Britannica Sources used in this answer

Why does a nearby object appear to shift more than a distant object when the observer moves?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Diagram illustrating how astronomers calculate the distance to a star using parallax: Earth is shown in two positions on opposite sides of its orbit around the Sun, with lines extending to a nearby star X and further to distant background stars. The apparent shift in star X's position, measured as angle A, is used to determine its distance, with the baseline labeled as 186,000,000 miles. — Parallax Calculating stellar distances.

The phenomenon where a nearby object appears to shift more than a distant one when an observer moves is known as movement parallax.1

This effect occurs because of the geometry of the observer's movement relative to the objects in their field of view. When you move your head or body, your position changes, creating a new vantage point. Because the nearby object is closer to you, the angular change in its position against the background is significantly larger than the angular change for an object that is far away.1

In essence, the closer an object is, the greater the disparity in the retinal images created by your movement. This difference in displacement serves as a vital depth cue, allowing the brain to perceive distance and three-dimensional space even when using only one eye. While binocular disparity relies on the fixed distance between two eyes, movement parallax allows for similar depth perception through the active movement of the observer.1