← All notes

Short answer

A light-year is a distance, not a time: how far light travels in one year, about 6 trillion miles (9 trillion km). [S2] In astronomical units it is about 63,197 au. [S1]

Why it is useful

• Light moves about 186,000 miles (300,000 km) per second, but space is so large that light still takes years to cross between stars. [S2]
• Looking far away means looking back in time: sunlight is about 8.3 minutes old when it reaches us, and light from the Andromeda galaxy left about 2.5 million years ago. [S2]
• The next closest star after the Sun is about 4.3 light-years away. [S2]

Claims

• id: C1 claim: A light-year is the distance light travels in one year. source: S1 evidence: "the distance light travels in one year"
• id: C2 claim: One light-year is about 63,197 astronomical units. source: S1 evidence: "about 63,197 au"
• id: C3 claim: One light-year is about 6 trillion miles. source: S2 evidence: "One light-year is about 6 trillion miles (9 trillion km)."
• id: C4 claim: The Sun's light takes about 8.3 minutes to reach us. source: S2 evidence: "the Sun's light takes about 8.3 minutes to reach us"

Sources

• [S1] Basics of Spaceflight: Glossary. NASA. https://science.nasa.gov/learn/basics-of-space-flight/glossary/ . License: US government work, public domain. Accessed 2026-11-04.
• [S2] What Is a Light-Year?. NASA. https://spaceplace.nasa.gov/light-year/en/ . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_6jq2xbr2,
  author = {{iinventedlight}},
  title = {{What is a light-year?}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/light-year-explained/},
  note = {Permanent ID iil:note:6jq2xbr2. License: CC-BY-4.0.},
}

Markdown

Spotted a mistake? Use the MCP feedback tool or email hello@iinventedlight.com.

For language models and agents: start at /llms.txt. This page in Markdown: /library/notes/light-year-explained.md.