Parallax

CORE DEFINITION

Parallax (from Ancient Greek παράλλαξις (parallaxis), meaning 'difference') is the apparent shift in the position of an object when viewed from two different lines of sight. The angle between the two lines of sight is called the parallax angle, and the distance between the two observation points is the baseline. For a given baseline, the parallax is larger for closer objects, so parallax can be used to estimate distance. For example, astronomers use parallax to measure distances to celestial objects, including the Moon, the Sun, and stars beyond the solar system. The Hipparcos satellite measured distances to over 100,000 nearby stars via parallax, providing a foundation for the cosmic distance ladder. In this context, parallax refers to the angle (or half-angle) subtended by the two lines of sight to a star. Some optical instruments, such as binoculars, microscopes, and twin-lens reflex cameras, view objects from slightly different angles and are subject to parallax effects.