In article ,
says...

Dean Wilkinson wrote:

Its called parallax. Your motion, the motion of the plane, and the
background were in the right configuration so that the parallax made the
plane appear to be stationary when in fact it was not...



okay! This is clicking for me, even though I know it may just be another
way of saying "optical illusion". I read this page,



Honestly, I hate to bicker, especially on a subject that has been beaten into
the ground, but I believe this response in incorrect. I don't think this
sighting illusion has anything to do with parallax.

As pilots, all of us learn in our early VFR training, that in scanning the sky
for possible traffic conflicts, the most dngerous "targets" are those that
appear not to be moving. No apparent motion indicates that the target is
heading pretty much straight toward us. The only modification in the visual
profile of an object in this case is a gradual increase in size. This can be
*very* gradual while the target is still some distance away.

The large airplane fools our brain - we don't *expect* it to be so big,
compared with other landmarks, so we guess it is closer than it actually is.
When the profile doesn't increase appreciably in size, compared with how close
we "believe" it to be, our brain concludes it is stationary, or moving very
slowly.

It is a simple illusion, easily explained and understood - yet very beautiful
and impressive to behold!

G Faris