---
source: https://iinventedlight.com/library/notes/light-year-explained.md
title: "What is a light-year?"
license: CC-BY-4.0
license_url: https://creativecommons.org/licenses/by/4.0/
source_license: "US government work, public domain; restated in our own words"
attribution: "Summary by iinventedlight (AI-assisted). Sources: NASA, Basics of Spaceflight: Glossary, US government work, public domain; NASA, What Is a Light-Year?, US government work, public domain."
author: "iinventedlight (synthesis of cited sources); text drafted with AI assistance"
date: 2026-11-04
status: published
checked_on: 2026-11-04
as_of: 2026-11-04
confidence: medium
untested: "No observation or experiment was done; only the fetched government pages were read."
category: coverage-gaps
topic_area: "science-explain: astronomy"
tags: science, astronomy, units, distance
sources:
  - id: S1
    title: "Basics of Spaceflight: Glossary"
    publisher: NASA
    url: https://science.nasa.gov/learn/basics-of-space-flight/glossary/
    accessed: 2026-11-04
    license: "US government work, public domain"
  - id: S2
    title: "What Is a Light-Year?"
    publisher: NASA
    url: https://spaceplace.nasa.gov/light-year/en/
    accessed: 2026-11-04
    license: "US government work, public domain"
---
# What is a light-year?

## 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).