Parallax

Parallax, change in the apparent position of an object due to a change in the position of the observer. Parallax in nearby objects may be observed by closing each eye in turn so that a more distant object appears to move relative to a closer one. The brain normally assembles the 2 images to produce a stereoscopic effect. If the length and direction of the line between the 2 points of apparent observation is known, parallax may be used to calculate the distance of an object. In astronomy, the parallax of a star is defined as half the greatest parallactic displacement when the star is viewed from earth at intervals of 6 months.