1. An apparent displacement of a distant object (with respect to a more distant background) when viewed from two different positions. If such an object is viewed from two points at either end of a base line, the angle between the lines joining the object to the ends of the base line is the angle of parallax. If the base line is the distance between the two eyes of an observer the angle is called the binocular parallax.
2. The angular displacement in the apparent position of a celestial body when observed from two different points. Diurnal parallax results from the earth’s daily rotation, the celestial body being viewed from the surface of the earth rather than from its centre. Annual parallax is caused by the earth’s motion round the sun, the celestial body being viewed from the earth rather than from the centre of the sun. Secular parallax is caused by the motion of the solar system relative to the fixed stars.