Parallax is the apparent shift in an objects position relative to more distant background objects caused by a change in the observers position. It is measured by the angle or half-angle of inclination between two different lines of sight. Parallax is a perspective effect that arises due to a change in viewpoint occurring due to the motion of the observer, of the observed, or both.

Parallax is used in many fields, including astronomy, photography, and range-finding. In astronomy, parallax is an essential tool for calculating distances of faraway stars. The parallax method is the first rung on the cosmic distance ladder, a succession of methods that astronomers use to estimate distances of objects in the universe. In photography, parallax is used to reproduce and display 3D images. In range-finding, parallax affects sighting devices of ranged weapons such as rifle scopes, binoculars, and bows.

Parallax also affects optical instruments such as microscopes and twin-lens reflex cameras that view objects from slightly different angles. Many animals, including humans, have two eyes with overlapping visual fields that use parallax to gain depth perception; this process is known as stereopsis. Humans are already very accustomed to parallax as our two eyes provide a small parallax effect known as stereo vision.

The first successful measurements of a stellar parallax were made by Friedrich Bessel in 1838, for the star 61 Cygni. To create the largest parallax effect, we need to have the largest shift in position for the observer. The largest shift possible from the Earth is 2 AU – the diameter of the Earths orbit.