Light-year converter
How the math works
One light-year is the distance light travels in a Julian year (365.25 days) at exactly 299,792.458 km/s — the speed of light defined by the International System of Units.
Multiplying that out: 299,792.458 × 86,400 × 365.25 ≈ 9.461 × 1012 km. An astronomical unit (AU), defined by the IAU in 2012 as exactly 149,597,870.7 km, is the average Earth–Sun distance. A parsec is the distance at which one AU subtends one arcsecond — about 3.26 light-years.
Why parsecs at all
Parsecs come from parallax measurements. If a star shifts by one arcsecond as Earth swings from one side of its orbit to the other, the star is at one parsec. Astronomers use parsecs and kiloparsecs because they map directly to the geometry used to measure stellar distances.