Plane Stopped in Midair

OK, so first off, here is a reference for you defining parallax:
http://www.phatnav.com/wiki/wiki.phtml?title=Parallax

Now, since when an observer is in motion, they expect to see objects
closer to them appear to move backwards with respect to objects in the
background (as they pass them) if they are stationary. If you are
observing a plane in flight, you expect it to move in its direction of
travel with respect to the background. If the plane is moving in the
same direction as your car, and the parallax motion of the plane
(backwards motion) equals the relative forward motion of the plane
with respect to the background, the plane will not move at all with
respect to the background and appear to "hang" in the air. This
effect is attributable to the phenomenon of parallax...

Do you still want to debate this subject?

Dean

(G Farris) wrote in message ...
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

In article ,
says...


Do you still want to debate this subject?


Not particularly.
I do maintain that you are mistaken about the application of parallax to
illusions related to apparent motion. Parallax, by definiton, requires two
distinct points of view. Thus, the distance between our two eyes would create
two slightly different views, and if we could measure it, we could use this
parallax to measure the distance to the object.

Nevertheless, parallax is not conventionally used to describe perceptions
related to motion. In the photogrammetric model, two views of the same subject
are taken and compared through a stereoscope to produce a pseudo-stereographic
image. Though the two images were taken over time, because of the movement of
the airplane, the information conveyed is considered to represent a geometric
offset, and not an expression of the airplane's movement. The same image could
have been created in the same instant by one, very large airplane, with a
camera at each extremity.

Parallax, is a term used to describe geometric differences in an observed
object from two differing viewpoints. It is not intended, nor is it
sufficient, to describe psychological illusions (expectations) or observations
or illusions related to motion (evolution of geometry over time) except
indirectly, when time is required to obtain two different views, as in the
photogrammetric example above. Definitions of parallax do not include time
constants - only geometric relationships.

Therefore, I maintain that the use of the term "parallax" to describe the
illusion discussed in this thread is imprecise at best.

G Faris

Morgans wrote:

Another thing to keep in mind, is that airplanes work by the laws of
physics.

True enough.

An airplane, be it large or small, can not have zero airspeed for
even an instant, or it will stop flying, and fall out of the air.

Dave Morgan, Sharkey Ward, and a host of other Royal Air Force, Royal
Navy, United States Marine Corps, and Spanish Navy pilots would beg
to differ.

What remains, is the fact that the jet you saw did not stop, and what you
perceived was due to your observation being in error, in some manner.

Given that the reported wind speed that day was quite modest (*way*
short of tornado-grade), that is clearly true of the "cargo jet"
which Debbie described.

I guess it is conceivable that its cargo included a Pegasus engine,
but the aeroplane certainly wasn't being held aloft by one ...

"Robert Briggs" wrote

I guess it is conceivable that its cargo included a Pegasus engine,
but the aeroplane certainly wasn't being held aloft by one ...

OK, No zero airspeed, unless you are being held up by the thrust of the
engine, alone. That ought to cover Shawn Tucker, Harriers, and Super
Hornets. g
--
Jim in NC


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