A light-year is a unit of length: the distance light travels through empty space in one year, about 9.46 trillion kilometers. Astronomers use it to describe the enormous distances between stars and galaxies. Despite its name, a light-year measures distance, not time.

Definition

The International Astronomical Union (IAU), which maintains astronomy's special units, defines the light-year as the distance light travels in a vacuum in one Julian year. A Julian year is exactly 365.25 days of 86,400 seconds each, or 31,557,600 seconds. Light in a vacuum travels exactly 299,792,458 meters per second, about 300,000 kilometers per second.

Multiplying the two gives exactly 9,460,730,472,580.8 kilometers, a little under 9.5 trillion. The IAU also gives it as about 63,241 astronomical units, where one astronomical unit is the average distance from Earth to the Sun. Because the definition fixes both the speed and the year, a light-year has no uncertainty at all.

To define the light-year, the IAU first had to say exactly what a year is, because there are several kinds of year of slightly different lengths. The IAU counts a year as a Julian year unless another kind is named.

Size comparisons

• A light-year is about 63,000 times the average distance from Earth to the Sun.
• If the Sun were shrunk to the size of a basketball, Earth would orbit it about 26 meters (85 feet) away, and one light-year would be about 1,600 kilometers (1,000 miles).
• The distance from Proxima Centauri to Alpha Centauri A, two stars in the nearest star system to the Sun, is about one-fifth of a light-year.
• Proxima Centauri is about 4.25 light-years from the Sun, about 40 trillion kilometers.
• The disk of the Milky Way, the galaxy that contains the Sun, is about 100,000 light-years across.

Light-years and parsecs

Professional astronomers more often use the parsec, which is about 3.26 light-years. The parsec is defined by an angle: it is the distance at which one astronomical unit would appear one arcsecond wide, where an arcsecond is 1/3,600 of a degree. The IAU describes the parsec as the usual choice for studying the Milky Way.

The IAU notes that the light-year is sometimes used in scientific papers, but that it appears mostly in writing for the general public. Within the Solar System, astronomers use the astronomical unit instead. Beyond it, the IAU says, distances are so great that the astronomical unit becomes too cumbersome.

Seeing into the past

Because light takes time to travel, distant objects are seen as they were when their light left them. Sunlight takes about 8.3 minutes to reach Earth, so the Sun always appears as it was about 8.3 minutes earlier.

The same is true on much larger scales. The Andromeda Galaxy, the nearest large galaxy, is about 2.5 million light-years away. The light seen from it today left it about 2.5 million years ago.

Sources

• Measuring the Universe. International Astronomical Union.
• Speed of light in vacuum. NIST CODATA, 2022.
• What Is a Light-Year?. NASA Space Place, 2020.
• Sun Fact Sheet. NASA, 2024.
• Proxima Centauri. SIMBAD Astronomical Database, Strasbourg Astronomical Data Center.
• Precision Millimeter Astrometry of the Alpha Centauri AB System. Akeson et al., The Astronomical Journal, 2021.
• Our Milky Way Galaxy: How Big is Space?. NASA, 2024.
• Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.