<P> Parallax is a displacement or difference in the apparent position 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 word παράλλαξις (parallaxis), meaning "alternation". Due to foreshortening, nearby objects show a larger parallax than farther objects when observed from different positions, so parallax can be used to determine distances . </P> <P> To measure large distances, such as the distance of a planet or a star from Earth, astronomers use the principle of parallax . Here, the term parallax is the semi-angle of inclination between two sight - lines to the star, as observed when Earth is on opposite sides of the Sun in its orbit . These distances form the lowest rung of what is called "the cosmic distance ladder", the first in a succession of methods by which astronomers determine the distances to celestial objects, serving as a basis for other distance measurements in astronomy forming the higher rungs of the ladder . </P> <P> Parallax also affects optical instruments such as rifle scopes, binoculars, 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 . In computer vision the effect is used for computer stereo vision, and there is a device called a parallax rangefinder that uses it to find range, and in some variations also altitude to a target . </P>

How to calculate distance of a star using parallax
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