Limits on human perception, and how they influence UFO sighting reports
To: Common Errors UFO Witnesses Make
The human visual system is primarily adapted to simple close-up work. It fails miserably when someone tries to do any of
the following tasks:
- The visual system can't determine the actual size, actual distance, actual speed, or actual altitude of an unknown
object. Human vision needs some of the following clues to accurately report any of these values:
The values given above are the maximum ranges observed experimentally. Many witnesses cannot achieve the given values,
either because their visual systems are below average, or because they don't have enough experience to use the clues
- Lens Accommodation: The object must be within 20 feet of the witness. People with presbyopia lose the ability to make
this test, and glasses cannot restore it.
- Eyeball convergence: The object must be within 30 feet of the witness, and the witness must have two good eyes. Many
people can't do this beyond 20 feet.
- Parallax: The object must be within 250 feet of the witness, and other objects at known distances must be in the
same central vision field. The witness must have two good eyes.
CAVEAT: Witnesses tend to place the UFO at the same distance as a nearby landmark is at, if they are unable to use
parallax correctly due to a substantial angle between the landmark and the UFO.
- Perspective: The object must be visibly in contact with a surface that maintains a perspective order to the objects
on it (e.g. the ground). The maximum distance depends on the contour of the surface, and on the periodicity of known
objects on the surface, but a mile is achievable with an adequate number of objects on flat ground, and many miles can
be observed accurately from certain high places.
CAVEAT: The sky cannot be considered as a perspective surface, since nothing can touch it at a known place. Many
visual illusions are caused by the sky appearing to be a plane surface.
CAVEAT: Faulty perspective effects can also cause the witness to overestimate the angular size of a UFO.
- Scattering: Intervening air scatters light into the path of observation, causing the object to look lighter and
bluer than it is. Unfortunately, if the true color of the object is unknown, then this effect is not very useful.
- Overlap: If the object passes in front of objects at known distances, and behind objects at known distances, then
the estimate of actual distance can be constrained to be between the known values.
CAVEAT: Halation from a very bright object can appear to overlap a nearer object. This happens because halation
occurs within the retina of the eye, inside photographic film, or within a TV camera's sensor array.
- Cast shadows: If a known light source causes the object to cast a shadow in a known location, then the location of
the object can be estimated.
CAVEAT: Remember that the sun is at infinity for the purpose of measuring shadows.
- Light propagation: If the unknown object is a light source, the way its light shines on other objects and casts
shadows can be used to locate its position relative to the other objects.
CAVEAT: Some other source may have actually shone the light on the other objects.
- Familiarity: If the observer knows the exact identity of the object, then its true size is in mind. The observed
angular size than gives him a rough (well within one order of magnitude) idea of distance, speed, and altitude. This
is how trained observers can spot airplanes and report their distances, speeds, and altitudes over a distance of several
CAVEAT: This training cannot extend to unknown objects, since the true size is unknown. It only works with familiar
CAVEAT: If the observer guesses at the identity or size of the object, this guess then distorts
the impressions of size, distance, speed, and altitude in all further observations, unless something occurs to strongly
contradict the guess.
CAVEAT: UFO witnesses tend to guess a size in the range of a human carrying vehicle.
- Instrumentation: If the observer has a device that can find distance by widened parallax (or widened convergence),
it can increase the distance that can be measured.
CAVEAT: This still requires objects of known distance in the visual field, unless instruments and calibrations for
convergence are present.
- Multiple Witnesses: While each witness can form an individual estimate of size or distance that is way off, if they
are far enough apart, and if their observations can be reconstructed to be synchronous in space and time, the object
can be accurately located.
CAVEAT: Once the true position is found, this does not mean that the visual impressions of size or speed can
simply be scaled to get the true values.
- The visual system can't measure angles without instruments. This is probably the greatest failing of human perception:
- The visual system tends to overestimate acute angles, and underestimate obtuse angles.
- In environments of perspective, the visual system tends to look at the trigonometric tangent of the angle instead
of the angle itself. This is one of the causes of the horizon moon illusion.
- If the observer was concentrating on an object, it is likely that its angular size will be overestimated later.
- Angular size, unless measured at the time of the sighting with an extended hand, is very difficult to reconstruct
later. People tend to overestimate it, especially if they are indoors at the time they try to do it. The exception is
if the witness returns to the scene to reconstruct the angular size. Then they are more accurate.
Question: How big do you think the angular size of the rising full moon is? Which object would you guess could be
held at arm's length from your eye and positioned to just be able to cover up the moon?
- A grain of sand?
- A pea?
- A dime?
- A quarter?
- A baseball?
- A basketball?
Take your pick. The answer is at the bottom of this page.
- The visual system has great difficulty reconstructing at a later time visually observed sizes, angular sizes,
lengths, complex shapes, angles, and colors.
- The visual system can't measure light intensity very well at all. Humans can do this only relative to the
background illumination. Notice the following effects:
- The eye goes through two stages of accommodation to low light levels. The first is the opening of the iris to let
more light in. The second (which takes about 15 to 30 minutes to work) is the increase of sensitivity in the rods, due
to the increase of rhodopsin. Modern people rarely experience the second stage, and some are startled when they can see
in what they think is total darkness.
- By contrast, the eye adjusts in a few seconds to brighter light.
- A very dim light source can seem to be extremely bright at night, especially if there are no other lights close
- When the visual system overestimates the size and distance of an unknown object, it also overestimates the true
brilliance of any light on the object.
- Intense light striking the retina scatters in it, making the visual image of a very bright object appear much
larger than it should be. This is called halation.
- The visual system is very inaccurate at comparing colors separated by space, background, or time.
- Color perception at night is very different from daytime color perception.
- Color saturation of bright objects seems higher at night, especially in small light sources.
- Night vision is monochromatic, unless localized light sources shed their own colors intense enough to be seen.
- In total rod vision, blues, greens, and whites seem to be about the same lighter color of white. Reds and yellows
appear darker than normal.
- Color perception tends to be relative to the overall color of the visual field.
- Two identical patches of color, placed against different backgrounds, can appear to be two very different colors.
- The human visual system is very hard pressed to perceive correctly any of the patterns commonly called optical
- When presented with a stimulus, the visual system tends to assign a familiar reason for an effect rather than one
that is rarely seen. Here are some examples:
- An object that is expanding is often taken to be approaching instead. Likewise, an object that is contracting will
likely be seen to be receding instead.
- A small object that is brightening is often taken to be approaching instead. Similarly, a small object that is
dimming will likely be seen to be receding instead.
- An object that is closer to the horizon in the visual field is often taken to be farther away than an identical
object that is higher or lower.
- The visual system tends to see a stationary point source of light on a uniform background as moving about its
location in fitful jerks. This is called autokinesis.
- The visual system tends to see a uniformly moving point source of light on a uniform background as moving along in
fitful jerks (called autostasis), or tracing a wobbly course.
Many cases are unsolved because the investigator puts too much stock in the size, the distance, the speed, or the
altitude reported by witnesses. When these are discounted as being the very inaccurate values they are, many times the
solution is immediately apparent.
Our hearing works well under ordinary circumstances, but is taxed under unusual circumstances. Here are some of the
effects that are infrequently encountered:
- Very low frequency and very high frequency sounds are hard to localize in direction. A low humming sound could be
coming from any direction.
- Sine waves are harder to localize than other sounds.
- Hearing gives only an approximate direction of a sound (within a few degrees). It can't pinpoint the sound with
accuracy, nor can it give any indication of distance to the sound source.
- Reflections from nearby surfaces can alter the perceived direction of a sound. So can sound absorbers and other
materials that affect sounds.
- Multiple sources of the same sound can cause the sound to appear to come from an entirely different direction.
Stereophonic sound is an example of this.
- Multiple sources of different sounds can cause sounds to appear to come from entirely different directions, or can
blur the locations of some of the sounds.
- There is really no way to determine if a sound came from a UFO or from some other source in the area. Many reports
are puzzling because a cause for the sighting was easy to find, except that the sound doesn't match the cause. But if the
sound came from some other cause, then the case is easy to solve.
- Sometimes several familiar sounds overlap as they reach our ears, to produce a strange sound. This is especially true
if the sounds beat against each other to produce a new frequency.
- Preconceived notions can make a case harder to identify. Have you ever heard of a noisy balloon? If not, a manned
hot-air balloon can be hard to identify, since it gives off an intermittent loud roar that is directed mostly straight
Human Sense of Smell
The human sense of smell is very weak compared to that of other animals. It is nondirectional, and in most cases,
detects whatever the wind brings in. Here are some common misperceptions:
- The witness assumes that the smell came from the UFO. Smells can come from many mundane activities. The UFO (or its
exhaust, if any) must be upwind of the witness to be smelled.
- Different witnesses will report different named scents for the same unfamiliar smell.
Other Human Senses
The other human senses also can cause either misperceptions or special effects. Here are a few common effects:
- Prickly or tingling sensations can be caused by things other than effects created by a UFO. Among them are:
- Staying in one position too long
- Readiness for something to happen
- Fear of the unknown
- Heightened alertness
- Realization that something important may be happening
- Perception that something strange may be happening
- Static electricity
- Allergic reactions
- The above states can also cause sudden feelings of temperature change, lack of balance, sweating, or
- Sudden jerky pains that go away quickly can be caused by muscle spasms during periods of heightened alertness or
- If the witness has convinced himself that the UFO is a space ship, the witness can then have the feeling of being
watched, or the notion that the UFO is reacting to his or her presence or actions. Random fluctuations in the light
emitted by a fire balloon have been interpreted by witnesses as "answers" to
signals made with a flashlight or thoughts the witness had.
- Animals may react to any strange light in the sky (especially a fire). But animals also react to any agitation in
the behavior of the humans that care for them.
- Many UFOs are not noticed until something goes wrong with a piece of equipment (such as a power tool or a car). Some
questions come to mind:
- If the equipment hadn't failed, would the witness have seen the UFO?
- Could the presence of the UFO just be a coincidence, since it was the failure that (in effect) made the witness
- In the case of a power failure, did the UFO cause the failure, or did the failure cause the UFO? The latter is true
in many cases.
On February 2, 1993, people reported a UFO over Jefferson County, Kentucky. The police dispatched a helicopter to
investigate. The following effects were reported by the helicopter crew:
- The object looked like a huge ball of fire.
- The object was larger than the helicopter.
- The object made "ramming attacks" at the helicopter as it tried to get close enough to identify it.
- The object changed brightness erratically.
People watching from the ground reported a different scenario:
- The object and the helicopter appeared as lights only.
- The object was smaller than the helicopter.
- The helicopter repeatedly went past the object. One witness reported that the helicopter didn't seem to be able to
find the UFO, since it kept going right past it.
- The object changed brightness erratically.
The report of this incident in the paper the next day caused a man to come forward, saying that he had launched the
object the previous night. It was a fire balloon made of a plastic dry cleaner bag, with a
structure made of soda straws at the bottom that held some candles. He had launched it to observe wind direction at
various altitudes, and had no idea that his balloon would cause so much trouble.
So how did a small fire balloon cause the effects the helicopter crew witnessed?
- "The object looked like a huge ball of fire." The crew overestimated the size of the fire balloon. This
is a common effect, because people tend to think of any self-luminous flying object as being controlled by men. It looked
like fire because it had fires inside it.
- "The object was larger than the helicopter." This shows the limitations of human vision clearly. The crew
made a wrong guess as to what kind of object they were witnessing. They assumed it was a vehicle capable of carrying a
man. This made their estimates of size, distance, and speed too large.
- "The object made 'ramming attacks' at the helicopter as it tried to get close enough to identify it." Since
the estimate of distance was too large, when the pilot tried to get near the object, the helicopter quickly overshot it.
Then the pilot turned the helicopter around, found the object, and gave chase again. Since they were expecting a larger,
more distant object, they interpreted the relative motion they saw as a ramming attack, rather than their own motion
past a nearly stationary object.
- "The object changed brightness erratically." Fire balloons do that.
This shows that even highly trained people can make huge errors in judgment of the size of an object in the absence of
other clues to its true location. Those policemen were not at fault in any way. Everyone has the same limitations.
Nobody can accurately judge the size, distance, speed, or altitude of an unknown object without
additional information. No amount of training can alter this fact.
So the next time a UFO witness gives numeric values for the size, distance, speed, or altitude of an unidentified
flying object, the next question to ask is: "How do you know that value?"
- UFO PAGE
- EFFECTS THAT MAKE UFOS
- UFO MANEUVERS
- WHY UFOS ARE NOT IDENTIFIED
- UFO POWER, COINCIDENCE, OR OTHER CAUSAL LINK?
- Pages on LIGHT BULBS, SPECTRA, and HUMAN VISION
Here is a list of the common errors UFO witnesses (and investigators) make when trying to perceive or report a UFO
UFO PERCEPTION ERRORS:
- Assuming a "reasonable" size or distance when vision can't determine the actual value.
- Assuming that lens accommodation or eyeball convergence can work farther away than 30 feet.
- Assuming that parallax can work against a uniform background.
- Assuming that parallax works when the reference object is not in the central vision field at the same time.
- Placing the UFO at the same distance as a nearby landmark.
- Assuming that the sky is a plane surface, and attempting to measure distance with that surface through perspective
- Overestimating angular size due to use of the sky as a perspective surface.
- Confusing scattering effects with the true color of the object.
- Discounting the effects of halation when determining whether an object crossed in front of or behind something.
- Assuming that an angular size estimate, a true size estimate, a distance estimate, a speed estimate, an angular
velocity estimate, and/or an altitude estimate must all fit together for the same object at the same distance. Humans
make inaccurate estimates.
- Reporting an angular size or a shape seen as a result of halation.
- Assuming that the UFO is the only light source casting shadows.
- Guessing that a UFO has one particular identity, and assuming a size, distance, speed, and/or altitude based upon
- Assuming that a UFO is a vehicle.
- Thinking that a man must be able to fit in the UFO.
- Assuming that great power is necessary to perform the observed motions.
- Once the true position is found by other means, scaling the size or speed reported to attempt to get the true
- Assuming intelligence is behind random motions.
- Not checking the visual capabilities of the witness.
- Expecting perfect vision (or perfectly corrected vision).
- Assuming the witness can measure angles without instruments.
- Overestimating the angular size of the UFO later, due to intense concentration on the object during the sighting.
- Assuming that a reconstruction of angular size is accurate.
- Assuming that the witness has accurately reported visually observed sizes, angular sizes, lengths, complex shapes,
angles, light intensity, and colors.
- Discounting the effects of dark-adaptation.
- Overestimation of brilliance by overestimation of size and distance.
- Forgetting that color perception at night is very different.
- Assuming that colors reported at night are as saturated as they appear.
- Forgetting that background affects color perception.
- Ignoring the effects of optical illusions.
- Confusing whether an object is expanding, brightening, or approaching.
- Confusing whether an object is contracting, dimming, or receding.
- Assuming that an object moving away from the horizon is approaching or contracting, and that an object moving toward
the horizon is receding or expanding.
- Putting too much faith in sizes, distances, speeds, or altitudes reported by witnesses.
- Expecting a witness to locate the sources of very low frequency or very high frequency sounds.
- Expecting a witness to locate the sources of sounds with an accuracy similar to that of vision.
- Assuming that the sound heard must have been produced by the UFO.
- Discounting reflections of sound.
- Assuming that any strange sound must have been produced by a UFO.
- Preconceiving how things are, and using those preconceptions in a UFO encounter.
- Deciding that a smell must have come from the UFO.
- Assuming that prickly or tingling sensations, temperature changes, lack of balance, sweating, spasms, or faintness
must have been caused by a UFO.
- Thinking that the UFO must be a space ship.
- Assuming that the UFO is reacting to witness presence or actions.
- Thinking that a feeling of "being watched" means something. Strange situations cause that feeling.
- Assuming that animals are reacting to the presence of a UFO.
- Expecting strange effects because a UFO is sighted.
- Assuming that any equipment failure or anomaly must have been caused by the UFO, because of its presence.
- Believing that a UFO caused a power failure, instead of the more likely case that the power failure caused the UFO?
- Assuming that none of the perceived motions of the UFO are caused by witness motion.
- Disbelieving the true solution when it is found.
Specific Errors in UFO Cases:
- Mistaking an airplane reflecting sunlight specularly and halating as a "giant cloud cigar" or a missile.
- Mistaking an advertizing plane (with traveling light-up letters) seen edge-on for a rotating disk with portholes.
- Mistaking a small fire balloon close to the witness for a large UFO much farther away.
- Mistaking a meteor or re-entry that has broken into several parts for an "airship" with lighted portholes.
- Mistaking a series of parachute flares as a single giant UFO with lights.
- Mistaking a gliding bird for a domed disk seen edge-on.
- Mistaking a kite for an erratically moving UFO.
- Mistaking a wind-turbine kite for a rotating or flashing UFO.
- Mistaking a bright planet or star for a much closer UFO, especially if it is scintillating.
- Mistaking an electrical arc or an exploding transformer for a UFO that caused a power failure.
- Mistaking a small, stationary object for a larger object, farther away, that is moving in the opposite direction to
- Mistaking a photographic frame where a flashbulb failed to go off for another photo on the same roll of film that was
taken of something too faint or distant to get on film.
Answer: The angular size of the rising full moon is about the size of a pea at arm's length. If you don't believe me,
Answer: The angular size of the full moon directly overhead is also about the size of a pea at arm's length. Try this