An astronomical unit is the mean Earth-Sun distance.
Astronomical units to Light-years Converter — AU to ly
Convert Astronomical unit (AU) to Light-year (ly) using the exact conversion factor (1 astronomical unit = 0.0000158125 light-year). See the formula, worked examples, conversion table, unit comparison, history, and common uses.
Astronomical units to Light-years 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.
A light-year is the distance light travels in a Julian year.
Result = input x 1.49597871e+11 / 9.46073047e+15.
Length uses the meter as the base unit.
Reserved above the conversion cards and below the converter.
About Converting Astronomical units to Light-years
Astronomical unit and Light-year both measure length. Astronomical unit is typically associated with used internationally in astronomy for solar-system-scale distances., while Light-year is typically associated with used internationally in astronomy and popular science for interstellar distances..
Formula
light-years = astronomical units × 0.0000158125074098
This factor comes from each unit's defined relationship to the category's base unit: 1 astronomical unit equals 149597870700 base units, and 1 light-year equals 9.46073e+15 base units, so dividing one by the other gives the direct astronomical unit-to-light-year factor of 0.0000158125074098.
Simple example
1 AU × 0.00001581250741 = 0.0000158125 ly
1 astronomical unit = 0.0000158125 light-years.
Real-world example
10 AU × 0.00001581250741 = 0.0001581251 ly
10 astronomical units = 0.0001581251 light-years.
Conversion table
| Astronomical unit (AU) | Light-year (ly) |
|---|---|
| 1 AU | 0.0000158125 ly |
| 5 AU | 0.0000790625 ly |
| 10 AU | 0.0001581251 ly |
| 21.0975 AU | 0.0003336044 ly |
| 42.195 AU | 0.0006672088 ly |
| 100 AU | 0.0015812507 ly |
Reverse conversion: Light-year to Astronomical unit
0.0000158125 ly × 63241.07708 = 0.9999995314 AU
0.0000158125 light-years = 0.9999995314 astronomical units.
astronomical units = light-years × 63241.0770843
For a page dedicated to this direction, see Light-year to Astronomical unit.
Astronomical unit vs. Light-year
| Aspect | Astronomical unit | Light-year |
|---|---|---|
| Family | Non-SI unit accepted for use with SI (astronomical) | Non-SI unit (astronomical) |
| Typical use | Measuring distances within the solar system (planetary orbits) | Measuring distances to stars and galaxies |
| Where it's used | Used internationally in astronomy for solar-system-scale distances. | Used internationally in astronomy and popular science for interstellar distances. |
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.
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.
Astronomical unit — sources
- IAU Resolution B2 (2012) — Fixes the astronomical unit at exactly 149,597,870,700 meters.
Light-year — sources
- IAU — Light year definition — Defines the light-year via the Julian year and the speed of light.
Reserved between the cards and the FAQ so the page stays balanced.
Frequently asked questions
How many light-years are in 1 astronomical unit?
1 astronomical unit equals 0.0000158125 light-years, using the exact defined conversion factor rather than an estimate.
How do I convert astronomical unit to light-year?
Multiply the astronomical unit value by 0.00001581250741. The converter above does this instantly to whatever precision you set.
How do I convert light-year back to astronomical unit?
Use the reverse factor: 1 light-year equals 63,241.07708 astronomical units. You can also use the swap control in the converter above to flip the direction instantly.
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).
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).
Is the astronomical unit to light-year conversion exact?
Yes. 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. 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 astronomical unit still used today?
Used internationally in astronomy for solar-system-scale distances.