Stellar Parallax Distance Calculator

A parsec is the distance at which a star shows a parallax angle of exactly one arcsecond as the Earth orbits the Sun. It equals about 3.26 light-years and is the natural unit for stellar distances measured by parallax.

Multiply the distance in parsecs by 3.26156 to get the distance in light-years. The calculator does this automatically and reports both values.

Use d = 1/p, where d is the distance in parsecs and p is the parallax angle in arcseconds. A parallax of 0.1 arcseconds gives a distance of 10 parsecs, or about 32.6 light-years. The smaller the parallax angle, the more distant the star.

Because distance is the reciprocal of the angle (d = 1/p), the fractional error in the distance equals the fractional error in the measured parallax. As stars get farther away their parallax shrinks toward the limit an instrument can resolve, so a fixed measurement error becomes a larger and larger share of the tiny angle and the distance grows very uncertain. That is why the technique works best for nearby stars, and why the Gaia mission measures angles down to tens of microarcseconds (millionths of an arcsecond) to reach stars thousands of parsecs away. Beyond a few thousand parsecs, parallax gives way to other distance methods such as standard-candle stars.