Relative Motion

Ordinarily, a moving object not 
only changes its direction with 
respect to us but changes its 
location with respect to all 
other stationary things in the 
scene. This relative change of 
location can affect the motion 
we perceive in various ways.

INDUCED MOTION    When the 
moon appears to move across
the clouds, the moon is not 
changing its direction with 
respect to us—–that is, its 
egocentric direction—–but the 
clouds are doing so. If changing 
egocentric direction were all 
there were to motion 
perception, the moon would 
appear stationary and the 
clouds would appear to move. 
The fact that we see the moon 
as moving suggests that the 
change in position of an object 
relative to background objects 
must be a strong determinant of 
perceived motion. This effect is 
called induced motion—–the 
inducing of motion in a 
stationary object by a nearby 
moving object. One might have 
predicted that this relative 
change would simply reinforce 
an impression that the clouds 
are moving. The clouds ought to 
appear to move because they 
change direction with respect 
to the observer. The presence of 
the moon might be expected to 
further support that 
appearance by the impression 
of relative change it yields. 
Why, then, should the 
introduction of relative change 
cause us to see the moon rather 
than the clouds as moving?