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SYFY WIRE Bad Astronomy

Science out an airplane window: The case of the floating, stationary, disconnected, warped propeller blades

By Phil Plait
airplane propeller distorted by aliasing and rolling shutter

So, I fly a bit, traveling here and there to give talks about astronomy and science. In April 2017 I flew to Nebraska to give a talk about the upcoming solar eclipse. Kearney, Neb., is a lovely town, but not a terribly big one, so I was on a smaller plane instead of a big jet. I had a window seat and —as I usually do — entertained myself by peering out at the planet below.

sundog, halo, and parhelic circle

It turns out that, on this trip, the sky was far more enthralling than the ground. I was happy to see that there was a thin cirrostratus cloud layer all around; the ice crystals in these clouds can produce all kinds of glorious optical effects. The Sun was behind us and out of view of my window, but I was still able to see a beautiful halo around it, and a fairly bright sundog (technically, a parhelion), caused by light bent inside the ice crystals. There was even a bit of parhelic circle, a line of bright white light piercing though the sundog and parallel to the horizon. All these effects are not uncommon, but still delightful to see.

I grabbed my camera to take some pictures, like the one here. Then I turned to look forward to see what lay ahead of the plane, and saw a huge cumulonimbus cloud to port, partially blocked by the propeller. I took some photos, and when I checked them, I got a fun surprise!

airplane propeller distorted by aliasing and rolling shutter

What the what? It’s a bit shocking to see this bizarre photo, yet look out the window and see the propeller still whizzing around, blurred out in a circle as it should be.

Happily, as a science nerd, I knew right away what I was seeing. It was two effects, really: aliasing together with a rolling shutter.

Of course I’ll explain. But first, here’s a brief video I took of this, which shows the effect spectacularly:

That is so cool! I know the airplane noise was loud, so here’s a transcript of what I said:

I’m flying from Denver into Nebraska; it’s about a couple of hours before sunset. There’s a nice halo around the sun and a sundog… part of the parhelic circle there, too. I can barely catch it here in the plane. I actually pulled my phone out cuz I wanted to get this giant convection cloud, but then I also got the propeller doing this weird aliasing effect! The shutter frame is beating, basically, resonating with the propeller spin rate and you see this really weird effect. You can see it better over in the window in front of me. Very, very neat.

I couldn’t go into too much detail while recording that, but happily I’ve written about this effect before. Back in 2014, a massively viral video showed footage from a camera that accidentally fell off a skydiver’s helmet. The camera starting spinning rapidly, and there were some pretty weird effects from that. As I explained in my article about it:

This happens because the spin rate of the camera matches (or is a simple multiple of) its readout rate. Think of it this way: Imagine standing with a camera in your hand. Take a picture, then spin all the way around and take another picture. Do this again and again, and string the pictures together to make a video. The video will not show your motion, because every time you took a picture you were facing the same direction! It’ll look like you were just standing there.

That’s aliasing. You’ve probably seen this before; for example, on a TV show when a car is shown driving along a road, it sometimes looks like the wheel is slowly spinning backward! That’s because the wheel makes almost — but not quite — one complete rotation before the next video frame is taken. When put together, the wheel looks like it’s spinning backward, even though it’s spinning forward.

In my case, the shutter rate was some simple multiple of the propeller spin rate, so every time it took a video frame, the propeller had moved into a different position, making it look very odd.

At the same time, the distortion in the blades was caused by the rolling shutter effect. In a lot of digital cameras, there’s a light-sensitive detector that records the incoming scene, but not all at once. Instead, the scene is scanned, essentially exposing one row of pixels at a time, moving on to the next row, and then stacking them to make the image. If something is moving rapidly during the exposure, it can get distorted. In the case of the propeller, you get those phantom blades, some of which are bent and disconnected from the plane. I found a page with a really good graphic of how this works at Resource magazine.

Put this all together, and you get a video that looks like the propeller doing whatever the heck it’s doing in the video. Floating, stationary, disconnected, warped propeller blades.

This is a pretty fun little trick, but there’s an extremely important lesson, here: Cameras lie. People think that (barring hoaxes), if they see something in a photo or a video, then it must be real. But that’s not the case at all! Cameras do not work like the human eye, and what you see in an image or in a video may not be what’s going on. A ton of things happen between the events of the light entering the lens and the image being displayed on your monitor, and any one of those distorts reality. The lens doesn’t faithfully reproduce how light is focused by your eye. The shutter speed can change what’s seen. The way the detector in the camera sees light is very different from your eye, so brightness is distorted and colors are messed up (many cameras are sensitive to infrared, too, and use a filter to more or less block it). In some cameras, if a source of light is too bright the detector saturates and has a hard time keeping up with the incoming light; that spot in the camera actually undercounts the light and it looks black (I think of it like trying to fill a bucket with a firehose; even though a lot of water is entering the bucket, most splashes out — that’s an analogy and imperfect, but gives you the idea). I worked on a camera on Hubble Space Telescope that suffered this exact problem, and it was very difficult to process images of bright stars because of it.

My point? Just because you see it in a photo doesn’t mean it’s real. Sure, some pictures are faked, but even the honest ones are not showing you what really happened. Unless you think engineers have somehow designed a magic airplane propeller made of several pieces that hover off to the side and are all warped and twisted yet can still fly you safely to Nebraska, which seems unlikely.

What you see is never, ever what you get. A good thing to keep in mind when someone shows you photos proving the existence of non-existent things. Ghosts, Nibiru, UFOs, what have you: It’s also best to maintain a healthy skepticism, and to understand not just what you’re seeing, but how you’re seeing it.

Image Credit: Phil Plait