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What is a light-year?

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Quick answer

A light-year is the distance light travels in one year — about 9. 5 trillion kilometers. Despite the word 'year,' it measures distance, not time, and it's used because cosmic distances are far too vast for everyday units. It's a unit of distance, not time.

Key things to understand

  1. It's a unit of distance, not time.
  2. Light travels about 9.5 trillion km (5.9 trillion miles) in a year.
  3. It exists because distances in space are enormous.
  4. The nearest star is about 4.2 light-years away.
  5. Seeing a star 100 light-years away shows it as it was 100 years ago.

People also ask

Is a light-year a measure of time?
No — despite the name, it measures distance: how far light travels in one year, about 9.5 trillion kilometers.
How far is one light-year?
Roughly 9.5 trillion kilometers (5.9 trillion miles) — light moving at about 300,000 km per second for a full year.
Why do astronomers use light-years?
Cosmic distances are so vast that kilometers become unwieldy; light-years (and parsecs) keep the numbers manageable.

Part of the guide: Space & Astronomy →