Light-years, parsecs and AU — the three scales of astronomy
The parsec is defined by geometry. The light-year is defined by time. The AU is defined by our own orbit. Three units, three eras, one very big sky.
Astronomers rarely use metres. Distances are enormous, and the units that survive in professional literature match the measurement technique. Radar-ranged planets get AU. Parallax-measured stars get parsecs. Everything else gets light-years, mostly for the press release.
The exact factors
1 AU = 149,597,870,700 m (exact, IAU 2012) > 1 light-year = 9.4607 × 10¹⁵ m ≈ 63,241 AU > 1 parsec = 3.0857 × 10¹⁶ m ≈ 3.2616 ly > 1 kiloparsec = 3,261.6 ly; 1 megaparsec = 3.26 million ly
Why parsecs and not light-years
A parsec is the distance at which 1 AU subtends 1 arcsecond of parallax. That definition matches directly to what a telescope measures — an angle over six months as Earth swings around the Sun. Converting parallax to a light-year distance adds a factor of 3.26 that carries no physical meaning, so professionals skip the step.
The AU and Kepler's third law
In AU-and-years units, Kepler's third law reduces to T² = a³ for objects orbiting the Sun. A planet at 4 AU orbits in 8 years. A comet at 100 AU aphelion takes 500-year loops. The unit choice makes the physics fall out cleanly — one reason the IAU keeps the AU alive alongside SI metres.
Worked example — Proxima Centauri
Proxima Centauri sits at a parallax of 0.7687 arcsec, giving 1/0.7687 = 1.301 parsecs = 4.244 light-years = 268,553 AU = 4.014 × 10¹⁶ m. Each unit tells a different story: the parsec tells you the measurement was clean, the light-year tells you the signal delay is 4.24 years, the AU compares it to the Solar System (2,700× wider than Neptune's orbit).
The five-second recap
AU for planets, parsec for stars, light-year for anything you need a general audience to feel. And 1 pc ≈ 3.26 ly, always.
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FAQ
Is the Kessel Run in 12 parsecs a mistake?+
A parsec is a distance, not a time — but Star Wars justified it as a shorter route around a black hole cluster. It still bothers astronomers.
How far is the nearest galaxy in parsecs?+
Andromeda is at about 780 kiloparsecs, or 2.54 million light-years.
Why 149,597,870,700 m for the AU?+
It was redefined as an exact conventional value in 2012 so that AU-based calculations no longer depend on measuring the Sun's mass.
This article was written by The Turbo Unit Converter engineering desk (Astronomy & physics) and last reviewed on 2026-06-28 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.