In the last post, Twinkle Twinkle Little UFO, we looked at the effects of atmospheric refraction on starlight and how that might contribute to misidentification of a star as a rotating, color-changing UFO hovering in the sky. But in many cases the suspected objects don’t remain stationary, they seem to move. So what’s going on?
Bright UFOs in the night sky have been reported by witnesses to dart around randomly, zig-zag, and drift one way or another. Some witness claim that the lights must be communicating with them because they seem so in-tune with the viewer. The motion may be less “in-tune” than you think, and more physically innate.
In this article we’ll be looking at a specific phenomenon that is the likely cause of this perceived motion: autokinetic illusion.
The other morning I was standing at my kitchen window looking out to the back yard and for a brief moment I thought saw something small flitting around out there, it never moved far. In fact it stopped moving and became apparent once I focused on it. Did it land?
It was a little red thing. Was it some kind of bird? Something else? Nope, it was a fine spec of tomato sauce stuck on the window which I subsequently cleaned off. But before cleaning it I did a quick experiment: I tried to steady my head and keep the spot fixed in my gaze relative to the background, but I couldn’t do it. That spot moved ever so slightly back and forth, not so much up and down though.
I realized that it was virtually impossible to keep my head perfectly still. I said the “spot moved” but to clarify, the spot didn’t really move, I did.
What I experienced, although mundane, was technically the result of autokinesis, because auto-kinesis literally means “self motion”. I was moving, and for a moment my brain misattributed this as the spot of tomato sauce moving. Until I realized what it was. True autokinetic illusion gets a bit more involved though.
Most of the time our brains do just fine figuring things out but, head movement aside, there is some pretty tricky eye-brain stuff going on that makes autokinetic illusions pretty convincing, even disorienting, and cool.
Human vision isn’t nearly as precise as, say, an owl or hawk, but our eyes are still pretty impressive. We have a pretty sophisticated tracking system in place that we rely on all the time. But misinterpretation, by our thinking brain, of the stuff our innate brain is telling us can lead us to make false assumptions. We might think we’re witnessing something that we really are not.
The two specific movements made by the eyes are saccades and microsaccades. The first are voluntary movements used to look around a scene or move your eyes across a page while reading. The second, microsaccades, are very small, involuntary jerking motions made by the eyes during visual fixation, when your gaze settles and you focus on a specific object or area.
The reason our eyes do this is because humans have a small area of visual acuity on the retina called the fovea, outside of this focal point visual detail is diminished. In a nutshell, when you look at something long enough, it must be important, so your eye jerks around in that specific area of your view, in an effort to collect as much detail through the fovea as it can. This helps your brain determine, to the highest degree possible, what you’re looking at and what’s going on with it.
In the absence of any other visual cues, the brain cannot determine the object’s position accurately, and it appears to move randomly. If you sit in a pitch black room and stare at a spot of light fixed on the far wall, that bright spot will eventually appear to move. Something called ocular drift may greatly contribute to the motion during this visual fixation.
During ocular drift the eyes follow a slow, meandering course. The spot might appear to slowly drift one way or the other, loop back a little, all very slowly. Periodically our eyes may attempt to refocus on the spot through microsaccades, which may lead to a jerking-drifting effect.
The jerking motion of the eye during microsaccades directly corresponds to the jerky or zig-zagging motion described by many UFO sighting witnesses. Again, as I mentioned in the previous article, we’re not talking about reports of silver daylight discs, triangle craft or other sightings of specific craft, just bright UFOs seen in the sky at night.
If you slightly avert your gaze the similarity to a twinkling star is more evident. These bright, whirling, rotating, multi-colored lights frequently reported dancing in the dark sky can be explained, and may simply be stars. The truth is no less amazing when you consider the natural science involved.
The optical illusions sometimes created by voluntary or involuntary eye movements, like saccades or microsaccades, can create some pretty amazing effects. One excellent example is the Rotating Snakes illusion devised by Japanese artist Akiyoshi Kitaoka. Warning: This image might make observers feel sick. if you are very sensitive to optical illusions you may want to scroll down and skip it.
The effect seen here is due to a phenomenon called “peripheral drift” an effect that shares the stage with autokinetic illusion due to the actual mechanism at work.
So when we see twinkling stars it is caused by refraction, and when those bright points of light seem to move it is an optical illusion caused by the movement of our eyes and sometimes our heads. These are personal experiences though, often seen through binoculars, and peripheral drift will not occur the same for everyone… so what’s going on when multiple people see the same thing? That’s for next time.
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References
https://en.wikipedia.org/wiki/Saccade
https://en.wikipedia.org/wiki/Microsaccade
https://en.wikipedia.org/wiki/Fixation_(visual)