RE: Satellite observing and the autokinetic illusion

From: Ted Molczan (seesat-list@rogers.com)
Date: Fri Sep 01 2006 - 11:41:07 EDT

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    Frits Westra asked:
    
    > The autokinetic illusion is generally described as the illusion of a  
    > _fixed_ light moving when gazed at steadily against a dark background.
    > 
    > However, I regularly receive reports from members of the 
    > public who appear to have observed a passing satellite (often the ISS)
    > at night, but who are reporting strange (impossible) non-linear movements
    > of the satellite. E.g. jumping back and forth and zizagging along the track.
    
    This question arises from time to time. I do not know about autokinetic, but it
    is a common optical illusion. I found it most pronounced when observing bright,
    slow-moving satellites, like the old Pageos balloon, which is no longer in
    orbit.
    
    Terence Dickinson offered this explanation in his astronomy field guide,
    NightWatch:
    
    "Whether satellites have a steady or fluctuating brightness, most novice
    observers agree that they do not appear to move across the sky in perfectly
    straight lines. There seems to be a perceptible waviness to their paths, a
    jerkiness in speed as they glide through the starry background. In fact, these
    oscillations are in the mind, not the sky. The satellites actually move in
    precise linear paths at an even velocity.
    
    The human brain likes to link patterns into a recognizable image. This is done
    instantaneously in daily life. However, looking at one moving light in a
    randomly dotted black sky, the brain constantly tries to produce these patterns
    but fails. What are thought to be oscillations in the satellite's path are
    really the unconscious workings of the mind trying to make sense out of an
    unfamiliar visual environment. The result is, in effect, an optical illusion."
    
    Ted Molczan
    
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