Parsec

Friedrich Bessel finally measured a stellar distance in 1838, picking 61 Cygni for its large proper motion and teasing out a parallax of about 0.31 arcseconds — an angle the width of a coin seen from ten kilometres. It proved, 295 years after Copernicus, that Earth really moves, and it revealed that even a nearby star sits some 10 light-years away. Herbert Hall Turner coined "parsec" in 1913 for the natural unit the geometry hands you: at one parsec the parallax is exactly one arcsecond, so distance in parsecs is simply the reciprocal of the parallax in arcseconds.

Since 2015 the parsec is exact, defined as 648 000/π astronomical units, which is exact because the au is. Nothing in the sky is closer than one: Proxima Centauri's parallax of 0.7685″ puts it at 1.30 pc. ESA's Gaia mission now measures parallaxes to a few microarcseconds, mapping nearly two billion stars and rebuilding every rung of the cosmic distance ladder on top of Bessel's method.