Astronomical distance converter for AU, light-years and parsecs
This astronomical distance converter handles the scales of space, from the radius of the Earth to megaparsecs between galaxies. It includes kilometers and miles for reference, light-travel units, the astronomical unit, the parsec family, and Earth and solar radii.
How each unit is defined
- Astronomical unit (au): set by an IAU resolution in 2012 at 149,597,870,700 meters, close to the mean Earth–Sun separation.
- Light-second and light-minute: the distance light travels in 1 s (299,792.458 km) or 60 s.
- Light-year (ly): how far light goes during one Julian year (365.25 days): 9,460,730,472,580.8 km.
- Parsec (pc): the distance at which 1 au subtends one arcsecond, which is 648,000/π au. The kiloparsec and megaparsec are 1000 and 1,000,000 parsecs.
- Earth radius: the mean radius, 6371 km. Solar radius: the nominal value, 695,700 km.
Examples
1 au = 8.316746397 lmin
1 pc = 3.261563777 ly
1 pc = 206264.8062 au
4.24 ly = 1.29998991 pc
384400 km = 1.282220382 ls
1 rs = 109.1979281 reSunlight takes about 8 minutes 19 seconds to reach us, since 1 au is 8.32 light-minutes. The nearest star beyond the Sun, Proxima Centauri, lies about 4.24 light-years away, which is 1.3 parsecs. The Moon's average distance of 384,400 km is only about 1.28 light-seconds.
Familiar distances for scale
These rounded reference values make it easier to sanity-check a result:
| Distance | Approximate value |
|---|---|
| Earth to Moon (average) | 384,400 km ≈ 1.28 light-seconds |
| Earth to Sun | 1 AU ≈ 8.3 light-minutes |
| Sun to Neptune | ≈ 30 AU ≈ 4.2 light-hours |
| Sun to Proxima Centauri | ≈ 4.25 light-years ≈ 1.30 pc |
| Sun to the Milky Way's center | ≈ 8.2 kpc ≈ 26,700 light-years |
| Milky Way to Andromeda | ≈ 0.77 Mpc ≈ 2.5 million light-years |
Each step up the table is a jump of several orders of magnitude, which is why astronomers switch units instead of writing ever longer numbers in kilometers.
Light-years versus parsecs
Popular writing favors light-years, while professional astronomy mostly uses parsecs, because a parallax angle in arcseconds converts straight to parsecs: distance in pc = 1 / parallax in arcsec. The angle converter handles arcseconds if you need them. Remember that the light-year is a distance, not a time.
Working with huge numbers
Very large or very tiny results appear in exponent notation, such as 2.4983761708e-10. At the default ten significant digits, 4.24 ly shows as 40113497200000 km; raise the precision if you need every digit. For shorter, everyday distances the length converter is simpler, and the speed converter includes the speed of light itself.