Astronomical Converter
Convert light-years, parsecs, AU, and kilometers.
Enter a value, choose the units, and see the result plus every other astronomical distance unit.
Light-year → Astronomical unit
Astronomical Distance Converter
Convert cosmic distances between kilometers, miles, light-seconds, light-minutes, light-hours, light-days, light-years, astronomical units, parsecs, kiloparsecs, megaparsecs, Earth radii, lunar distances, and solar radii. Enter a distance once and see how it looks on every scale – from the Moon's orbit to neighboring galaxies.
The main astronomical units
| Unit | Definition | Kilometers |
|---|---|---|
| Astronomical unit (au) | Mean Earth–Sun distance, fixed by the IAU in 2012 | 149,597,870.7 |
| Light-year (ly) | Distance light travels in 365.25 days | ≈ 9.461 × 10¹² |
| Parsec (pc) | Distance with 1″ parallax | ≈ 3.086 × 10¹³ |
| Lunar distance (LD) | Mean Earth–Moon distance | 384,399 |
Distances that make sense of the scale
- The Moon is about 1.28 light-seconds away – the delay you hear in Apollo radio transcripts.
- The Sun is 1 au, or about 8.3 light-minutes.
- Neptune orbits at roughly 30 au, about 4.2 light-hours.
- Proxima Centauri, the nearest star, is about 4.2465 light-years or 1.302 parsecs away.
- The Andromeda Galaxy is roughly 0.77 megaparsecs, about 2.5 million light-years.
Worked example
A news article says an asteroid will pass at 5 lunar distances. Select Lunar distance, enter 5, and read about 1.92 million km, 0.0128 au, or 6.4 light-seconds.
Why light-time units are useful
Distances expressed as light travel time tell you the communication delay with spacecraft. A rover on Mars, between about 3 and 22 light-minutes away depending on orbital positions, cannot be driven in real time. For everyday distances use the Length Converter; for orbital speeds, try the Speed Converter.
Frequently Asked Questions
How far is a light-year in kilometers?
One light-year is exactly 9,460,730,472,580.8 km – about 9.46 trillion km – the distance light travels in one Julian year of 365.25 days.
What is the difference between a parsec and a light-year?
A parsec is the distance at which one astronomical unit subtends an angle of one arcsecond. It is about 3.2616 light-years or 206,265 AU. Astronomers prefer parsecs because they come directly from parallax measurements.
How long does sunlight take to reach Earth?
One astronomical unit is about 499 light-seconds, so sunlight takes roughly 8 minutes and 19 seconds to reach Earth.