What Is a Light Year? Definition and Examples

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The light year is a unit of length used to measure vast distances, like between galaxies.
The light year is a unit of length used to measure vast distances, like between galaxies. (photo: Guillermo Ferla)

The light year (ly) is a unit of length that is the distance light travels in a vacuum in one Earth year. One light year is approximately 9.46 trillion kilometers (9.46 x 1012 km) or 5.88 trillion miles (5.88 x 1012 mi). The light year is used to describe distances to stars without having to use very large numbers.

Abbreviations for light year and its multiple are:

  • ly – one light year
  • kly – 1000 light years or one kilolight year
  • Mly – 1,000,000 light years or one megalight year
  • Gly – 1,000,000,000 light year or one gigalight year

Examples of Distances in Light Years

Here is a map of stars within 12 light years of Earth and the Sun.
Here is a map of stars within 12 light years of Earth and the Sun. (image: Richard Powel)

Here is a light of astronomical objects and their distance from the Sun in light years:

ObjectDistance (light years)
approximate distance to the Moon4.04×10−8 ly
Voyager 1 spacecraft on October, 20180.0028 (almost 20 light hours)
Oort cloud1.6
Edge of Sun’s gravitational pull2.0
Proxima Centauri (closest star)4.2
Sirius8.6
Distance to center of the Milky Way26,000
Length of the Milky Way100,000
Andromeda Galaxy2.5 million
3C 273 (brightest quasar)2.4×109
Visible edge of the universe4.57×1010

How Long Is a Light Year?

One common misconception about the light year is thinking it is a unit of time. This arises because the unit has the word “year” in it. The length of a light year is a length or distance, not a time (9.46 x 1012 kilometers or 5.88 x 1012 miles).

Julian Year vs Gregorian Year

The light year is defined with the speed of light defined as 299792458 m/s and the year being a Julian year (365.25 days).

There are different ways to measure the length of a year on Earth. The light year is defined as distance light travels in a Julian year (365.25 days). This is slightly different from the Gregorian year (365.2425 days). The Gregorian year is the type of year most of the world uses, based on the Gregorian calendar. Before 1984, astronomers defined the light year using a measured speed of light (as opposed to a defined speed) and a tropical year (time it takes for the Earth to return to the same position, like summer solstice to summer solstice, which is 31556925.9747 ephemeris seconds). Before 1984, a light year was approximately 9.460530×1012 km. For the most part, the change doesn’t make much of a difference, but it’s good to know!

Light Year, Parsec, and AU

In addition to the light year, two other units of length are used in astronomy:

The astronomical unit (AU or au) is the distance from the Sun to the Earth. The distance between the Sun and Earth changes throughout the year because Earth’s orbit is an ellipse, but is equal to approximately 93 million miles or 150 million kilometers. In 2012, the AU was defined as exactly 149,597,870,700 meters. This is approximately 92,9555807 million miles. The modern AU definition is based on the definition of the meter, while retaining the spirit of the original definition. Because the AU is a relatively short distance (in astronomy), scientists use the astronomical unit to measure distances within the solar system or around other stars.

The parsec (pc) is a unit of length defined as exactly 648000/π astronomical units. It is the distance from the Sun to an astronomical object with a parallax angle of one arcsecond. One parsec is equal to about 3.3 light years, 210,000 AU, 31 trillion kilometers, or 19 trillion miles. It is used to measure large distances in astronomy. Multiples of parsecs are used for enormous distances, like kiloparsecs (kpc) within the Milky Way, megaparsecs (Mpc) for mid-distance galaxies, and gigaparsecs (GPc) for distant galaxies and quasars.

In summary:

  • A light year (ly) is the distance light travels in one Earth year. It is 9.4607×1015 meters or 5.8786×1012 miles, about 63 astronomical units or about 0.3 parsecs. It is an intermediate unit of astronomical distance.
  • An astronomical unit (AU) is approximately the distance from the Sun to the Earth. It is defined as exactly 149,597,870,700 meters or about 92,9555807 million miles. It is the shortest astronomical unit of distance.
  • A parsec (pc) is the distance from the Sun to a distance object with a parallax angle of one arcsecond. It is about 3.3 light years, 31 trillion kilometers, or 19 trillion miles.

References

  • Cox, Arthur N., ed. (2000). Allen’s Astrophysical Quantities (4th ed.). New York: AIP Press / Springer. ISBN 978-0387987460.
  • Hussmann, H.; Sohl, F.; Oberst, J. (2009), “Astronomical units.” in Joachim E Trümper (ed.). Astronomy, Astrophysics, and Cosmology – Volume VI/4B Solar System. Springer. ISBN 978-3-540-88054-7.
  • Luque, B.; Ballesteros, F. J. (2019). “To the Sun and beyond.” Nature Physics. 15: 1302. doi:10.1038/s41567-019-0685-3
  • McNamara, D. H.; Madsen, J. B.; Barnes, J.; Ericksen, B. F. (2000). “The Distance to the Galactic Center.” Publications of the Astronomical Society of the Pacific 112 (768): 202. doi:10.1086/316512
  • Seidelmann, P. Kenneth (ed.) (1992). Explanatory Supplement to the Astronomical Almanac. Mill Valley, California: University Science Books. ISBN 978-0-935702-68-2.