UnitConvConvert
ly
Lengthscientificastronomy

What is a light year?

Astronomical distance unit — 9.461 × 10¹⁵ metres

Definition

A light year is the distance light travels in one Julian year — approximately 9.461 × 10¹⁵ meters, or 63,241 AU. It is used in astronomy to describe distances between stars and galaxies. The nearest star to the Sun, Proxima Centauri, is 4.24 light years away.

Common uses

  • Interstellar and galactic distances in astronomy
  • Popular-science descriptions of the observable universe
  • Exoplanet distance reporting in the media

🌍 Real-world scale

Proxima Centauri, the nearest star beyond the Sun, is 4.24 ly away. The Milky Way is about 100,000 ly across.

Convert from light years

14 conversions
Light Year to Meter1 ly = 9.4607e15 mLight Year to Kilometer1 ly = 9,460,700,000,000 kmLight Year to Centimeter1 ly = 9.4607e17 cmLight Year to Millimeter1 ly = 9.4607e18 mmLight Year to Mile1 ly = 5,878,606,438,399.75 miLight Year to Yard1 ly = 1.0346e16 ydLight Year to Foot1 ly = 3.1039e16 ftLight Year to Inch1 ly = 3.7247e17 inLight Year to Nautical Mile1 ly = 5,108,369,330,453.56 nmiLight Year to Micrometer1 ly = 9.4607e21 µmLight Year to Nanometer1 ly = 9.4607e24 nmLight Year to Furlong1 ly = 47,028,851,507,197.96 furLight Year to Fathom1 ly = 5.1732e15 ftmLight Year to Angstrom1 ly = 9.4607e25 Å

Convert to light years

14 conversions
Meter to Light Year1 m = 1.0570e-16 lyKilometer to Light Year1 km = 1.0570e-13 lyCentimeter to Light Year1 cm = 1.0570e-18 lyMillimeter to Light Year1 mm = 1.0570e-19 lyMile to Light Year1 mi = 1.7011e-13 lyYard to Light Year1 yd = 9.6652e-17 lyFoot to Light Year1 ft = 3.2217e-17 lyInch to Light Year1 in = 2.6848e-18 lyNautical Mile to Light Year1 nmi = 1.9576e-13 lyMicrometer to Light Year1 µm = 1.0570e-22 lyNanometer to Light Year1 nm = 1.0570e-25 lyFurlong to Light Year1 fur = 2.1264e-14 lyFathom to Light Year1 ftm = 1.9330e-16 lyAngstrom to Light Year1 Å = 1.0570e-26 ly

Tips

  • The light year is a unit of distance, not time, despite the name.
  • Professional astronomers usually prefer parsecs (1 pc ≈ 3.26 ly) for precision work.
  • Light from the Andromeda galaxy takes 2.5 million years to reach us — we see it as it was 2.5 Myr ago.

Common mistakes

  • Confusing light year with light second — light second ≈ 300,000 km.
  • Treating light years as time rather than distance in casual conversation.
  • Using light years for solar-system distances — use astronomical units (AU) instead.

FAQ about the light year

What is a light year?
A light year is the distance light travels in one Julian year — approximately 9.461 × 10¹⁵ meters, or 63,241 AU. It is used in astronomy to describe distances between stars and galaxies. The nearest star to the Sun, Proxima Centauri, is 4.24 light years away.
Where is the light year used?
The light year is commonly used for: Interstellar and galactic distances in astronomy; Popular-science descriptions of the observable universe; Exoplanet distance reporting in the media.
What is an example of a light year in real life?
Proxima Centauri, the nearest star beyond the Sun, is 4.24 ly away. The Milky Way is about 100,000 ly across.
What category does the light year belong to?
The light year is a unit of length, part of the 15-unit length category on UnitConv.
How accurate are light year conversions?
All conversion factors follow NIST and BIPM international standards to 8 significant figures — the highest practical accuracy.
Explore all 15 length units and their conversions.📏 Length hub →