Parallax

a difference in the apparent direction of a heavenly body, due to different positions of the viewpoint. Persons standing at opposite points on the earth's surface will see the same star in slightly different directions. In astronomy parallax is measured by the angle included between lines drawn from the object to the observer, and from the object to the center of the earth or other point taken as a standard center. Evidently a heavenly body immediately above the observer has no parallax. The farther away a body, the less its parallax. The parallax of the moon is 57'. That is, to the lunar observer, the earth would seem to have a diameter of 114'. If star observations be made at intervals of six months when the viewpoints are at opposite diameters of the earth's annual orbit, the parallax becomes appreciable, even for the most distant star. By the aid of trigonometry this apparent shifting of direction is made use of by astronomers in calculating the distance of heavenly bodies.