Units · Universe scale
Light-Year
The distance light travels in a vacuum in one year: approximately 9.461 × 10¹² km (≈ 63,241 AU). It is a unit of distance, not time.
In a sentence
Proxima Centauri, the nearest star to the Sun, is about 4.24 light-years away.
Source: International Astronomical Union
Distance units compared
Astronomers use different distance units depending on the scale. Within the Solar System, the Astronomical Unit (AU) is most practical. For stellar and galactic distances, the parsec is preferred by researchers because it arises naturally from parallax measurements. The light-year is used in popular science because it communicates the concept of "looking back in time" intuitively.
Convert between all three units instantly with our Distance Converter tool. For a deeper look at how these units are used in practice, see our guide to Measuring cosmic distances.
Frequently asked questions
- No — despite containing the word "year", a light-year is a unit of distance. It is how far light travels in one year, not how long anything takes. The confusion arises because the word "year" is in the name. The IAU defines the light-year formally as a unit of length.
- 1 AU (Astronomical Unit) = 149,597,870.7 km (mean Earth–Sun distance). 1 light-year ≈ 63,241 AU ≈ 9.461 × 10¹² km. 1 parsec ≈ 3.26 light-years ≈ 206,265 AU. Astronomers tend to use AU within the Solar System, parsecs for stellar and galactic distances, and light-years in popular science because they communicate the travel-time analogy intuitively. Convert between them instantly with our Distance Converter tool.
- By definition, exactly one year — specifically one Julian year of 365.25 days. Light travels at 299,792.458 km/s in a vacuum. Multiply that by the number of seconds in a Julian year (31,557,600 s) and you get 9.4607 × 10¹² km. The speed of light in a vacuum is a fundamental constant defined by the BIPM SI system.
- Proxima Centauri, the nearest known star to the Sun, is approximately 4.24 light-years away. At light speed it takes 4.24 years to reach — at the fastest spacecraft ever launched (Voyager 1, ~17 km/s), it would take roughly 75,000 years. This illustrates why interstellar travel remains an enormous challenge. NASA's guide to our Solar System's neighbourhood provides further context.
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