An apparent change in the position of an object, arising from a change of the observer's station, and which diminishes with the altitude of an object in the vertical circle. Its effect is greatest in the horizon, where it is termed the horizontal parallax, and vanishes entirely in the zenith. The positions of the planets and comets, as viewed from the surface of the earth, differ from those they would occupy if observed from its centre by the amount of parallax, the due application of which is an important element. The stars are so distant that their positions are the same from whatever part of the earth they are seen; but attempts have been made to detect the amount of variation in their places, when observed from opposite points of the earth's orbit, the minute result of which is termed the annual parallax; and the former effect, due to the observer's station on our globe, is called the diurnal parallax.