parallax

Parallax is the apparent shift in the position of an object relative to distant background when viewed from different vantage points. In everyday experience it explains why nearby objects seem to move against faraway scenery when you change your viewpoint. In astronomy, parallax uses Earth's orbit around the Sun as a baseline to observe the slight apparent motion of nearby stars against more distant stars over six months.

The measured quantity is the parallax angle, p, subtended by the baseline of 1 astronomical unit (the average Earth–Sun distance). Distances are commonly expressed in parsecs, where one parsec is the distance at which a star subtends an angle of one arcsecond. The relationship is d (parsecs) = 1 / p (arcseconds). Smaller parallax angles correspond to greater distances. Measuring stellar parallax requires precise angular measurements and is hindered by atmospheric distortion for ground-based observations; space-based observatories provide greater accuracy.

Historically, the first successful stellar parallax measurement was made by Friedrich Wilhelm Bessel in 1838 for 61 Cygni, confirming that stars lie at vast distances. Subsequent measurements by Struve and others improved accuracy. Space missions such as Hipparcos and Gaia have dramatically increased the number of stars with reliable parallaxes and refined our understanding of the structure of our galaxy.

Parallax concepts extend beyond astronomy. In photography and cinema, parallax describes apparent shifts between images from different viewpoints and underpins stereoscopic imaging. In computer graphics, parallax scrolling and parallax mapping create depth cues by manipulating the relative motion or rendering of layered surfaces.