Convert Light Year to A.u. of Length
Convert light years to a.u. of length instantly. 1 light year = 1.787819e+26 a.u. of length — use the live calculator, the exact formula, a conversion table and worked examples. Also check the A.u. of Length to Light Year converter for the reverse conversion.
Units explained
Light Year
A light year is the distance light travels in one Julian year in vacuum — exactly 9,460,730,472,580,800 meters (about 9.46×10¹⁵ m). It is the most common unit for expressing inter-stellar distances in non-technical astronomy.
Defined as the speed of light (299,792,458 m/s, exact) multiplied by one Julian year (365.25 days × 86400 seconds/day). Both factors are exact, making the light year exact by definition.
Light years are used in popular astronomy, science journalism, and stellar distance descriptions. The nearest star (Proxima Centauri) is 4.24 ly away; the Milky Way is about 100,000 ly across.
Concept of distance-as-time used since the 19th century; the modern exact definition follows from the 1983 SI definition of the meter via the speed of light.
A.u. of Length
The atomic unit of length (a.u.) is equal to the Bohr radius — approximately 5.292×10⁻¹¹ m. Despite the abbreviation matching 'astronomical unit', this is a different unit used in atomic and molecular physics.
The atomic unit system was developed in the 1950s for quantum-chemistry calculations. The a.u. of length is defined as the Bohr radius (a₀), the most probable distance between the proton and electron in a hydrogen atom in its ground state.
The atomic unit of length is used throughout quantum chemistry, electronic-structure theory, and computational physics. Most quantum-mechanical equations simplify when expressed in atomic units rather than SI units.
Atomic unit system developed by Hartree in the 1920s; value fixed via CODATA 2018 determination of fundamental physical constants.
Light Year to A.u. of Length conversion formula
The relationship between light years and a.u. of length:
To convert light years to a.u. of length, multiply the value in light years by 1.787819e+26. To reverse, multiply a.u. of length by 5.593408e-27.
How to use this converter
Type a value into the calculator. The result in a.u. of length updates as you type. Tap a quick value, copy the result with one click, or use the swap arrow to jump straight to the A.u. of Length to Light Year converter for the reverse direction.
Step-by-step: convert light years to a.u. of length
- Write down the value in light years (ly).
- Multiply that value by the factor 1.787819e+26.
- The product is the equivalent value in a.u. of length (a.u.).
- To reverse, multiply the a.u. of length value by 5.593408e-27.
Worked examples
Example 1 — Convert 1 ly to a.u.:
1 × 1.787819e+26 = 1.787819e+26 a.u.
Example 2 — Convert 100 ly to a.u.:
100 × 1.787819e+26 = 1.787819e+28 a.u.
Light Year to A.u. of Length conversion table
Standard reference values for converting light years to a.u. of length:
| Light Year [ly] | A.u. of Length [a.u.] |
|---|---|
| 0.01 | 1.787819e+24 |
| 0.1 | 1.787819e+25 |
| 1 | 1.787819e+26 |
| 2 | 3.575638e+26 |
| 3 | 5.363457e+26 |
| 4 | 7.151276e+26 |
| 5 | 8.939095e+26 |
| 10 | 1.787819e+27 |
| 20 | 3.575638e+27 |
| 30 | 5.363457e+27 |
| 40 | 7.151276e+27 |
| 50 | 8.939095e+27 |
| 100 | 1.787819e+28 |
| 500 | 8.939095e+28 |
| 1000 | 1.787819e+29 |
Frequently asked questions
How many a.u. of length is 1 light year?
How do I convert light years to a.u. of length?
How do I convert a.u. of length back to light years?
How many a.u. of length is 100 light years?
Popular length unit conversions
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Metric / SI (4 units)
Imperial / US Customary (4 units)
Astronomical (8 units)
Sources & references
Conversion factor (1 ly = 1.787819e+26 a.u.) verified against the following authoritative sources:
- BIPM — The International System of Units (SI Brochure 9th ed.)
Official BIPM publication defining the seven SI base units (including the meter) and the rules for their use. The global authority on units of measurement.
- NIST — Guide to the SI
US National Institute of Standards and Technology reference covering the SI base and derived units with definitions and usage rules for US technical practice.
- NIST Special Publication 811 — Guide for the Use of the International System of Units
Detailed NIST guide covering exact conversion factors between SI and US customary units along with formatting and rounding conventions.
- NIST — Refinement of values for the yard and pound (Federal Register 1959)
The treaty (signed by US
- International Astronomical Union — System of Astronomical Constants
The IAU defines astronomical units including the AU (149597870700 m exactly) light-year and parsec used in astronomy and astrophysics.