motion parallax, is the change of angular position of two observations of a single object relative to each other as seen by an observer, caused by the motion of the observer. Simply put, it is the apparent shift of an object against the background that is caused by a change in the observer's position. The term is derived from the Greek παραλλαγή (parallagé), meaning "alteration".
Parallax is often thought of as the 'apparent motion' of an object against a distant background because of a perspective shift, as seen in Figure 1. When viewed from Viewpoint A, the object appears to be closer to the blue square. When the viewpoint is changed to Viewpoint B, the object appears to have moved in front of the red square.
In astronomy, parallax is the only direct method by which distances to objects beyond the Solar System can be measured. The Hipparcos satellite has used the technique for over 100,000 nearby stars. This provides the basis for all other distance measurements in astronomy, the cosmic distance ladder.