A light-year is a unit of distance (not time) equal to the distance that light travels in one year in a vacuum — approximately 9.461 trillion kilometres (5.879 trillion miles). It is the standard unit for expressing astronomical distances, from nearby stars to the edges of the observable universe.
Understanding Light-Year
Astronomical Distance Scales
| Object | Distance | Light Travel Time |
|---|---|---|
| Moon | 384,400 km | 1.3 seconds |
| Sun | 150 million km (1 AU) | 8.3 minutes |
| Mars (closest) | 55 million km | 3 minutes |
| Proxima Centauri (nearest star) | 4.24 ly | 4.24 years |
| Centre of Milky Way | 26,000 ly | 26,000 years |
| Andromeda Galaxy | 2.5 million ly | 2.5 million years |
| JWST deepest observation | 13.4 billion ly | 13.4 billion years |
Light-Year vs Other Units
Astronomers also use parsecs (1 pc ≈ 3.26 ly), derived from parallax measurements, and astronomical units (1 AU ≈ 150 million km, the Earth–Sun distance). Parsecs are preferred in professional astrophysics; light-years are more common in public communication because they intuitively connect distance to time — saying a star is "10 light-years away" immediately conveys that its light takes 10 years to reach us.
Looking Back in Time
Because light travels at a finite speed, every astronomical observation is a view into the past. The Sun we see is 8.3 minutes old. The exoplanets in the TRAPPIST-1 system are seen as they were 40 years ago. JWST can observe galaxies from when the universe was less than 400 million years old — over 13 billion light-years away. The observable universe has a radius of approximately 46.5 billion light-years, though the actual distance is larger due to the expansion of space.