A light-year is the distance light travels in a Julian year.
Light-years to Astronomical units Converter — ly to AU
Convert Light-year (ly) to Astronomical unit (AU) using the exact conversion factor (1 light-year = 63,241.07708 astronomical unit). See the formula, worked examples, conversion table, unit comparison, history, and common uses.
Light-years to Astronomical units converter
Converter
Length Converter
Convert SI, imperial, US customary, astronomical, scientific, and historical length units.
Context values
Some unit families need one extra value for a physically accurate result.
An astronomical unit is the mean Earth-Sun distance.
Result = input x 9.46073047e+15 / 1.49597871e+11.
Length uses the meter as the base unit.
Reserved above the conversion cards and below the converter.
About Converting Light-years to Astronomical units
Light-year and Astronomical unit both measure length. Light-year is typically associated with used internationally in astronomy and popular science for interstellar distances., while Astronomical unit is typically associated with used internationally in astronomy for solar-system-scale distances..
Formula
astronomical units = light-years × 63241.0770843
This factor comes from each unit's defined relationship to the category's base unit: 1 light-year equals 9.46073e+15 base units, and 1 astronomical unit equals 149597870700 base units, so dividing one by the other gives the direct light-year-to-astronomical unit factor of 63241.0770843.
Simple example
1 ly × 63241.07708 = 63,241.07708 AU
1 light-year = 63,241.07708 astronomical units.
Real-world example
10 ly × 63241.07708 = 632,410.7708 AU
10 light-years = 632,410.7708 astronomical units.
Conversion table
| Light-year (ly) | Astronomical unit (AU) |
|---|---|
| 1 ly | 63,241.07708 AU |
| 5 ly | 316,205.3854 AU |
| 10 ly | 632,410.7708 AU |
| 21.0975 ly | 1,334,228.624 AU |
| 42.195 ly | 2,668,457.248 AU |
| 100 ly | 6,324,107.708 AU |
Reverse conversion: Astronomical unit to Light-year
1 AU × 0.00001581250741 = 0.0000158125 ly
1 astronomical unit = 0.0000158125 light-years.
light-years = astronomical units × 0.0000158125074098
For a page dedicated to this direction, see Astronomical unit to Light-year.
Light-year vs. Astronomical unit
| Aspect | Light-year | Astronomical unit |
|---|---|---|
| Family | Non-SI unit (astronomical) | Non-SI unit accepted for use with SI (astronomical) |
| Typical use | Measuring distances to stars and galaxies | Measuring distances within the solar system (planetary orbits) |
| Where it's used | Used internationally in astronomy and popular science for interstellar distances. | Used internationally in astronomy for solar-system-scale distances. |
Understanding the Light-year
Light-year (ly) measures distance. A light-year is the distance light travels in one Julian year (365.25 days) in vacuum: exactly 9,460,730,472,580,800 meters. It is classified as a Non-SI unit (astronomical).
Where it's used: Used internationally in astronomy and popular science for interstellar distances.
Uses of the Light-year today
- Measuring distances to stars and galaxies
- Popular science communication about the scale of the universe
- Astrophysics research
History of the Light-year
The light-year emerged in 19th-century astronomy as scientists began measuring stellar distances via parallax (Friedrich Bessel's 1838 measurement of 61 Cygni is often cited as the first reliable stellar distance). Expressing these vast distances in light-travel-time became a practical convention, and the unit's value follows directly from the defined speed of light and the length of a Julian year.
Historical origin. Light-year was not the invention of a single named person; it developed through the process described above.
Understanding the Astronomical unit
Astronomical unit (AU) measures distance. The astronomical unit is defined as exactly 149,597,870,700 meters, close to the average Earth-Sun distance. It is classified as a Non-SI unit accepted for use with SI (astronomical).
Where it's used: Used internationally in astronomy for solar-system-scale distances.
Uses of the Astronomical unit today
- Measuring distances within the solar system (planetary orbits)
- Astronomical calculations and spacecraft navigation
- Popular science communication about the solar system
History of the Astronomical unit
The astronomical unit originated from 17th-19th century attempts to measure the Earth-Sun distance, refined over time using transits of Venus and later radar and spacecraft ranging.
In 2012, the International Astronomical Union fixed the AU at exactly 149,597,870,700 meters, replacing its earlier definition (tied to gravitational calculations) with a simple fixed distance, independent of the slightly-variable actual Earth-Sun distance.
Historical origin. Astronomical unit was not the invention of a single named person; it developed through the process described above.
Light-year — sources
- IAU — Light year definition — Defines the light-year via the Julian year and the speed of light.
Astronomical unit — sources
- IAU Resolution B2 (2012) — Fixes the astronomical unit at exactly 149,597,870,700 meters.
Reserved between the cards and the FAQ so the page stays balanced.
Frequently asked questions
How many astronomical units are in 1 light-year?
1 light-year equals 63,241.07708 astronomical units, using the exact defined conversion factor rather than an estimate.
How do I convert light-year to astronomical unit?
Multiply the light-year value by 63241.07708. The converter above does this instantly to whatever precision you set.
How do I convert astronomical unit back to light-year?
Use the reverse factor: 1 astronomical unit equals 0.0000158125 light-years. You can also use the swap control in the converter above to flip the direction instantly.
What is a light-year?
Light-year (ly) measures distance. A light-year is the distance light travels in one Julian year (365.25 days) in vacuum: exactly 9,460,730,472,580,800 meters. It is classified as a Non-SI unit (astronomical).
What is a astronomical unit?
Astronomical unit (AU) measures distance. The astronomical unit is defined as exactly 149,597,870,700 meters, close to the average Earth-Sun distance. It is classified as a Non-SI unit accepted for use with SI (astronomical).
Is the light-year to astronomical unit conversion exact?
Yes. The astronomical unit is defined as exactly 149,597,870,700 meters, close to the average Earth-Sun distance. That fixed definition is what the converter above uses, so any rounding you see is a display choice, not a limitation of the math.
Where is the light-year still used today?
Used internationally in astronomy and popular science for interstellar distances.