Parallax is a phenomenon whereby an object observed in the foreground has a different position relative to the background when the viewing point is changed. The greater the distance between the vantage points the greater the parallax effect. Parallax is the basis of stereoscopic vision, used by the visual systems of humans and other animals to estimate the distance to an object.
Parallax is also used by astronomers to estimate the distance to nearby stars. A photograph is taken of the star. Six months later the Earth is on the opposite side of the Sun and a second photograph is taken. This provides a baseline of twice the distance from the Earth to the Sun, i.e. 186 million miles. The change in the apparent position of the star can be used to calculate its distance. The angular displacement of the star, expressed in radians is equal to the baseline divided by the distance to the star.