What stellar parallax is
Stellar parallax is the apparent shift in a nearby star’s position against distant background stars, as observed from opposite points in Earth’s orbit around the Sun. This tiny angle lets astronomers directly measure distances to nearby stars.
The formula
Distance in parsecs d = 1 / p, where p is the parallax angle measured in arcseconds. This is the definition of the parsec itself: one parsec is the distance at which a star would show a parallax of exactly one arcsecond. The result can then be converted to light-years (1 parsec ≈ 3.26156 light-years) or kilometers.
This is the standard astronomical trigonometric parallax formula, most accurate for relatively nearby stars where parallax angles are measurable.
Last reviewed August 2026