Parallax

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 [1,2]

In Computation, the 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 arcsecond,[3] 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 (see derivation below), the distance to an object (measured in parsecs) is the reciprocal of the parallax (measured in arcseconds): d (\mathrm{pc}) = 1 / p (\mathrm{arcsec}). For example, the distance to Proxima Centauri is 1/0.7687=1.3009 parsecs (4.243 ly).[4]


Another case that parallax can be observed


Software requirements are the most important part of a software project because some mistakes or errors in one can cause many problems in the others projects parts.

Software requirements can present parallax situations considering owner knowledge.

[1] Shorter Oxford English Dictionary. 1968. "Mutual inclination of two lines meeting in an angle"
[2] "Parallax". Oxford English Dictionary (Second Edition ed.). 1989. "Astron. Apparent displacement, or difference in the apparent position, of an object, caused by actual change (or difference) of position of the point of observation; spec. the angular amount of such displacement or difference of position, being the angle contained between the two straight lines drawn to the object from the two different points of view, and constituting a measure of the distance of the object."
[3] a b Zeilik & Gregory 1998, p. 44.
[4] a b Benedict, G. Fritz et al. (1999). "Interferometric Astrometry of Proxima Centauri and Barnard's Star Using HUBBLE SPACE TELESCOPE Fine Guidance Sensor 3: Detection Limits for Substellar Companions". The Astronomical Journal 118 (2): 1086–1100. arXiv:astro-ph/9905318. Bibcode 1999astro.ph..5318B. doi:10.1086/300975.