Definition
A visual illusion in which a pilot misjudges the distance to an object or light source because its apparent size differs from what was expected. A larger-than-expected light or object appears closer than it actually is, while a smaller-than-expected one appears farther away, leading to incorrect spatial judgments — particularly at night.
Plain English
Your brain decides how far away something is partly by how big it looks. If a light or object is bigger or smaller than you expect, you can misjudge the distance — thinking it is closer or farther than it really is.
Context Anchor
Encountered during night flying when judging distance from lights, airports, terrain, or other lighted objects with few outside visual references.
Derivation
“Illusion” comes from a Latin idea meaning to deceive or play a trick on. That helps here because the object has not moved or changed distance; the pilot’s eyes and brain are being tricked by how large or small it appears.
Why Pilots Care
Misjudging distance can result in flying a high or low approach, increasing the risk of landing short, long, or hard.
Grounding Statement
In darkness, when there are few outside cues, apparent size can become a poor substitute for true distance.
Intuition Check
Do not assume that a bigger-looking object is always actually closer. In this illusion, apparent size is misleading the distance judgment.
Example Sentence 1
On a dark night approach, the pilot recognized a possible size-distance illusion when the runway lights appeared unusually large and seemed closer than the GPS distance indicated.
Example Sentence 2
The size-distance illusion can also occur when bright city lights appear smaller than expected, leading a pilot to descend too low over unlit terrain.