# The illusion of the hanging aircraft

1. Jan 20, 2012

### checkitagain

As I driving on an interstate yesterday in the area of an airport,
I saw over to my right a larger commerical jet flying in the
opposite direction that I was driving. It was slightly angled up,
so I took that to mean it was just flying away from the
airport.

I noticed how it just seemed to hang in the sky, despite
its speed. I glanced over to my left another couple of
times to verify what I had seen. Also, I have seen this at
other times (other years, for instance).

Have other users noticed this illusion of the aircraft hanging,
as if it were on a cable?

$Edit:$

Don't assume any headwind here. Suppose the aircraft were
moving dirrectly overhead the vehicle (about 150 to 200 feet up)
at 145 miles per hour, but traveling in the opposite direction.
and you were driving 55 miles per hour. But the relative speed
of the plane should be 200 mph passing you. The plane's
length shouldn't account for it to be moving relatively slowly to you.

Last edited: Jan 20, 2012
2. Jan 20, 2012

### Andre

Mind this, a fly with a lenght of 10mm flying at one meter per second, covers his own lenght in 1/100 second. An airliner with a lenght of 51.4 meters having an approach speed of 125 knots in a 25 knots headwind does 51.4 meter per second and hence covers his own lenght in one second, 100 times slower than the fly.

3. Jan 20, 2012

### Jimmy Snyder

The flight to Aruba is like that.

4. Jan 20, 2012

### DaveC426913

Parallax.

The simple fact is that you need a viewing reference frame in which is judge how fast - or whether - an object is moving. Objects behind or under the plane such as the runway are too ambiguous to use as reference points. A runway is a long featureless object, parallel to the direction of the plane's motion and you are in motion too; it is virtually impossible for your brain to measure an object moving relative to it.

What ends up happening is that you use closer objects and vertical objects such as buildings and posts as your reference frame. So as you move East, the plane moves West, as does your frame of reference.

On the other hand, if there are no adequate objects in your field of view, then you have no frame of reference at all with which to detect change in position, and therefore are unable to detect any movement.

Either way the result of this is that, from you point of view, the plane appears to be motionless.

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Last edited: Jan 20, 2012
5. Jan 20, 2012

### Containment

Perhaps it had some really heavy people aboard causing it to stall while climbing to higher elevations??? It could have been one of those army hover planes you see in the adds before a movie?