# Light-year
A light-year (symbol ly) is a unit of distance equal to the length light travels in vacuum in one year, about 5.88 trillion miles (9.46 trillion kilometers)[1](https://science.nasa.gov/mission/voyager/voyager-1/voyager-1-what-is-a-light-day/). Despite the word "year" in its name, it measures length, not time[2](https://assets.cambridge.org/97811076/87561/excerpt/9781107687561_excerpt.pdf). Astronomers use it for interstellar and intergalactic distances, where figures in miles or kilometers become unwieldy[3](https://science.nasa.gov/solar-system/cosmic-distances/).
| Key fact | Detail |
|---|---|
| Definition | The distance light travels in vacuum in one year[3](https://science.nasa.gov/solar-system/cosmic-distances/) |
| Length | About 5.88 trillion miles (9.46 trillion km), roughly 63,000 astronomical units[1](https://science.nasa.gov/mission/voyager/voyager-1/voyager-1-what-is-a-light-day/) • [3](https://science.nasa.gov/solar-system/cosmic-distances/) |
| Basis | Speed of light in vacuum fixed at exactly 299,792,458 m/s by the 1983 definition of the meter[4](https://www.bipm.org/en/committees/cg/cgpm/17-1983/resolution-1) |
| Relation to parsec | One parsec equals 3.26 light-years; the parsec is the unit preferred in professional astronomy[5](https://www.esa.int/Science_Exploration/Space_Science/Gaia/Cosmic_distances) • [6](https://iauarchive.eso.org/publications/proceedings_rules/units/) |
| Nearest star | Proxima Centauri, about 4.24 light-years away[1](https://science.nasa.gov/mission/voyager/voyager-1/voyager-1-what-is-a-light-day/) |
| Galactic scale | The Milky Way is about 100,000 light-years across; the Andromeda galaxy lies about 2.5 million light-years away[7](https://www.nasa.gov/science-research/astrophysics/how-big-is-space-we-asked-a-nasa-expert-episode-61/) • [1](https://science.nasa.gov/mission/voyager/voyager-1/voyager-1-what-is-a-light-day/) |
| Lookback time | Seeing an object 4.3 light-years away means seeing it as it was 4.3 years ago[3](https://science.nasa.gov/solar-system/cosmic-distances/) |
## Definition and exact length
The light-year is defined by two fixed quantities: the speed of light and the length of a year. Since 1983, the meter itself has been defined so that the speed of light in vacuum is exactly 299,792,458 meters per second[4](https://www.bipm.org/en/committees/cg/cgpm/17-1983/resolution-1). Multiplying that speed by the seconds in a year gives the distance; NASA states the result as about 5.88 trillion miles, or 9.46 trillion kilometers[1](https://science.nasa.gov/mission/voyager/voyager-1/voyager-1-what-is-a-light-day/). ESA gives the equivalent figure of about 9.5 million million kilometers[5](https://www.esa.int/Science_Exploration/Space_Science/Gaia/Cosmic_distances), and NASA's Cosmic Distances page rounds it to about 6 trillion miles (9 trillion kilometers), equal to roughly 63,000 astronomical units[3](https://science.nasa.gov/solar-system/cosmic-distances/). One astronomical unit (AU) is the average Sun-to-Earth distance, about 93 million miles (150 million kilometers)[3](https://science.nasa.gov/solar-system/cosmic-distances/).
The same logic produces smaller light-based units. Light covers one light-second, about 186,000 miles, each second; the Sun is about 8.3 light-minutes from Earth, and Neptune averages four light-hours and 10 light-minutes away[1](https://science.nasa.gov/mission/voyager/voyager-1/voyager-1-what-is-a-light-day/). A light-day, the distance light covers in 24 hours, is 16,094,799,096 miles (25,902,068,356 kilometers)[1](https://science.nasa.gov/mission/voyager/voyager-1/voyager-1-what-is-a-light-day/).
## Why astronomers use it
Distances between stars expressed in miles or kilometers run into the trillions, numbers too large to compare easily. The [Alpha Centauri](https://www.edgepedia.org/alpha-centauri) system, the nearest star system to ours, is about 4.3 light-years away, a more manageable figure than 25 trillion miles, 40 trillion kilometers, or 272,000 AU[3](https://science.nasa.gov/solar-system/cosmic-distances/). NASA's Space Place puts the next closest star at about 4.3 light-years[8](https://spaceplace.nasa.gov/light-year/en/), and the Voyager mission page gives 4.24 light-years for [Proxima Centauri](https://www.edgepedia.org/proxima-centauri) and its planets[1](https://science.nasa.gov/mission/voyager/voyager-1/voyager-1-what-is-a-light-day/).
Light-years also encode time. Because light from an object travels at light speed, seeing the Sun, Jupiter, or a distant star means seeing it as it was when the light left it, whether 8 minutes, tens of minutes, or 4.3 years ago[3](https://science.nasa.gov/solar-system/cosmic-distances/). Looking farther out in space therefore means looking farther back in time[3](https://science.nasa.gov/solar-system/cosmic-distances/). Light from some stars in the [Milky Way](https://www.edgepedia.org/milky-way) takes tens of thousands of years to reach us[9](https://imagine.gsfc.nasa.gov/ask_astro/stars.html).
## Relation to the parsec and other units
Professional astronomy mostly uses the parsec (pc), defined as the distance at which one astronomical unit subtends an angle of one arcsecond[5](https://www.esa.int/Science_Exploration/Space_Science/Gaia/Cosmic_distances). One parsec equals 3.26 light-years, or nearly 31 trillion kilometers[5](https://www.esa.int/Science_Exploration/Space_Science/Gaia/Cosmic_distances). For intergalactic distances astronomers use megaparsecs, each equal to one million parsecs, and at the largest scales they describe distance by redshift, the shift of a galaxy's light toward longer, redder wavelengths caused by the expansion of the universe[3](https://science.nasa.gov/solar-system/cosmic-distances/).
According to the International Astronomical Union, the light-year suits non-technical accounts of astronomy and occasionally appears in scientific publications as a distance measure, whereas the parsec is the proper length unit for research on Galactic structure[6](https://iauarchive.eso.org/publications/proceedings_rules/units/). The British astrophysicist [Arthur Eddington](https://www.edgepedia.org/arthur-eddington), a prominent scientist of the early 20th century, preferred the parsec and called the light-year "inconvenient"[10](https://www.livescience.com/56115-what-is-a-light-year.html).
## Measuring distances in light-years
The distances quoted in light-years rest on a chain of methods. For stars within about 100 light-years, astronomers derive distances by stellar parallax, measuring the apparent shift of a star against a distant background as Earth orbits the Sun[11](https://starchild.gsfc.nasa.gov/docs/StarChild/questions/parallax.html). In 1838, Friedrich Bessel achieved the first successful measurement of stellar parallax, targeting the star 61 Cygni.[12](https://astro.unl.edu/naap/distance/parallax.html) Parallax works reliably for stars within a few hundred light-years of Earth; using a technique called spatial scanning, the [Hubble Space Telescope](https://www.edgepedia.org/hubble-space-telescope) can precisely measure distances to stars up to 10,000 light-years away, ten times farther than previously possible[13](https://www.nasa.gov/news-release/nasas-hubble-extends-stellar-tape-measure-10-times-farther-into-space/). Farther out, [Cepheid variable](https://www.edgepedia.org/cepheid-variable) stars and other mileposts extend what astronomers call the cosmic "distance ladder"[13](https://www.nasa.gov/news-release/nasas-hubble-extends-stellar-tape-measure-10-times-farther-into-space/). ESA's Gaia mission has measured parallaxes for more than a billion stars, plotting more than four million stars within five thousand light-years of the Sun in its second data release[14](https://www.esa.int/ESA_Multimedia/Images/2013/06/Measuring_stellar_distances_by_parallax) • [15](https://www.esa.int/Science_Exploration/Space_Science/Gaia/How_does_Gaia_study_the_Milky_Way).
## Scale of the universe in light-years
The light-year gives a sense of cosmic scale. [The Milky Way](https://www.edgepedia.org/the-milky-way) is about 100,000 light-years across and probably contains 100 to 400 billion stars[7](https://www.nasa.gov/science-research/astrophysics/how-big-is-space-we-asked-a-nasa-expert-episode-61/) • [16](https://science.nasa.gov/exoplanets/what-is-a-light-year/). The nearest large galaxy, Andromeda, is about 2.5 million light-years away[1](https://science.nasa.gov/mission/voyager/voyager-1/voyager-1-what-is-a-light-day/). Deep-field images from the [James Webb Space Telescope](https://www.edgepedia.org/james-webb-space-telescope) show galaxies whose light was emitted about 13.5 billion years ago, but because the universe is expanding, those galaxies are actually much farther away than 13.5 billion light-years; the observable universe is estimated to be about 92 billion light-years across[7](https://www.nasa.gov/science-research/astrophysics/how-big-is-space-we-asked-a-nasa-expert-episode-61/).
## History of the term
The term appeared in German astronomical writing in the mid-19th century. In 1851 it made a first appearance in Germany, in an astronomical publication known as *Lichtjare*[10](https://www.livescience.com/56115-what-is-a-light-year.html). In 1868 an English journal labeled the light-year as a unit used by the Germans[10](https://www.livescience.com/56115-what-is-a-light-year.html). The concept depended on the then-recent recognition that light has a finite speed, and on Bessel's 1838 parallax measurement, which proved that stars were unimaginably far away[10](https://www.livescience.com/56115-what-is-a-light-year.html) • [12](https://astro.unl.edu/naap/distance/parallax.html). Light-years later became a popular unit of measurement even in the scientific literature[10](https://www.livescience.com/56115-what-is-a-light-year.html).
## References
1. [Voyager 1: What Is a Light-Day, NASA Science](https://science.nasa.gov/mission/voyager/voyager-1/voyager-1-what-is-a-light-day/)
2. [The Cosmic Century excerpt, Cambridge University Press](https://assets.cambridge.org/97811076/87561/excerpt/9781107687561_excerpt.pdf)
3. [Cosmic Distances, NASA Science](https://science.nasa.gov/solar-system/cosmic-distances/)
4. [Resolution 1 of the 17th CGPM (1983), Definition of the metre, BIPM](https://www.bipm.org/en/committees/cg/cgpm/17-1983/resolution-1)
5. [Cosmic distances, ESA](https://www.esa.int/Science_Exploration/Space_Science/Gaia/Cosmic_distances)
6. [Recommendations concerning Units (SI Units), IAU](https://iauarchive.eso.org/publications/proceedings_rules/units/)
7. [How Big is Space? We Asked a NASA Expert: Episode 61, NASA](https://www.nasa.gov/science-research/astrophysics/how-big-is-space-we-asked-a-nasa-expert-episode-61/)
8. [What Is a Light-Year? NASA Space Place](https://spaceplace.nasa.gov/light-year/en/)
9. [Imagine the Universe! Ask an Astrophysicist, NASA Goddard](https://imagine.gsfc.nasa.gov/ask_astro/stars.html)
10. [What Is a Light-Year? Live Science](https://www.livescience.com/56115-what-is-a-light-year.html)
11. [Parallax, StarChild, NASA Goddard](https://starchild.gsfc.nasa.gov/docs/StarChild/questions/parallax.html)
12. [Parallax, Cosmic Distance Ladder, NAAP, University of Nebraska-Lincoln](https://astro.unl.edu/naap/distance/parallax.html)
13. [NASA's Hubble Extends Stellar Tape Measure 10 Times Farther Into Space, NASA](https://www.nasa.gov/news-release/nasas-hubble-extends-stellar-tape-measure-10-times-farther-into-space/)
14. [Measuring stellar distances by parallax, ESA](https://www.esa.int/ESA_Multimedia/Images/2013/06/Measuring_stellar_distances_by_parallax)
15. [How does Gaia study the Milky Way? ESA](https://www.esa.int/Science_Exploration/Space_Science/Gaia/How_does_Gaia_study_the_Milky_Way)
16. [What is a light-year? NASA Science](https://science.nasa.gov/exoplanets/what-is-a-light-year/)
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*Topic: Encyclopedia › Physical world and mathematics › Measurement and time › Units and unit systems › Units by physical quantity › Units of length and distance*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 15, 2026 · Edited: Sep 30, 2026; Oct 15, 2026 · Last review: Oct 15, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license