Plane Stopped in Midair

[DM]

Yesterday I saw a cargo jet (a major air express company) come to what
seemed like a dead stop in midair as it was making its ascent. After
about 20 - 30 seconds of hanging without dropping out of the sky, it
continued climbing and apparently did not crash. There's been nothing
about it in the local news but I've still been very concerned. Could
someone here explain how such a thing is possible?

Some details: the temperature was about 50 degrees F, the sky was mostly
clear, and the time was around 0645. I was traveling by car at about 40
MPH on a street that is parallel to a regular flight path. From this
street it's common to see 3 or 4 planes per minute either climbing or
descending; the airport is about a mile or two away from this particular
street.

As I was moving relatively slowly compared to the how fast the jet
should have been moving, I noticed that I was gaining on it. I quickly
eyeballed the area for tall buildings and other geographical reference
points so I could be sure that I had a good perspective and wasn't just
"seeing things". The object was either not moving or it was moving
*very* slowly, and it was not a helicopter. For a few seconds I was
stopped at an intersection looking at this hanging plane and at the
people in the other cars around me. No one else seemed to be paying any
attention to it besides me.

The main reason this bothered me so much is because had the plane
fallen, it would have landed less than a half mile from where I and
about 30 other running cars were, in addition to several warehouse-type
buildings and auto repair garages, plus a 6 or 8 lane freeway filled
with morning traffic. Since the plane had just taken off and was
probably full of fuel, and was still low enough for its markings to be
readable from the ground, the crash probably would have been
extraordinarily disastrous.

I've done a lot of Googling to try to get an understanding of what I saw
and really haven't learned anything meaningful. I'm hoping someone here
can explain how a "regular" jet--versus a specialized military jet--can
apparently stop in midair and not drop from the sky. As a daily traveler
near a major metro airport, I'd really like to be reassured that this is
not a common occurrence.

Debbie

[BeaglePig]

DM > wrote in
:

> Yesterday I saw a cargo jet (a major air express company) come to what
> seemed like a dead stop in midair as it was making its ascent. After
> about 20 - 30 seconds of hanging without dropping out of the sky, it
> continued climbing and apparently did not crash. There's been nothing
> about it in the local news but I've still been very concerned. Could
> someone here explain how such a thing is possible?
>
> Some details: the temperature was about 50 degrees F, the sky was
> mostly clear, and the time was around 0645. I was traveling by car at
> about 40 MPH on a street that is parallel to a regular flight path.
> From this street it's common to see 3 or 4 planes per minute either
> climbing or descending; the airport is about a mile or two away from
> this particular street.
>
> As I was moving relatively slowly compared to the how fast the jet
> should have been moving, I noticed that I was gaining on it. I quickly
> eyeballed the area for tall buildings and other geographical reference
> points so I could be sure that I had a good perspective and wasn't
> just "seeing things". The object was either not moving or it was
> moving *very* slowly, and it was not a helicopter. For a few seconds I
> was stopped at an intersection looking at this hanging plane and at
> the people in the other cars around me. No one else seemed to be
> paying any attention to it besides me.
>
> The main reason this bothered me so much is because had the plane
> fallen, it would have landed less than a half mile from where I and
> about 30 other running cars were, in addition to several
> warehouse-type buildings and auto repair garages, plus a 6 or 8 lane
> freeway filled with morning traffic. Since the plane had just taken
> off and was probably full of fuel, and was still low enough for its
> markings to be readable from the ground, the crash probably would have
> been extraordinarily disastrous.
>
> I've done a lot of Googling to try to get an understanding of what I
> saw and really haven't learned anything meaningful. I'm hoping someone
> here can explain how a "regular" jet--versus a specialized military
> jet--can apparently stop in midair and not drop from the sky. As a
> daily traveler near a major metro airport, I'd really like to be
> reassured that this is not a common occurrence.
>
> Debbie
>


It is just your perception, and the relative size of the plane, verses
the size your brain thinks it "out to be". The plane was likely also
either angled at something less than a 45degree angle either toward you,
or awayfrom you making it difficult to percieve movement.

It was not hovering or anything like it.

BeaglePig

[Christopher Brian Colohan]

DM > writes:
> Yesterday I saw a cargo jet (a major air express company) come to what
> seemed like a dead stop in midair as it was making its ascent. After
> about 20 - 30 seconds of hanging without dropping out of the sky, it
> continued climbing and apparently did not crash. There's been nothing
> about it in the local news but I've still been very concerned. Could
> someone here explain how such a thing is possible?
>
> Some details: the temperature was about 50 degrees F, the sky was
> mostly clear, and the time was around 0645. I was traveling by car at
> about 40 MPH on a street that is parallel to a regular flight
> path. From this street it's common to see 3 or 4 planes per minute
> either climbing or descending; the airport is about a mile or two away
> from this particular street.

One detail you left out -- how windy was it? I'm going to guess it
was a really windy day. If the plane was taking off into a strong
headwind then it would not have to go nearly as fast (relative to the
ground) to stay in the air. The only thing the plane cares about is
how fast it is going relative to the wind (airspeed). You may have
seen a plane which was travelling at a much slower groundspeed than
you are used to seeing, and thought it was almost stopped as a result.

Also, if seems to be windy at ground level where you are, it may be
much windier once you get up above any trees/buildings/etc which only
slow the wind down.

Chris
--
Chris Colohan Email: PGP: finger
Web: www.colohan.com Phone: (412)268-4751

[John T Lowry]

"DM" > wrote in message
...
> Yesterday I saw a cargo jet (a major air express company) come to what
> seemed like a dead stop in midair as it was making its ascent. After
> about 20 - 30 seconds of hanging without dropping out of the sky, it
> continued climbing and apparently did not crash. There's been nothing
> about it in the local news but I've still been very concerned. Could
> someone here explain how such a thing is possible?
>
> Some details: the temperature was about 50 degrees F, the sky was
> mostly clear, and the time was around 0645. I was traveling by car at
> about 40 MPH on a street that is parallel to a regular flight path.
> From this street it's common to see 3 or 4 planes per minute either
> climbing or descending; the airport is about a mile or two away from
> this particular street.
>
> As I was moving relatively slowly compared to the how fast the jet
> should have been moving, I noticed that I was gaining on it. I quickly
> eyeballed the area for tall buildings and other geographical reference
> points so I could be sure that I had a good perspective and wasn't
> just "seeing things". The object was either not moving or it was
> moving *very* slowly, and it was not a helicopter. For a few seconds I
> was stopped at an intersection looking at this hanging plane and at
> the people in the other cars around me. No one else seemed to be
> paying any attention to it besides me.
>
> The main reason this bothered me so much is because had the plane
> fallen, it would have landed less than a half mile from where I and
> about 30 other running cars were, in addition to several
> warehouse-type buildings and auto repair garages, plus a 6 or 8 lane
> freeway filled with morning traffic. Since the plane had just taken
> off and was probably full of fuel, and was still low enough for its
> markings to be readable from the ground, the crash probably would have
> been extraordinarily disastrous.
>
> I've done a lot of Googling to try to get an understanding of what I
> saw and really haven't learned anything meaningful. I'm hoping someone
> here can explain how a "regular" jet--versus a specialized military
> jet--can apparently stop in midair and not drop from the sky. As a
> daily traveler near a major metro airport, I'd really like to be
> reassured that this is not a common occurrence.
>
> Debbie

A pilot acquaintance of mine in Montana (Larry) flies a Super Cub to
inspect pipelines. Sometimes, when it's windy, he confounds drivers on
the Interstate by flying backwards above them. Not a problem; it's only
airspeed that matters.

John Lowry
Flight Physics

[Dylan Smith]

In article >, BeaglePig wrote:
> It is just your perception, and the relative size of the plane, verses
> the size your brain thinks it "out to be". The plane was likely also
> either angled at something less than a 45degree angle either toward you,
> or awayfrom you making it difficult to percieve movement.

Especially the giant Antanovs - they look like they are barely moving on
approach, despite really moving at 150 knots or so. By comparison,
a C140 approaching at 55 knots looks like it's just racing along.

--
Dylan Smith, Castletown, Isle of Man
Flying: http://www.dylansmith.net
Frontier Elite Universe: http://www.alioth.net
"Maintain thine airspeed, lest the ground come up and smite thee"

[jharper aaatttt cisco dddooottt com]

One thing to add to what the other posters have said...
in a climbing turn a plane can easily appear to be
stationary for a short while. Not sure about the optics
of this but from my office area I can see planes taking
off from San Jose and starting the right turn for the
standard departure course, and this illusion is quite
common. Combine that with a strong headwind and the effect
of the size of the plane, and you could easily believe it
had stopped.

John

[jharper aaatttt cisco dddooottt com]

One other thing... it's essentially impossible to
stop a fixed-wing airplane in midair. (You can have
zero ground speed due to headwinds, but not zero
airspeed). If you try, the nose drops and it glides
towards the ground. If you try to stop that, it will
eventually perform an aerodynamic stall, which WILL
make the nose drop, but it still won't come to a halt.
About the only way to reach zero airspeed is to
pull the plane into a near-vertical climb. And even
then it reacts by dropping the nose and building
up some speed again (or if you are truly vertical,
it can fall tail-first and then it will at some point
snap into a more normal posture and then start
to fly nose-first again). It's fun to do though, in
the right kind of airplane - a small aerobatic one -
and with enough altitude.

John

[C Kingsbury]

Debbie,

A large transport jet (such as a DC-10 operated by FedEx) can climb, lightly
loaded, at speeds around 150mph. This would be unusual (170-200mph is more
normal) but well short of impossible. It might have been a maintenance
flight, for instance.

Now, with that in mind let's apply a few more factors. First, windspeed.
Let's say the wind is blowing at 40mph. This would not be unusual for the
Northern half of the country this time of year. An airplane flying into that
wind would only need to move at 110mph relative to the ground. Windspeed can
vary strongly from the ground to the first few thousand feet as well, so you
might not have noticed the wondspeed on the ground. Now, you also mentioned
you were in the car going at least 40mph, so relative to you, the plane
would only appear to be moving at 70mph.

Last, you need to do a little trigonometry, I'm afraid. Imagine putting a
ladder up against the side of your house. The ladder goes up ten feet, and
at its base is set back perhaps three or four feet from the house. Now, when
you climb that ladder, you are traveling ten feet, but you only move forward
three or four. A similar thing is going on when you compare your speed in
the car to the plane's speed in the air. So we can reduce the difference to
perhaps 55 or 60mph.

What you perceive as speed is going to be based on changes in angle, in
other words, you expect to see the plane "pulling ahead of you" at a certain
rate. It turns out that the human eye is quite poor at judging small angles.
A speed difference of 60mph at a distance of half a mile is going to produce
very small angular changes, which your eye will not perceive well. So, even
though the plane is moving at sufficient speed to fly quite safely, to your
eye it will appear to be standing still.

I can assure you that even us pilots, who know all these things, still often
see the very same illusion you describe, especially with very large
airplanes. It's simply a combination of factors that produce an overwhelming
optical illusion.

-cwk.

[Darrell S]

"As I was moving relatively slowly" quoting from your post, it sounds like
you were in your car driving. The APPARENT relative movement of an aircraft
being observed from a moving vehicle changes drastically if either the
aircraft or the car change relative headings. That could make it appear as
if the forward motion of the jet had stopped.
As for then "it continued climbing" is hard to explain. Perhaps you just
thought it had stopped climbing due to the apparent stoppage of any forward
movement.

--

Darrell R. Schmidt
B-58 Hustler History: http://members.cox.net/dschmidt1/
-

"DM" > wrote in message
...
> Yesterday I saw a cargo jet (a major air express company) come to what
> seemed like a dead stop in midair as it was making its ascent. After about
> 20 - 30 seconds of hanging without dropping out of the sky, it continued
> climbing and apparently did not crash. There's been nothing about it in
> the local news but I've still been very concerned. Could someone here
> explain how such a thing is possible?
>
> Some details: the temperature was about 50 degrees F, the sky was mostly
> clear, and the time was around 0645. I was traveling by car at about 40
> MPH on a street that is parallel to a regular flight path. From this
> street it's common to see 3 or 4 planes per minute either climbing or
> descending; the airport is about a mile or two away from this particular
> street.
>
> As I was moving relatively slowly compared to the how fast the jet should
> have been moving, I noticed that I was gaining on it. I quickly eyeballed
> the area for tall buildings and other geographical reference points so I
> could be sure that I had a good perspective and wasn't just "seeing
> things". The object was either not moving or it was moving *very* slowly,
> and it was not a helicopter. For a few seconds I was stopped at an
> intersection looking at this hanging plane and at the people in the other
> cars around me. No one else seemed to be paying any attention to it
> besides me.
>
> The main reason this bothered me so much is because had the plane fallen,
> it would have landed less than a half mile from where I and about 30 other
> running cars were, in addition to several warehouse-type buildings and
> auto repair garages, plus a 6 or 8 lane freeway filled with morning
> traffic. Since the plane had just taken off and was probably full of fuel,
> and was still low enough for its markings to be readable from the ground,
> the crash probably would have been extraordinarily disastrous.
>
> I've done a lot of Googling to try to get an understanding of what I saw
> and really haven't learned anything meaningful. I'm hoping someone here
> can explain how a "regular" jet--versus a specialized military jet--can
> apparently stop in midair and not drop from the sky. As a daily traveler
> near a major metro airport, I'd really like to be reassured that this is
> not a common occurrence.
>
> Debbie

[DM]

Christopher Brian Colohan wrote:
> DM > writes:
>
>>Yesterday I saw a cargo jet (a major air express company) come to what
>>seemed like a dead stop in midair as it was making its ascent. After
>>about 20 - 30 seconds of hanging without dropping out of the sky, it
>>continued climbing and apparently did not crash. There's been nothing
>>about it in the local news but I've still been very concerned. Could
>>someone here explain how such a thing is possible?
>>
>>Some details: the temperature was about 50 degrees F, the sky was
>>mostly clear, and the time was around 0645. I was traveling by car at
>>about 40 MPH on a street that is parallel to a regular flight
>>path. From this street it's common to see 3 or 4 planes per minute
>>either climbing or descending; the airport is about a mile or two away
>>from this particular street.
>
>
> One detail you left out -- how windy was it? I'm going to guess it
> was a really windy day. If the plane was taking off into a strong
> headwind then it would not have to go nearly as fast (relative to the
> ground) to stay in the air. The only thing the plane cares about is
> how fast it is going relative to the wind (airspeed). You may have
> seen a plane which was travelling at a much slower groundspeed than
> you are used to seeing, and thought it was almost stopped as a result.
>
> Also, if seems to be windy at ground level where you are, it may be
> much windier once you get up above any trees/buildings/etc which only
> slow the wind down.
>
> Chris

interesting, but no, it wasn't particularly windy on the ground.

Debbie

[DM]

jharper aaatttt cisco dddooottt com wrote:

> One thing to add to what the other posters have said...
> in a climbing turn a plane can easily appear to be
> stationary for a short while. Not sure about the optics
> of this but from my office area I can see planes taking
> off from San Jose and starting the right turn for the
> standard departure course, and this illusion is quite
> common. Combine that with a strong headwind and the effect
> of the size of the plane, and you could easily believe it
> had stopped.
>
> John
>
yes, but as best as I could tell, the plane wasn't turning. It appeared
to be taking a straight path up, or trying to anyway.

Debbie

[DM]

jharper aaatttt cisco dddooottt com wrote:

> One other thing... it's essentially impossible to
> stop a fixed-wing airplane in midair.

yep, that's why I was expected the plane to fall from the sky.

(You can have
> zero ground speed due to headwinds, but not zero
> airspeed). If you try, the nose drops and it glides
> towards the ground. If you try to stop that, it will
> eventually perform an aerodynamic stall, which WILL
> make the nose drop, but it still won't come to a halt.
> About the only way to reach zero airspeed is to
> pull the plane into a near-vertical climb. And even
> then it reacts by dropping the nose and building
> up some speed again (or if you are truly vertical,
> it can fall tail-first and then it will at some point
> snap into a more normal posture and then start
> to fly nose-first again). It's fun to do though, in
> the right kind of airplane - a small aerobatic one -
> and with enough altitude.
>
> John

if it had been an airshow kind of situation I wouldn't have given it a
thought but this was a commercial jet and it was over a populated area.
No fun...

Debbie

[DM]

C Kingsbury wrote:

> Debbie,
>
> A large transport jet (such as a DC-10 operated by FedEx) can climb, lightly
> loaded, at speeds around 150mph. This would be unusual (170-200mph is more
> normal) but well short of impossible. It might have been a maintenance
> flight, for instance.
>
> Now, with that in mind let's apply a few more factors. First, windspeed.
> Let's say the wind is blowing at 40mph. This would not be unusual for the
> Northern half of the country this time of year. An airplane flying into that
> wind would only need to move at 110mph relative to the ground. Windspeed can
> vary strongly from the ground to the first few thousand feet as well, so you
> might not have noticed the wondspeed on the ground. Now, you also mentioned
> you were in the car going at least 40mph, so relative to you, the plane
> would only appear to be moving at 70mph.
>
> Last, you need to do a little trigonometry, I'm afraid. Imagine putting a
> ladder up against the side of your house. The ladder goes up ten feet, and
> at its base is set back perhaps three or four feet from the house. Now, when
> you climb that ladder, you are traveling ten feet, but you only move forward
> three or four. A similar thing is going on when you compare your speed in
> the car to the plane's speed in the air. So we can reduce the difference to
> perhaps 55 or 60mph.
>
> What you perceive as speed is going to be based on changes in angle, in
> other words, you expect to see the plane "pulling ahead of you" at a certain
> rate. It turns out that the human eye is quite poor at judging small angles.
> A speed difference of 60mph at a distance of half a mile is going to produce
> very small angular changes, which your eye will not perceive well. So, even
> though the plane is moving at sufficient speed to fly quite safely, to your
> eye it will appear to be standing still.
>
> I can assure you that even us pilots, who know all these things, still often
> see the very same illusion you describe, especially with very large
> airplanes. It's simply a combination of factors that produce an overwhelming
> optical illusion.
>
> -cwk.

yes, I've imagined the whole trig thing. That's why I was looking at the
buildings and other higher geographic markers to get a fix on how it
appeared to be moving relative to those things. Plus, for a moment I was
stopped and pretty much next to it, although of course I was on the
ground and it was probably a couple of thousand feet up or less. And, if
it makes a difference in possible explanations, the plane was headed
east and I was moving west. I'm 99.999% sure it wasn't moving or was
moving very slowly.

Note, I've driven in this particular area for more than a decade and
I've seen hundreds of planes moving in the sky from many vantage points
under many different weather and cloud conditions. I'm pretty sure of
what I saw. Your explanation makes great sense but I'm still not
convinced that it was an optical illusion. Really hope so though because
it was quite disturbing.

Debbie

[DM]

Darrell S wrote:

> "As I was moving relatively slowly" quoting from your post, it sounds like
> you were in your car driving. The APPARENT relative movement of an aircraft
> being observed from a moving vehicle changes drastically if either the
> aircraft or the car change relative headings. That could make it appear as
> if the forward motion of the jet had stopped.
> As for then "it continued climbing" is hard to explain. Perhaps you just
> thought it had stopped climbing due to the apparent stoppage of any forward
> movement.
>
yes, I was paying particular attention to how high it was, for obvious
reasons. When I fixed on its altitude relative to buildings, power
poles, the freeway, etc., for a few seconds the altitude didn't seem to
be changing. And for a few seconds my car wasn't moving at all and it
didn't seem to be moving either, or if it was it was going slower that
you'd expect a plane in takeoff to be moving. Then suddenly did it start
moving forward and up. It was just like a helicopter, except it wasn't a
helicopter.

Debbie

[DM]

DM wrote:

> snipped

thank you to everyone who replied. I've been considering all the
explanations offered and appreciate the information. I'm pretty close to
accepting the "optical illusion" idea but am not quite there yet. I was
looking hard at that plane because I really didn't want to believe what
I was seeing. If it weren't for the fact that I could read the name of
the carrier on the plane, I'd probably be going for a UFO explanation!

Fly safe everybody.

Debbie

[Trent Moorehead]

"DM" > wrote in message
...
> Yesterday I saw a cargo jet (a major air express company) come to what
> seemed like a dead stop in midair as it was making its ascent. After
> about 20 - 30 seconds of hanging without dropping out of the sky, it
> continued climbing and apparently did not crash.

As others have said, it's just an optical illusion, unless the wind picked
up suddenly to bring the aircraft's ground speed to zero (unlikely).

At Raleigh-Durham International, there used to be a road that paralleled one
of the main runways. A couple of times, I just happened to be on that road
at the end where a large commercial jet (B737or similar) was just starting
its takeoff roll. I floored the car and left the jet behind. About the time
I got to the speed of around 70 mph, that jet blew past me about midway down
the runway going so much faster it was hard to believe. It brought home the
fact that those jets are moving very fast when they look like they are going
the slowest.

-Trent
PP-ASEL

[Peter Duniho]

"DM" > wrote in message
...
> thank you to everyone who replied. I've been considering all the
> explanations offered and appreciate the information. I'm pretty close to
> accepting the "optical illusion" idea but am not quite there yet.

Airliners simply don't stop in midair. You have no choice but to accept the
"optical illusion" idea.

> [...]
> Fly safe everybody.

We do. Now, since you brought it up, how about you quit panicking every
time you see an airplane go overhead doing something other than what you
think it ought to be doing. We're not the dangerous boogey man that you,
the general public, apparently think we are. Tell your friends.

Thanks.

Pete

[alexy]

DM > wrote:

>
>yes, I've imagined the whole trig thing. That's why I was looking at the
> buildings and other higher geographic markers to get a fix on how it
>appeared to be moving relative to those things. Plus, for a moment I was
>stopped and pretty much next to it, although of course I was on the
>ground and it was probably a couple of thousand feet up or less. And, if
>it makes a difference in possible explanations, the plane was headed
>east and I was moving west. I'm 99.999% sure it wasn't moving or was
>moving very slowly.

Put me in for $100 on that 99,999-to-1 odds bet! <g>

Seriously, it sounds like you are trying to understand what you
perceived, and I think the responses here have been right on target.
I'm betting on the laws of physics and hope some explanation here will
lead to an "aha" for you, to understand what caused your perception.
--
Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently.

[C Kingsbury]

"Peter Duniho" > wrote in message
...
>
> We do. Now, since you brought it up, how about you quit panicking every
> time you see an airplane go overhead doing something other than what you
> think it ought to be doing. We're not the dangerous boogey man that you,
> the general public, apparently think we are. Tell your friends.
>

Methinks the man doth protest too much.

-cwk.

[G.R. Patterson III]

DM wrote:
>
> interesting, but no, it wasn't particularly windy on the ground.

I have the winds aloft forecast for yesterday morning. Where was this?

George Patterson
If a man gets into a fight 3,000 miles away from home, he *had* to have
been looking for it.

[Andrew Gideon]

John T Lowry wrote:

> A pilot acquaintance of mine in Montana (Larry) flies a Super Cub to
> inspect pipelines. Sometimes, when it's windy, he confounds drivers on
> the Interstate by flying backwards above them. Not a problem; it's only
> airspeed that matters.

Hmm. Perhaps the idea of a nose-dragger originated when a tail-dragger
landed in a stiff wind.

- Andrew

[Peter Duniho]

"C Kingsbury" > wrote in message
link.net...
> Methinks the man doth protest too much.

Are you saying that we ARE the dangerous boogey man that the general public
thinks we are? Why on earth would you say such a thing?

[Peter Duniho]

"Todd Pattist" > wrote in message
...
> Give her a break, Peter, she asked politely and responded to
> the answers.

Yes, she did respond to the answers, refusing to accept them.

> It's something that concerned her, and if GA
> is to survive, we don't want to be scaring the general
> public.

The general public is already scared. For no good reason. That's my point.

> The illusion she encountered is quite overpowering.

Most optical illusions are. So what? That's what makes it an *illusion*.
An easily ignored illusion isn't really much of an illusion.

> I look at planes every day. Every week I drive by Stewart
> where they land the C5A. Its size gives me the same
> "stopped in midair" illusion every time I see it. It
> always looks like it's just hanging in midair.

But, it is NOT just hanging in midair. It's one thing to ask for expert
advice to confirm one's intuitive sense that an airplane can't just hang in
midair. It's yet another to repeatedly reply that the expert advice is
simply unbelieveable.

I'm getting a little sick and tired of folks who run in a panic every time
they see an airplane doing something they don't understand. "Debbie" has
made very clear that this isn't just about her trying to understand a visual
paradox.

Her *primary* concern is that an airliner might come to a complete stop over
a populated area, and then having done so, fall straight down and kill her.
Statements like "it was quite disturbing", "No fun", "I've been very
concerned", "this bothered me", "the crash would have been extraordinarily
disastrous", and "I'd like to be reassured this is not a common occurrence"
make it very clear where she's coming from, and she's not coming from "I'm
curious about this optical illusion" frame of mind.

Pete

[C Kingsbury]

"Peter Duniho" > wrote in message
...
> "Todd Pattist" > wrote in message
> ...

> I'm getting a little sick and tired of folks who run in a panic every time
> they see an airplane doing something they don't understand.

Would that they all came to r.a.p. and asked nicely about what they saw
instead of demanding immediate satisfaction from equally uninformed
government officials.

You catch more flies with honey than vinegar.

-cwk.

[Nathan Gilliatt]

In article >,
Dylan Smith > wrote:
> Especially the giant Antanovs - they look like they are barely moving on
> approach, despite really moving at 150 knots or so. By comparison,
> a C140 approaching at 55 knots looks like it's just racing along.

I remember reading something about this. The conclusion was that we
perceive speed as a function of aircraft lengths per second. So a large
jet, such as a 747, seems to be crawling, while a smaller aircraft, such
as a Lear, will seem to be moving much faster while actually at the same
speed. Recalculate their speed in terms of aircraft lengths per second,
and you can see the correlation, since a 747 is several Lears long.

I would imagine you'd experience the same thing if you were to compare
the visual perception of the speeds of an Amtrak train and a fast car
travelling at the same rate.

- Nathan

[G.R. Patterson III]

C Kingsbury wrote:
>
> You catch more flies with honey than vinegar.

and B.S. beats them both.

George Patterson
If a man gets into a fight 3,000 miles away from home, he *had* to have
been looking for it.

[Aviv Hod]

Nathan Gilliatt wrote:
> In article >,
> Dylan Smith > wrote:
>
>>Especially the giant Antanovs - they look like they are barely moving on
>>approach, despite really moving at 150 knots or so. By comparison,
>>a C140 approaching at 55 knots looks like it's just racing along.
>
>
> I remember reading something about this. The conclusion was that we
> perceive speed as a function of aircraft lengths per second. So a large
> jet, such as a 747, seems to be crawling, while a smaller aircraft, such
> as a Lear, will seem to be moving much faster while actually at the same
> speed. Recalculate their speed in terms of aircraft lengths per second,
> and you can see the correlation, since a 747 is several Lears long.
>
> I would imagine you'd experience the same thing if you were to compare
> the visual perception of the speeds of an Amtrak train and a fast car
> travelling at the same rate.
>
> - Nathan

The vehicle length per second is often cited as a reason of why
motorcycle racing is so exciting to watch. The bikes are tiny in
comparison to cars, so they look like they're going even faster than a
car going the same speed. It is quite thrilling to see these racers
pushing the edge while scraping the ground with their knee pucks!

-Aviv

[Peter Duniho]

"C Kingsbury" > wrote in message
ink.net...
> Would that they all came to r.a.p. and asked nicely about what they saw
> instead of demanding immediate satisfaction from equally uninformed
> government officials.

What exactly would a person like Debbie request of the "equally uninformed
government officials"? To pass a regulation prohibiting airliners from
stopping in mid-air?

> You catch more flies with honey than vinegar.

We weren't catching any flies with the honey.

[Dean Wilkinson]

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...


"DM" > wrote in message
...
> Yesterday I saw a cargo jet (a major air express company) come to what
> seemed like a dead stop in midair as it was making its ascent. After
> about 20 - 30 seconds of hanging without dropping out of the sky, it
> continued climbing and apparently did not crash. There's been nothing
> about it in the local news but I've still been very concerned. Could
> someone here explain how such a thing is possible?
>
> Some details: the temperature was about 50 degrees F, the sky was mostly
> clear, and the time was around 0645. I was traveling by car at about 40
> MPH on a street that is parallel to a regular flight path. From this
> street it's common to see 3 or 4 planes per minute either climbing or
> descending; the airport is about a mile or two away from this particular
> street.
>
> As I was moving relatively slowly compared to the how fast the jet
> should have been moving, I noticed that I was gaining on it. I quickly
> eyeballed the area for tall buildings and other geographical reference
> points so I could be sure that I had a good perspective and wasn't just
> "seeing things". The object was either not moving or it was moving
> *very* slowly, and it was not a helicopter. For a few seconds I was
> stopped at an intersection looking at this hanging plane and at the
> people in the other cars around me. No one else seemed to be paying any
> attention to it besides me.
>
> The main reason this bothered me so much is because had the plane
> fallen, it would have landed less than a half mile from where I and
> about 30 other running cars were, in addition to several warehouse-type
> buildings and auto repair garages, plus a 6 or 8 lane freeway filled
> with morning traffic. Since the plane had just taken off and was
> probably full of fuel, and was still low enough for its markings to be
> readable from the ground, the crash probably would have been
> extraordinarily disastrous.
>
> I've done a lot of Googling to try to get an understanding of what I saw
> and really haven't learned anything meaningful. I'm hoping someone here
> can explain how a "regular" jet--versus a specialized military jet--can
> apparently stop in midair and not drop from the sky. As a daily traveler
> near a major metro airport, I'd really like to be reassured that this is
> not a common occurrence.
>
> Debbie

[WRE]

Perhaps the bong you were holding obscured your view.....j/k

"DM" > wrote in message
...
> Yesterday I saw a cargo jet (a major air express company) come to what
> seemed like a dead stop in midair as it was making its ascent. After about
> 20 - 30 seconds of hanging without dropping out of the sky, it continued
> climbing and apparently did not crash. There's been nothing about it in
> the local news but I've still been very concerned. Could someone here
> explain how such a thing is possible?
>
> Some details: the temperature was about 50 degrees F, the sky was mostly
> clear, and the time was around 0645. I was traveling by car at about 40
> MPH on a street that is parallel to a regular flight path. From this
> street it's common to see 3 or 4 planes per minute either climbing or
> descending; the airport is about a mile or two away from this particular
> street.
>
> As I was moving relatively slowly compared to the how fast the jet should
> have been moving, I noticed that I was gaining on it. I quickly eyeballed
> the area for tall buildings and other geographical reference points so I
> could be sure that I had a good perspective and wasn't just "seeing
> things". The object was either not moving or it was moving *very* slowly,
> and it was not a helicopter. For a few seconds I was stopped at an
> intersection looking at this hanging plane and at the people in the other
> cars around me. No one else seemed to be paying any attention to it
> besides me.
>
> The main reason this bothered me so much is because had the plane fallen,
> it would have landed less than a half mile from where I and about 30 other
> running cars were, in addition to several warehouse-type buildings and
> auto repair garages, plus a 6 or 8 lane freeway filled with morning
> traffic. Since the plane had just taken off and was probably full of fuel,
> and was still low enough for its markings to be readable from the ground,
> the crash probably would have been extraordinarily disastrous.
>
> I've done a lot of Googling to try to get an understanding of what I saw
> and really haven't learned anything meaningful. I'm hoping someone here
> can explain how a "regular" jet--versus a specialized military jet--can
> apparently stop in midair and not drop from the sky. As a daily traveler
> near a major metro airport, I'd really like to be reassured that this is
> not a common occurrence.
>
> Debbie

[Morgans]

"Todd Pattist" > wrote

> Give her a break, Peter, she asked politely and responded to
> the answers.

They only break pete deserves, is from you breaking you finger to get him
into the loony bin. He proves time and time again why this feature is on
most mail readers.
--
Jim in NC


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[Morgans]

"WRE" (remove nospam)> wrote in message
...
> Perhaps the bong you were holding obscured your view.....j/k

Oh, that was funny, and useful, too. Not.
--
Jim in NC


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[DM]

Peter Duniho wrote:
> "Todd Pattist" > wrote in message
> ...
>
>>Give her a break, Peter, she asked politely and responded to
>>the answers.
>
>
> Yes, she did respond to the answers, refusing to accept them.
>
>
>>It's something that concerned her, and if GA
>>is to survive, we don't want to be scaring the general
>>public.
>
>
> The general public is already scared. For no good reason. That's my point.
>
>
>>The illusion she encountered is quite overpowering.
>
>
> Most optical illusions are. So what? That's what makes it an *illusion*.
> An easily ignored illusion isn't really much of an illusion.
>
>
>>I look at planes every day. Every week I drive by Stewart
>>where they land the C5A. Its size gives me the same
>>"stopped in midair" illusion every time I see it. It
>>always looks like it's just hanging in midair.
>
>
> But, it is NOT just hanging in midair. It's one thing to ask for expert
> advice to confirm one's intuitive sense that an airplane can't just hang in
> midair. It's yet another to repeatedly reply that the expert advice is
> simply unbelieveable.
>
> I'm getting a little sick and tired of folks who run in a panic every time
> they see an airplane doing something they don't understand. "Debbie" has
> made very clear that this isn't just about her trying to understand a visual
> paradox.
>
> Her *primary* concern is that an airliner might come to a complete stop over
> a populated area, and then having done so, fall straight down and kill her.
> Statements like "it was quite disturbing", "No fun", "I've been very
> concerned", "this bothered me", "the crash would have been extraordinarily
> disastrous", and "I'd like to be reassured this is not a common occurrence"
> make it very clear where she's coming from, and she's not coming from "I'm
> curious about this optical illusion" frame of mind.
>
> Pete
>
>
ouch, knew I shouldn't have check back here. Pete, sweetie, you're
overparsing my words. I wasn't running in a panic, I was just trying to
get an explanation for what I saw. At the moment I was trying every
which way to come up with the easy one so I could just shrug the whole
thing off. I couldn't, and spent several hours researching on the Web
before I finally decided to come here. I respect the professional views
provided, and I'm pretty much over it. I love planes, I love to fly, I
don't wear tinfoil hats. Peace out.

Debbie

[DM]

G.R. Patterson III wrote:

>
> DM wrote:
>
>>interesting, but no, it wasn't particularly windy on the ground.
>
>
> I have the winds aloft forecast for yesterday morning. Where was this?
>
Phoenix

> George Patterson
> If a man gets into a fight 3,000 miles away from home, he *had* to have
> been looking for it.

Debbie

[DM]

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,

http://en.wikipedia.org/wiki/Parallax

and the concept makes sense to me (not to imply that most everyone
else's explanation didn't).

Debbie

[G.R. Patterson III]

DM wrote:
>
> Phoenix

So much for that idea. Winds at 5,000' above sea level were out of the northwest at 5
knots. At 10,000' they were out of the west at 20 knots. It's unlikely that a 20 knot
headwind made much difference in the appearance of the speed of a large plane.

George Patterson
If a man gets into a fight 3,000 miles away from home, he *had* to have
been looking for it.

[Morgans]

"DM" > wrote

> okay! This is clicking for me, even though I know it may just be another
> way of saying "optical illusion". I read this page,
>
> http://en.wikipedia.org/wiki/Parallax
>
> and the concept makes sense to me (not to imply that most everyone
> else's explanation didn't).
>
> Debbie

Another thing to keep in mind, is that airplanes work by the laws of
physics. 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. Given
enough altitude, it can dive, regain airspeed, and start flying again, but
you would have seen that happen, and you did not.

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.

Have a good one!
--
Jim in NC


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[G Farris]

In article >,
says...

>yes, I've imagined the whole trig thing. That's why I was looking at the
> buildings and other higher geographic markers to get a fix on how it
>appeared to be moving relative to those things. Plus, for a moment I was
>stopped and pretty much next to it, although of course I was on the
>ground and it was probably a couple of thousand feet up or less. And, if
>it makes a difference in possible explanations, the plane was headed
>east and I was moving west. I'm 99.999% sure it wasn't moving or was
>moving very slowly.
>

You were moving in opposite directions - This is notoriously the most
difficult movement to perceive. The enormous size of the object is the other
factor that confuses our judgement. Sooner or later you are going to have to
accept the 0.001% and admit that you were victim - just as all of us have been
- to an illusion. It's actually a very common illusion, and if it's the first
time you see it, in your frequent drives near that airport, it very well may
not be the last. You must accept that your eye *cannot* reasonably appreciate
the size of the thing. You cannot know how high it is, or how far away it is
or its position relative to ground landmarks or where it would land on the
ground if it were to fall at that instant.

I have seen such illusions many times - but your suggestion that the plane
was stopped, or nearly so, is quite impossible.

G Faris

[Cub Driver]

bOn Thu, 11 Nov 2004 09:10:35 -0700, DM >
wrote:

>Could
>someone here explain how such a thing is possible?

It is indeed possible for an airplane to hover. I saw a Feiseler
Storch (German liaison aircraft of WWII) sit over a runway without
forward motion. Its stall speed of say 30 mph was the same as the wind
speed.

But what you saw was more likely an optical illlusion. The bigger the
aircraft, the slow it seems to move because it is farther from you,
and it is *very* difficult to know how far an airplane is away from
you. I once watched a slow-moving USAF cargo plane searching for
bodies in the water over an inland bay. It appeared to be motionless
because it was twice as large as the planes I usually see land at this
airport.


all the best -- Dan Ford
email: (put Cubdriver in subject line)

Warbird's Forum www.warbirdforum.com
Piper Cub Forum www.pipercubforum.com
the blog www.danford.net

[Aviv Hod]

DM wrote:

>> Her *primary* concern is that an airliner might come to a complete
>> stop over a populated area, and then having done so, fall straight
>> down and kill her. Statements like "it was quite disturbing", "No
>> fun", "I've been very concerned", "this bothered me", "the crash would
>> have been extraordinarily disastrous", and "I'd like to be reassured
>> this is not a common occurrence" make it very clear where she's coming
>> from, and she's not coming from "I'm curious about this optical
>> illusion" frame of mind.
>>
>> Pete
>>
> ouch, knew I shouldn't have check back here. Pete, sweetie, you're
> overparsing my words. I wasn't running in a panic, I was just trying to
> get an explanation for what I saw. At the moment I was trying every
> which way to come up with the easy one so I could just shrug the whole
> thing off. I couldn't, and spent several hours researching on the Web
> before I finally decided to come here. I respect the professional views
> provided, and I'm pretty much over it. I love planes, I love to fly, I
> don't wear tinfoil hats. Peace out.
>
> Debbie

Debbie,
I had a feeling that Pete was overreacting, and I'm glad that you
found the time to respond and clarify your frame of mind. Yes, in fact
I think that most folks on this group are VERY glad that you politely
asked a good question about aviation in one of the most appropriate
newsgroups there are about the subject.

Please don't let some people's overreactions turn you off from this
generally friendly but sometimes rowdy bunch. If you have any more
questions about aviation, or if you just want to learn more about this
whole miraculous invention of flight, I invite you to follow us for a
while, tune out the negative comments, and join the conversation once
again! Who knows, maybe we can convince you that everyone owes it to
themselves to take an introductory flight if they haven't already done
so. If you love planes and you love to fly, you definitely should. A
good spot to start is www.beapilot.com

I hope you join us for a little while longer!

-Aviv

[alexy]

Todd Pattist > wrote:

>DM > wrote:
>
>>ouch, knew I shouldn't have check back here. Pete, sweetie, you're
>>overparsing my words.
>
>Debbbie, I'm glad you didn't let Peter get to you. He's one
>of the resident "sweeties" here who has been coaxed by the
>semi-anonymity of Usenet to ease off the polite social norms
>just a bit farther than most.

Nicely put. And while it was good to see the chivalrous rush to defend
the "damsel in distress", Debbie's response to the isolated rude
response sounds like someone who can take care of herself in a usenet
"dust-up"!
--
Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently.

[Robert M. Gary]

DM > wrote in message >...
> Yesterday I saw a cargo jet (a major air express company) come to what
> seemed like a dead stop in midair as it was making its ascent. After
> about 20 - 30 seconds of hanging without dropping out of the sky, it
> continued climbing and apparently did not crash. There's been nothing
> about it in the local news but I've still been very concerned. Could
> someone here explain how such a thing is possible?

If the plane did get too slow ( a very unlikely possibility) the nose
would drop and the plane would accelerate again. That would assume the
pilots ignored the clax horn, the shaking controls and the indicator
on the airspeed.
I grew up in a military environment and often saw very large planes
(like giant C-5 cargo planes) coming and going. They always looked
like they were stopped in midair. The huge size makes your brain think
they can't be that big and therefor must be closer, smaller, and
moving slower than they really are.

-Roebrt

[Robert M. Gary]

"Dean Wilkinson" > wrote in message >...
> 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...

Are you sure? Parallax is when you try to show a student how to make a
coordinated turn and you end up with the ball out to the left because
you aren't looking at the turn coordinator straight on.

-Robert, CFI

[Darrell S]

That's probably right. An optical illusion. One night in the late 1950s I
was flying back from Las Vegas to Laredo in a T-33. We were around 30,000'.
I was in the front seat and saw a saucer shaped red/orange object way out at
11 o'clock coming straight at me. (around this time there had been numerous
flying saucer reports in Texas). I pointed it out to the pilot in the back
seat and we both became alarmed. It maintained shape and increased in size,
indicating it was moving right towards us.

Just before I might have taken evasive action the "saucer" lost its disk
shape and we could see it was the moon rising between cloud layers. Since
we were looking through lots of atmosphere horizontally, it had a red/orange
color. When it first rose above the lower cloud level it was a small disk.
As it continued to rise we saw more of the moon, still in the disk shape
which produce the illusion it was moving straight towards us. Once more
than half the moon was above the cloud layer we could see it wasn't a disk
at all but the top of a round moon rising.

--

Darrell R. Schmidt
B-58 Hustler History: http://members.cox.net/dschmidt1/
-

"DM" > wrote in message
...
> DM wrote:
>
>> snipped
>
> thank you to everyone who replied. I've been considering all the
> explanations offered and appreciate the information. I'm pretty close to
> accepting the "optical illusion" idea but am not quite there yet. I was
> looking hard at that plane because I really didn't want to believe what I
> was seeing. If it weren't for the fact that I could read the name of the
> carrier on the plane, I'd probably be going for a UFO explanation!
>
> Fly safe everybody.
>
> Debbie

[Peter Duniho]

"Todd Pattist" > wrote in message
...
> Debbbie, I'm glad you didn't let Peter get to you. He's one
> of the resident "sweeties" here who has been coaxed by the
> semi-anonymity of Usenet to ease off the polite social norms
> just a bit farther than most.

Oh please. You haven't got a clue regarding the effect of "semi-anonymity
of Usenet" on me. Get off your damn high horse.

If some crackpot came up to me in person and told me that she'd seen an
airliner stop in mid-air, and oh-my-gosh she sure hopes THAT doesn't happen
too terribly often, and then I explained to her that it was simply an
optical illusion, and then she continued on insisting that
oh-no-I-know-what-I-saw-the-airliner-DID-stop-in-mid-air, I would say
straight to her face exactly what I wrote here.

If you knew me as a friend in person, you would not write something so
foolish.

It's great you have more patience with people scared to death of airplanes
for no reason, but that doesn't mean you have the inside scoop on why I say
the things I do. Frankly I'm sick up putting up with them, and given the
frustration I experience from people like this on a reasonably regular
basis, what I wrote here has been quite mild indeed.

Pete

[Dean Wilkinson]

Parallax is the scientific term for relative displacement of objects
observed at different distances. Parallax is used often by astronomers to
ascertain the distance of objects within our own solar system, and even
nearby stars by observing them from different points in the earth's orbit
around the sun.

I the case of the turn coordinator, the reason the ball appears off center
when observed from the side is due to parallax. This does not mean that
this is the only instance in which the concept of parallax is applied. The
original poster described another circumstance that results from parallax;
i.e. a moving object appearing to be stationary when compared to the
background when the observer is also moving at a rate and in a direction
that allows the parallax to create the illusion that the airplane is
stationary when in fact it is not...

I just wanted to point out to the group that this is a well known phenomenon
that has a scientific term to describe it.

Dean Wilkinson

"Robert M. Gary" > wrote in message
om...
> "Dean Wilkinson" > wrote in message
>...
> > 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...
>
> Are you sure? Parallax is when you try to show a student how to make a
> coordinated turn and you end up with the ball out to the left because
> you aren't looking at the turn coordinator straight on.
>
> -Robert, CFI

[mhquay]

DM wrote:
> *Yesterday I saw a cargo jet (a major air express company) come to
> what
> seemed like a dead stop in midair as it was making its ascent. After
> about 20 - 30 seconds of hanging without dropping out of the sky, it
> continued climbing and apparently did not crash. There's been
> nothing
> about it in the local news but I've still been very concerned. Could
> someone here explain how such a thing is possible?
>
> As a daily traveler
> near a major metro airport, I'd really like to be reassured that this
> is not a common occurrence.
>
> Debbie *

Hi Debbie

The human mind is a funny thing. It sometimes sees what it wants to
see. Your first thought of "Gee, that aircraft is not moving"
compounded by all the other reasons given on other postings is why you
perceived something that was not happening.

I can remember a time when I crewed on a yacht on an ocean race up the
coast of Western Australia. Prior to the start we were advised of the
hazard of a number of naval vessels engaged in exercises in the general
vicinity of the course we were to track.

About 24 hours into the race the crew on watch reported a ship ahead.
(It was broad daylight and excellent visibility) Not long after one of
the crew stated that it was coming toward us. Ten minutes later another
one of the crew claimed "Yes, it looks like a war ship." Ten minutes
later yet another crew member said it was an aircraft carrier. A
further ten minutes and all on board had confirmed it was an aircraft
carrier and in fact we could clearly see the aircraft parked on the
deck.

When a rusting old oil tanker passed about 2 miles abeam we were all
quite shocked at the illusion our own minds had created.

The lesson I learnt from this:
Trust logic - not what you think you see.
Trust your instruments - not what you feel is right
Seeing is NOT believing.

Phil


--
mhquay
------------------------------------------------------------------------
Posted via OziPilots Online [ http://www.OziPilotsOnline.com.au ]
- A website for Australian Pilots regardless of when, why, or what they fly -

[Peter Duniho]

"Todd Pattist" > wrote in message
...
> To give you your due, I find most of your posts truthful and
> accurate. You're not particularly tactful or patient,
> however.

You'd be surprised at how patient I am with someone who is willing to admit
that they need information and is asking for it. You would probably NOT be
surprised at how IMpatient I am with someone who claims to need information,
and then starts arguing with a person who attempts to provide that
information.

[Tlewis95]

As a glider pilot I get a lot of questions and people freaking out!. For
instance, when a friend landing a glider in a field due to loss of thermals and
not being able to make it to an airport ( a perfectly common thing in
sailplanes, ive landed in fields 7 times) the person driving on a nearby road
claimed an airplane "crashed" in the field. Soon the NTSB and several police
cars were at the site. The glider was undamaged, the pilot was eating an aplle
under the gliders wing, and the crew arrived 10 minutes later with the trailer
and in 30 more minutes they were all gone.

Just shows how basic (very basic) aviation should be taught in public schools.

See ya

Trace Lewis

[G Farris]

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

[Dean Wilkinson]

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

[G Farris]

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

[Robert Briggs]

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 ...

[Morgans]

"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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