Parallax is the apparent shift in a star's position when viewed from opposite sides of Earth's orbit, serving as astronomy's most fundamental direct distance measurement method. Imagine holding your finger at arm's length and alternately closing each eye – your finger appears to jump back and forth against the background. Stars exhibit this same effect as Earth travels around the Sun, though the shifts are incredibly tiny.</p><p>The nearest star, Proxima Centauri, shows a parallax of just 0.77 arcseconds – equivalent to viewing a coin from 4 miles away! Astronomers measure these minute angular shifts using the parallax formula: distance (in parsecs) equals 1 divided by the parallax angle (in arcseconds). One parsec equals 3.26 light-years, making Proxima Centauri about 4.2 light-years distant.</p><p>Friedrich Bessel achieved the first successful stellar parallax measurement in 1838 for the star 61 Cygni, finally proving Earth orbits the Sun and opening the door to mapping our galaxy's three-dimensional structure. Ground-based telescopes can reliably measure parallax out to about 100 light-years, while space missions like Hipparcos and Gaia have revolutionized the field, measuring parallaxes for over a billion stars with unprecedented precision. This cosmic yardstick remains our only direct method for determining stellar distances, forming the foundation of astronomy's distance ladder.