Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines.
Distance measurement by parallax is a special case of the principle of triangulation, which states that one can solve for all the sides and angles in a network of triangles if, in addition to all the angles in the network, the length of at least one side has been measured. Thus, the careful measurement of the length of one baseline can fix the scale of an entire triangulation network. In parallax, the triangle is extremely long and narrow, and by measuring both its shortest side (the motion of the observer) and the small top angle (always less than 1 arcsec,leaving the other two close to 90 degrees), the length of the long sides (in practice considered to be equal) can be determined.
Assuming the angle is small, the distance to an object is the reciprocal of the parallax, as shown.
|d||distance to an object(parsecs) (dimensionless)|