Parallax: How We Measure Star Distances

Parallax is the apparent shift of a star’s position when viewed from two different points.
Astronomers use Earth’s orbit as a baseline—comparing views six months apart.
The closer the star, the larger its parallax shift; distant stars barely move at all.
One parsec—the standard cosmic distance unit—is defined by a parallax angle of one arcsecond.
The nearest star system, Alpha Centauri, has a measurable parallax of less than one arcsecond.
Parallax was first measured successfully in 1838 by Friedrich Bessel, proving stars lie at vast distances.
Ground-based telescopes can only measure parallax for relatively nearby stars.
Space missions like Hipparcos and Gaia have mapped star positions with unprecedented precision.
Parallax provides the foundation for the “cosmic distance ladder,” helping measure galaxies and beyond.
Every star map begins with parallax—the simplest yet most powerful tool for stellar distances.