Motion parallax



Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις (parallaxis), meaning "alteration".

Nearby objects have a larger parallax than more distant objects when observed from different positions, so parallax can be used to determine distances. In astronomy, parallax is the only direct method by which distances to objects (typically stars) beyond the Solar System can be measured. The Hipparcos satellite has used the technique for over 100,000 nearby stars. This provides the basis for all other distance measurements in astronomy, the cosmic distance ladder. Here, the term "parallax" is the angle or semi-angle of inclination between two sightlines to the star.

Parallax also affects optical instruments such as binoculars, microscopes, and twin-lens reflex cameras which view objects from slightly different angles. Many animals, including humans, have two eyes with overlapping visual fields to use parallax to gain depth perception; this process is known as stereopsis.

Visual perception
Because the eyes of humans and other highly evolved animals are in different positions on the head, they present different views simultaneously. This is the basis of stereopsis, the process by which the brain exploits the parallax due to the different views from the eye to gain depth perception and estimate distances to objects. Animals also use motion parallax, in which the animal (or just the head) moves to gain different viewpoints. For example, pigeons (whose eyes do not have overlapping fields of view and thus cannot use stereopsis) bob their heads up and down to see depth.