Stellar Parallax Distance Calculator
Calculate stellar distance from parallax angle in arcseconds or milliarcseconds.
Convert between parsecs, light-years, and AU for nearby star measurements.
Trigonometric parallax is the most direct method for measuring stellar distances — no assumptions about stellar brightness required. As Earth orbits the Sun, nearby stars appear to shift slightly against the background of distant stars. The size of that shift reveals the distance.
The geometry
The baseline is Earth’s orbital radius: 1 AU = 1.496 x 10^11 m. The parallax angle p is half the total angular shift observed over 6 months (Earth moving from one side of the orbit to the other).
d (parsecs) = 1 / p (arcseconds)
This defines the parsec: the distance at which a star would show a parallax of exactly 1 arcsecond. One parsec = 3.2616 light-years = 206,265 AU.
Unit conversion
d (light-years) = 3.2616 / p (arcseconds)
d (AU) = 206,265 / p (arcseconds)
Measuring parallax
The closest star, Proxima Centauri, has p = 0.7687 arcseconds, giving d = 1.295 parsecs = 4.22 light-years. This is visible only with careful telescope measurements — 0.77 arcseconds is about the apparent thickness of a human hair at 10 meters distance.
The Hipparcos satellite (1989-1993) measured parallaxes of about 100,000 stars with ~1 milliarcsecond precision. The Gaia mission (launched 2013) measures 1-7 billion stars with 10-20 microarcsecond precision, extending the parallax distance ladder to 10,000+ parsecs.
Limits
Ground-based telescopes can measure parallax to about 50-100 parsecs before atmospheric turbulence makes measurements unreliable. Beyond about 1,000 parsecs (Gaia limit for precision work), other distance indicators (Cepheid variables, type Ia supernovae) must take over.