Definition
A depth cue in which objects closer to the observer appear to move across the field of view faster than objects farther away when the observer is in motion. Pilots use this difference in apparent movement to judge the relative distance of terrain features, traffic, and obstacles.
Plain English
When you are moving, nearby things rush past quickly while distant things drift slowly. Your eyes use that difference in speed to tell what is close and what is far away.
Context Anchor
Seen in discussions of distance estimation, depth perception, and visual cues used while taxiing, landing, or judging height above the ground.
Derivation
From Latin movere ('to move') and Greek parallaxis ('alteration' or 'change'). The term literally describes the change in apparent position caused by motion, which is exactly how pilots use it to judge distance from a moving aircraft.
Why Pilots Care
Provides pilots with a reliable visual cue for judging distances to terrain, runways, or other aircraft when other references are limited.
Analogy
When you ride in a car, fence posts near the road seem to rush by, while mountains in the distance barely seem to move. The fence posts and mountains are not moving; your changing viewpoint creates the effect.
Grounding Statement
Looking out the side window of a car: roadside posts whip past, distant hills barely move. That speed difference is motion parallax, and pilots use the same effect from the cockpit.
Intuition Check
Motion parallax does not mean the outside objects are actually moving. It means their apparent position changes because your viewpoint is moving.
Example Sentence 1
Flying low over open water, the pilot lost the benefit of motion parallax because there were no nearby reference points moving past the aircraft.
Example Sentence 2
While flying low over the terrain, motion parallax helped confirm the relative height of nearby ridges.