In this column, posted up today on the SD home page, Cecil discusses how the “Wheel seems to move in reverse” effect generally only happens in movies, because it is a result of the strobe effect of recording things with movie cameras (rather than getting a “moving picture”, as it were, the camera instead takes 24 still pictures each second.)
Anyhow, he invited readers to come up with examples of where this happens outside of watching a movie or spinning something in front of a TV set. I am certain that I’ve seen this effect on airliners while watching the engines (mostly on planes with the engines in the back, such as an MD-80).
Mind you, I haven’t been on one of these since the early 90’s, but I distinctly remember seeing the effect and asking my dad about it. Is it possible that the blades of the turbofan engines on an airliner spin so fast that you get the same effect?
I see this sometimes on the road. I usually see it on cars with open hubcaps, in low light with street lights on, and doing 30-45mph, and I’m next to them. I’ve always wondered about it and I know my father sees it as well because I asked him about it when I was little.
I don’t know much about airplane engines, but there would have to be a strobing effect also for this to occur. (perhaps sunlight glinting off compressor vanes?).
I’ve seen this effect late at night when the frequency of the streetlights matches the tread on my bicycle tire as it rotates. If you look from just the right angle, your front tire can appear to be still as you ride down the street.
Many design guidelines for lighting in workshops where there is rotating machinary call for diffuser on flourescent lights to scatter teh light or additional components to boost the frequency at which the lights operate to avoid the roating components appearing to be stationary.
I’ve given the circumstances, just because some people do see it and others do not doesn’t mean I’m wrong. I think it’s because the wheels are turning about the same as one would see at 24 frames per second, along with the street lights that flicker off of some types of hub caps, make them seem like they are going in reverse. I do not see it at all speeds, nor with every car, only some. I’ve also been seeing it for years, long before there were the second set of caps that spin.
I don’t get out much in the evenings right now so I haven’t noticed it in awhile, but the next time I do I’ll give every possible detail I can.
Your eyes are constantly in motion, even when you think you’re looking in one direction. These movements, called saccades, require your brain to suppress (saccadic masking) the data generated during these short bursts Your brain then tries to fill in what it thinks should have been there. So even in broad daylight you can see this strobing effect. What works best for me is to move my eyes back and forth between the right and left sides of the rim of a car’s wheel that’s traveling beside me, since saccades are more prevalent when you’re changing your viewpoint.
Did you pay ANY attention to the added portion of the column? You know, the part where he talks about having gone out and experimented with Ms. Adams? The part where he concedes to the correctness of the idea? :rolleyes:
Then what are you trying to say? All I said is I’ve seen it a number of times, the way you wrote your post I took as say I couldn’t. I will say that I didn’t see the second part because the first part fit on my screen so I didn’t scroll.
I think there is something missing from Cecil’s explanation.
The human vision system itself has a sampling rate. Whatever that sampling rate is in frames per second, when the rpm of a wheel is equal to or a multiple of that sampling rate, the wheel will appear motionless. Similarly, the coincidence of sample with wheel position can cause the wheel to appear to roll faster, slower, or in a reverse direction to its movement in reality.
Oddly enough, I don’t recall that entire second portion even being there the last time I looked at this article (when I posted the thread).
Maybe it was a server-side problem, maybe my internet connection just burped. It’s remotely possible that I just didn’t think to scroll down, but I’d rather dismiss any suspicions of possible PEBMAC problems until we’ve blamed everything else.
