Light-Year (ly)
The distance light travels in vacuum in one Julian year (365.25 days).
Key Facts
- 1 ly = 9,460,730,472,580.8 km (~9.461 trillion km)
- 1 ly = 63,241 AU
- 1 ly = 0.3066 parsecs
- Light speed (c) = 299,792.458 km/s
Intuitive Scale
- Light from the Sun reaches Earth in 8.3 minutes (1 AU)
- Light from dest-proxima-centauri reaches us in 4.24 years
- Light from dest-andromeda reaches us in 2.537 million years — we see it as it was when early humans first used stone tools
Why It Matters for Travel
The light-year is both a distance unit and a speed limit reminder. Nothing with mass can reach c (see concept-relativistic-travel). Every light-year of distance represents a minimum travel time of 1 year — and that’s at an impossible speed.
Practical interstellar speeds in the near future (0.1c–0.2c) mean every light-year takes 5–10 years to cross. This makes the light-year the fundamental planning unit for interstellar mission design.
Related Units
| Unit | Value | Use Case |
|---|---|---|
| Light-second | 299,792 km | Cislunar distances |
| Light-minute | 18 million km | Inner solar system |
| Light-hour | 1.08 billion km | Outer solar system |
| Parsec | 3.26 ly | Professional astronomy |
| AU | 1/63,241 ly | Solar system distances |