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Parallax is an apparent displacement or difference in the apparent position of an object viewed along two different lines of sight. It is a phenomenon that occurs when an observer's position changes, causing the object to appear to shift against the background. This effect is commonly observed in everyday life, such as when looking out of a moving car or train, where nearby objects appear to move faster than distant objects.

In astronomy, parallax is a crucial method for measuring the distances to stars and other celestial objects. The most common type of parallax used in astronomy is called stellar parallax, which involves observing a star from two different points in Earth's orbit around the Sun. By measuring the apparent shift in the star's position relative to distant background objects, astronomers can calculate the star's distance using trigonometry.

Parallax is also utilized in various fields, including navigation, photography, and virtual reality, to create a sense of depth and immersion. In navigation, parallax is used to determine the position of a ship or aircraft by comparing the observed positions of celestial bodies with their known positions in the sky. In photography, parallax can be used to create depth-of-field effects, where the focus shifts between foreground and background elements. In virtual reality, parallax is employed to simulate the natural movement of objects as the viewer's perspective changes, enhancing the overall immersive experience.

The principle of parallax is based on the fact that the angle subtended by an object at the eye changes as the observer's position changes. This change in angle is what creates the perception of displacement. The amount of parallax depends on the distance between the observer's two positions and the distance to the object. Objects that are closer to the observer will exhibit a greater parallax shift than those that are farther away.