Astronomical Units, Light-Years & Parsecs: Observation, Measurement & Evidence

One astronomical unit is about 149.6 million kilometers. One light-year is about 9.46 trillion kilometers. One parsec is about 3.26 light-years and is the distance at which one astronomical unit subtends an angle of one arcsecond.

Space astrometry missions measure tiny stellar position changes repeatedly to infer parallax. Radar ranging and spacecraft navigation establish Solar-System distances extremely precisely, tying astronomical-unit-scale geometry to physical units.

A parsec comes from parallax geometry: as Earth moves from one side of its orbit to the other, a nearby star appears to shift against distant background stars. Half of the total angular shift defines the parallax angle, and distance in parsecs is the reciprocal of parallax in arcseconds for nearby stars where Euclidean geometry is adequate.

Modern astrometry measures parallaxes far smaller than one arcsecond, requiring repeated calibrated position measurements and careful treatment of proper motion, binary motion, and instrument systematics. Radar ranging provides the geometric scale of the Solar System, and that scale ultimately supports the AU baseline used in parallax. Consistency between geometric distances, cluster sequences, and calibrated luminosity methods validates the ladder. Uncertainty grows important because inverting a noisy parallax can bias distance estimates when fractional errors become large.