Parallax, the apparent change in position of an object when it is viewed from two different locations. A large parallax can be seen when a pencil is held upright at arm's length and viewed first with one eye and then the other. Parallax decreases as the object is moved farther away.

Unless allowance for parallax is made in closeup photography with a camera whose view finder is separate from its picturetaking lens, important parts of the picture may be omitted.

In astronomy, parallax is used to compute the distance to heavenly bodies. When the size of the parallax and the length of the baseline (an imaginary line connecting the two points of observation) are known, the distance can be computed with trigonometry. The radius of the earth and the radius of the earth's orbit are used as baselines for determining astronomical parallaxes. Parallax is measured in degrees, minutes, and seconds of arc.