduce the reflection inside the pipe, since after painting the inside, I still had left-over paint I decided to paint the exterior as well.

In retrospect, painting the exterior was a bad idea, every time I handle the pipes, I get a bunch of black smudges in my clothes and hands, and legs, and face…

As you can see in the above image, the view port of the projector was made with a Y pipe fitting. This turned out fantastic, because you were able to get really close to the projecting surface, even if there was some 3 meters of pipe until it got to the pinhole.

After the paint job was done, the only thing missing was to make the projection surface, which was a simple construction of cardboard, tape and paper, and the pinhole itself, which was cardboard, tape and aluminum foil.

It turns out that it’s pretty hard to make a small round hole in aluminum foil. This was the failed attempt that I had to redo.

It was finally time to test it (still on Sunday). I took to the roof of the building — at which point I realized just how heavy this thing was.

Thankfully there was the rail of those stairs, It was absolutely impossible to hold it still by myself.

And it turns out that Mathematics works, and the image projected was around an order of magnitude smaller than my original insane plan, but thankfully the device was also an order of magnitude smaller.

Yay, the sun *is* round in normal days.

So I disassembled the whole thing, put in the trunk of the car and just had to wait until the following day. We made plans to use a foldable staircase to hold the thing up, but that failed miserably. Thankfully there was a support pole for a tree in the park, which worked out great.

And then finally the nicest part came. We got to see the eclipse up-close, since you could just look through the PVC fitting, the image was bright and large:

The sun, after being eaten by the moon.

But the nicest part of all was actually that we started to gather a crowd around the projector, since this was actually nicer than staring at the sun with the eclipse glasses. I also turned into an impromptu science teacher, proudly explaining the math, the physics and the construction of the 3m-tall pinhole projector.

Now I just need to find storage for this for the next 100 years, for the next eclipse.

To close up, a photo that gives a better perspective of what looking at the projector actually felt like:

Yes, my son was walking in his socks in the park… Who knows what’s going on there…

Let me know what you think, and if you are in the NYC area and want to hold on to the 3m-long-pinhole-projector for the next eclipse...