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 ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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