Parallax is the apparent change in the position of an object resulting from a change in the position of the observer. The effect is useful in calculating the distances of stars from Earth, as stars appear to shift in relation to each other from different points in Earth’s orbit. Parallax can affect the accuracy of photographers and snipers, who may see a target from an angle different than that of their instruments. What does parallax have to do with why pigeons bob their heads?
Source: The Free Dictionary