EarthSky
Glossary · Galaxies & Cosmology
Light-year and parsec
Two units for the same job, one intuitive and one that falls out of the measurement.
A light-year is the distance light covers in a year, about 9.5 trillion kilometres. A parsec is the distance at which one astronomical unit subtends one arcsecond — 3.26 light-years — and it exists because it is what the parallax measurement gives you directly, with no conversion. Professionals use parsecs for that reason and popular writing uses light-years because it explains itself. Both describe distances so large that they are also measures of time: to look far is to look back.
Where it has come up lately
Phys.org
Astronomy Magazine
EarthSky
EarthSky
Phys.org
Related terms
Fast radio burst
A millisecond flash of radio energy from another galaxy, cause still unsettled.
Redshift
The stretching of light from distant objects, and the universe's odometer.
Hubble tension
Two good measurements of the universe's expansion rate that stubbornly disagree.
Dark energy
The name for whatever is making the expansion of the universe speed up.
Questions
What is Light-year and parsec?
Two units for the same job, one intuitive and one that falls out of the measurement.
Why does it matter?
Professionals use parsecs for that reason and popular writing uses light-years because it explains itself. Both describe distances so large that they are also measures of time: to look far is to look back.