Airplane moonlight reflection - physics, or optical illusion?

[QuikSand]

I did a quick search online for information on this, but struck out. If your google-fu exceeds mine and you can just link me up, great.


I was flying last night on an eastbound plane, during a fairly clear night with a fairly bright moon. As I looked out my window, I noticed a pretty obvious and bright light moving along the ground where I had been looking. At first, I assumed it was some sort of low-flying surveillance helicopter (i.e. the sort of thing used every so often when there's an evening public safety "event") but it then became clear that the light was moving along with our plane, and did so for a long time.

So, I'm not a science guy, exactly, but spent enough time contemplating it that I feel fairly comfy with physics and related stuff. However, I can't get this out of my head. (Yes, I know what you're thinking here... "magnets, how do they work?")

My plane is at its full cruising altitude, I presume 30,000 feet, give or take for a 4-hour trans-continent flight. The airplane is actually painted blue and red, not the most reflective colors, even, in case that mattered. I am seeing a bright light that seems to briefly illuminate specific buildings and blocks of the ground level. I just don't see how that works, at all. I would expect any moonlight reflection from the plane to become so completely diffuse over a period of *miles* that it wouldn't make any effect anywhere, period.

So, was I seeing some sort of optical illusion? That the ground wasn't really being lit up as it appeared? Or is it really true that moonlight can bounce of an airplane and create an apparently bright and fairly focused light miles and miles away, that covers the ground at hundreds of miles an hour?

If it's the latter... were I looking for it, would I notice this from the ground as just a momentary glint of light, and that's it? And that would explain my secondary question about "why aren't people constantly bugged about about these high-intensity searchlights constantly streaming across their lawns and neighborhoods?"

I'm puzzled by what seems to be the best answer to this. Anyone have anything?

[sabotai]

Originally Posted by QuikSand

So, was I seeing some sort of optical illusion? That the ground wasn't really being lit up as it appeared? Or is it really true that moonlight can bounce of an airplane and create an apparently bright and fairly focused light miles and miles away, that covers the ground at hundreds of miles an hour?

Well, I guess it could if it hit a reflective surface on the plane at the right angle. Did the plane fly in a consistently straight line? Did the light turn with the plane if it did change direction?

Where was the light? Was it next to the plane? In front of it?

How focused was the light? Are you sure it wasn't just the moonlight reflecting off the ground?

[dacman]

If it was moonlight reflection, and I'm not saying it was, I would suppose that it was the light reflecting off the ground and up to the plane, not vice versa. I've seen that effect. At night with a strong moonlight, it can be pretty strong.

[Mustang]

I think you missed a great opportunity to do some reflectoporn at 30K feet.

[Passacaglia]

Originally Posted by Mustang

I think you missed a great opportunity to do some reflectoporn at 30K feet.

No way. You do *not* want to get banned from an airplane at 30K feet.

[DanGarion]

You sure it wasn't just a reflection in the window...?

[QuikSand]

Okay, here's my best layman's description of what I was seeing:

As the plane was flying, I was looking out a window from the first row of the plane, on the starboard/right side. Looking down at the ground, as we flew over cities, the lighting of a streetlight or a house was only visible at a fuzzy object - in other words, we were too high up to see much detail.

However, at one point I noticed a light that was traveling along with the plane (took me a bit to realize as much) that appeared to be a bit behind us and maybe a couple of miles south of (away from) us, but moving at the same speed. What I saw was not a light that seemed to be pointed at me -- but a light that seemed to be pointing *downward* at the ground. As I watched it, it seemed like it covered roughly a block or territory at a time, and made that area look genuinely bright for the instant it was located there. Hard to describe it very well, I suppose, but once I noticed it, the area illuminated by this light at any moment was *clearly* a notch or two brighter than, say, a retail outlet with its light on (which was only fuzzily lit from my viewpoint). The light visible to me was very clear, but was also very focused -- there wasn't a gradual blurring and dropoff of intensity to zero -- it was a lot more like a searchlight. It was as if a giant and sharply-focused searchlight was being held over the ground, and moved block by block across the city, at the speed that the plane was traveling. A couple times, as the light crossed over a large building, I'd see a distortion in its shape, presumably as it picked up the contours of the building or objects in its focus at that moment.

That's about the best I can do with my description.

[QuikSand]

Originally Posted by DanGarion

You sure it wasn't just a reflection in the window...?

No, I'm not really sure of anything. I did the relatively obvious things like move my head/eyes to see if there was a dramatic change in the appearance (what I'd expect if I were actually getting a reflection from inside the plane, or even from something on its exterior/wing) but from any angle it looked just the same -- like someone was a couple hundred feet off the ground shining a bright searchlight onto the ground, right alongside our plane.

[EagleFan]

Sure, the supercomputer can't fiure it out and we're supposed to...

[JediKooter]

I'm thinking it may just be the moon light. Much like if you have a large body of water and there's a full moon. Regardless of where you are located, speed you are going and what altitude you are, there will always be a very 'focused' beam of moonlight on that body of water and it will appear to be moving with you.

Let me see if I can find a picture of what I'm talking about...be right back


Edit: Here we go. The moonlight is shinning everywhere, but, from our perspective, it looks like it's focused in a narrow band. If there was a lot of moisture in the air, I think that would also help with the effect.

[Mustang]

Originally Posted by DanGarion

You sure it wasn't just a reflection in the window...?

[sabotai]

I think a "block" sized area is too small to be moonlight reflecting off the ground, since it was a city and not water. My only guess would be that the moonlight was reflecting off of something on the plane (not the whole plane itself). Not sure what one the plane would be reflective enough for that effect, though (I'm not a physics expert nor an airplane expert), but if it was small, just a slight change in the plane's direction (so slight you may not even notice it) or even the motion of the moon could make the light appear to "turn off".

[JediKooter]

I don't think the ground is going to reflect as much of the moonlight as water would, so a block size doesn't seem to be unreasonable. Unless the belly of the plane was polished or unpainted (like American Airlines), I can't see it being a reflection from the plane at 30,000 feet. The light would be too diffused by the time it reaches the ground and would be practically canceled out by the moons own light. I am guessing though and this is actually pretty fun trying to figure this out!

[sabotai]

Originally Posted by JediKooter

The light would be too diffused by the time it reaches the ground and would be practically canceled out by the moons own light.

QS seems to be describing a pretty focused beam of light (if it's coming from the plane), so it wouldn't be "diffused" (that's not the right word to use....but I can't think of what the right word is....) by the time it reached the ground.

I'm imagining something similar to Iridium flairs. Here's a video of one: http://www.youtube.com/watch?v=1PFUP5LPyuA

That's caused by sun light hitting the antenna on the satellite - well, 3 of them, 188 cm wide x 86 cm long x 4 cm thick each, and from the perspective of the satellite, the bright "spotlight" would be constant (at least for awhile until the angle changed or it moved out of the sun's light). If there's something far less reflective, but similar, on the airplane, moon light could cause something like that but much dimmer.

But yeah, I have no idea if anything on an airplane could do a much smaller but similar effect. It's pure speculation.

[JediKooter]

Originally Posted by sabotai

QS seems to be describing a pretty focused beam of light (if it's coming from the plane), so it wouldn't be "diffused" (that's not the right word to use....but I can't think of what the right word is....) by the time it reached the ground.

So more like a spot light then? Kind of like what police helicopters have. I know the landing lights are pretty bright, but, they wouldn't be on at 30K, just the ones on the wings would be on.

I'm imagining something similar to Iridium flairs. Here's a video of one: http://www.youtube.com/watch?v=1PFUP5LPyuA

That was pretty cool looking. It actually looked like it was coming right at the person with the video camera.

That's caused by sun light hitting the antenna on the satellite - well, 3 of them, 188 cm wide x 86 cm long x 4 cm thick each, and from the perspective of the satellite, the bright "spotlight" would be constant (at least for awhile until the angle changed or it moved out of the sun's light). If there's something far less reflective, but similar, on the airplane, moon light could cause something like that but much dimmer.

But yeah, I have no idea if anything on an airplane could do a much smaller but similar effect. It's pure speculation.

So that opens up a couple of questions. 1. How long was the light 'pacing' the plane and 2. what was the intensity of the light that was hitting the ground?

As for intensity, something like this?

[Sun Tzu]

Does anyone else keep reading the title of this thread as "Albion moonlight reflection" ???

[Bad-example]

I called a pilot friend and asked him about this. He says that he has seen something similar and attributed it to reflected moonlight.

My suggestion that aliens existing as lightwaves could have been shadowing the plane didn't impress him.

[cartman]

Not sure if this linkage helps:

The Eye and Night Vision

VISUAL ILLUSIONS AT NIGHT

Reduced visual references, due to low ambient light levels, can lead to several types of visual illusions that may cause spatial disorientation. There are two forms of visual processing, central (foveal) and peripheral (ambient). Whereas central vision is predominately concerned with object recognition and discrimination, peripheral vision provides motion detection and spatial orientation information. At night, under reduced illumination levels, the normal peripheral vision cues may be degraded or absent, and spatial disorientation may be more difficult to overcome. Spatial disorientation may arise from labyrinthine, proprioceptive, or visual mechanisms.

Autokinetic Effect

Autokinesis, or the autokinetic effect, is the phenomenon of perceived movement exhibited by a static dim light when it is stared at in the dark. This effect can be demonstrated by staring at a lighted cigarette in a dark room. The dim light will appear to move about, even though it is stationary. Although the exact cause of this illusion is not known, it is related to tiny fixation movements of the eye and the loss of the surrounding references which normally stabilize visual perception. This illusion will also occur when there are two dim lights in darkness, but it normally disappears when three or more lights are present. Pilots on night flights have mistaken stars or ground lights for other aircraft and have become disoriented, with fatal consequences. Therefore, pilots should be aware of this visual illusion. The autokinetic effect can be reduced by maintaining good visual scanning techniques rather than staring at the light source, or by increasing the intensity of the lights, if that is possible.

Black Hole Illusion

The black hole illusion may occur on a dark night over water or unlighted terrain where the horizon is not easily discernible. The worst case occurs when there are no visual references except for runway lights. Without peripheral cues for orientation, the pilot tends to perceive that his/her aircraft is stable, but that the runway itself is out of position, usually sloping down. The black hole illusion makes the landing approach dangerous, and often results in a landing far short of the runway. A particularly hazardous type of black hole approach occurs when the earth appears to be totally dark except for the runway and the lights of a city on rising terrain beyond the runway. By maintaining a constant vertical visual angle on the distant city lights, the pilot's approach may fall below the intended glideslope as the aircraft gets closer to the runway.

Distance Illusion Phenomenon

The distance illusion phenomenon is a dangerous illusion (DIP) that can occur when one aircraft is trailing another in a black hole environment with few peripheral vision cues. A likely scenario for the DIP occurs the trailing aircraft pilot, for position reference, places the image of the lead aircraft on a certain spot in his canopy. The pilot orients the lead aircraft in exactly the same spot on the canopy, but, over time, inadvertently falls back to a greater separation distance. If the trailing aircraft started out 2 nautical miles behind and 300 feet below the lead aircraft and then fell back to 4 nautical miles, that would mean that the trailing aircraft was now 600 feet below the lead. If the lead aircraft gradually descends to 600 feet or less, the trailing aircraft may impact the ground. The potential mishap exists only if the pilot maintains the lead aircraft in exactly the same spot on the canopy, but fails to monitor the actual altitude or fails to realize that the separation distance has increased.

False Perceptions

Pilots are especially susceptible to misperception of the horizon while flying at night. To a pilot, isolated ground lights can look like stars and create the false impression of a nose-high pitch or wing-low attitude. If no stars are visible because of overcast conditions, unlighted areas of terrain may blend with the dark overcast to create the illusion that the unlighted terrain is part of the sky. An extremely hazardous takeoff occurs over an ocean or other body of water that cannot be distinguished visually from the night sky; pilots who have falsely perceived that the shoreline receding beneath them was the horizon have met with disastrous consequences.

At night, if runway lights viewed from an approaching aircraft are displaced laterally, the pilot may have the impression that the runway is closer than it really is, because it appears wider. This false perception may result in an early flare and a tendency to land short. Also, the intensity of runway lights may appear to vary, depending on their color and the adaptation states of the eyes. These differences in the brightness of runway lights may lead to false perceptions regarding altitude.

Pilots are aware of certain vestibular illusions that may be more difficult to overcome during nighttime operations. Somatogravic illusions are false perceptions of the body's orientation to gravity. Somatogyral illusions are experienced by pilots during maneuvers of sustained angular motion such as coordinated turns, spins, or rolls. These illusions result from the inability of the semicircular canals of the inner ear to register sustained angular velocity. The lack of visual cues at night may make these illusions more troublesome.

[Glengoyne]

Was John Travolta piloting the plane? On Board?

[PilotMan]

I feel so lonely and unloved. Nobody thinks to ask me this stuff. *sniff*

On a bright night, ie. the moon is big and it's relatively clear. You can see something like this.

It's the reflection of the moon off of the earth, and reflected back to you in the airplane. It appears to move fast because you are moving fast, and even though your perception is changing because you are moving there is still plenty of moonlight to reflect back up to you. Moonlight is everywhere, not just where you perceive it to be at that particular moment.

It is especially noticeable over water, for example, during the flooding in Tennessee recently, you can really see how much water was down there by seeing the light reflected back to you from the moon.

I hope this is what you were looking for, I have experienced what you have described before.

[EagleFan]

I think we just got our answer. Thanks PM.

[Subby]

This thread is an excellent example of what is awesome and what is absolutely brutal about FOFC.

[JonInMiddleGA]

Originally Posted by PilotMan

I feel so lonely and unloved. Nobody thinks to ask me this stuff. *sniff*

I thought of you pretty much immediately, but it wasn't my thread to title

[JediKooter]

Sounds like PilotMan and I are in agreement then.

[sabotai]

Weird that the moon's reflection would make such a bright spotlight on the ground like that, but that's why stuff like this is so interesting.

[JediKooter]

[QuikSand]

Thanks for the response. Very weird explanation, but it's by far the best I've got.

[AlexKintner]

Just joined this forum specifically to respond to this thread.

I experienced the EXACT same phenomenon last night (12/17/2023) on a flight from Las Vegas to Tampa. I too initially thought I was seeing a helicopter far below the aircraft shining a spotlight towards the ground. I too soon realized this was impossible as the light was travelling the exact same trajectory and speed as the airplane. It was to the south and slightly behind the airplane and was visible for at least 5 minutes.

The light was very focused and very bright. Last night was a 25% waxing crescent... not sure about the moon explanation.