Astronomical Distance Converter (AU, light-year, parsec)

Understanding the astronomical distance converter — concept and principle

Distances in space are so large that kilometres stop being useful. Astronomers use the astronomical unit (au) inside the Solar System, light-years in popular writing and parsecs in professional catalogues. This converter moves a distance between au, light-years, parsecs, light-minutes, kilometres, metres and miles, so you can see, for example, how many kilometres away the nearest star is or how many au a parsec contains.

How it is calculated

Every value is converted to metres and then divided by the size of the target unit. The definitions used are:

• 1 au = 149,597,870,700 m exactly (IAU 2012)
• 1 light-year = speed of light (299,792,458 m/s) × one Julian year (365.25 days) = 9,460,730,472,580,800 m
• 1 parsec = 648,000/π au ≈ 206,264.806 au ≈ 3.26156 light-years (IAU 2015)
• 1 light-minute = 299,792,458 × 60 m ≈ 17.99 million km
• 1 mile = 1,609.344 m

The principle

The au is the scale of the Earth–Sun distance, fixed as an exact number of metres. The light-year and light-minute are distances, not times: how far light travels in that time, which works because the speed of light has an exact defined value. The parsec comes from parallax. As Earth moves around the Sun, a nearby star seems to shift against distant background stars; a star at one parsec shifts by one arcsecond when the baseline is 1 au. One radian contains 206,264.806 arcseconds (648,000/π), which is why 1 pc equals that many au.

Worked example

Default input: 4.24 light-years, about the distance to Proxima Centauri, in kilometres.

1. Metres: 4.24 × 9,460,730,472,580,800 ≈ 4.0113497 × 10¹⁶ m.
2. Kilometres: ÷ 1,000 = 40,113,497,200,000 km (about 40 trillion km).
3. In au: ÷ 149,597,870,700 m ≈ 268,142.17 au.
4. In parsecs: ≈ 1.29998991 pc; in light-minutes: 4.24 × 525,960 = 2,230,070.4.

How to read the result

Useful benchmarks: sunlight takes about 8.3 minutes to reach Earth (1 au); Neptune is about 30 au from the Sun; the nearest star is about 1.3 pc; the centre of the Milky Way is roughly 8 kpc away; the Andromeda galaxy is about 2.5 million light-years away. Distances to stars are measured, not defined, so published values carry uncertainty that is far larger than the rounding here.

Common mistakes

• Treating a light-year as a unit of time.
• Using a 365-day year; the IAU light-year uses the Julian year of 365.25 days.
• Mixing up long and short scales: 10¹² is a trillion in English and 1조 in Korean.

Frequently asked questions

How many light-years is a parsec?

About 3.2616 light-years.

How far is 1 au in kilometres?

Exactly 149,597,870.7 km, close to the mean Earth–Sun distance.

How far is the nearest star?

Proxima Centauri is about 4.24 light-years, or 1.30 parsecs, away.

Why do astronomers prefer parsecs?

Because parsecs come straight from measured parallax: distance (pc) = 1 ÷ parallax (arcseconds).

Sources: IAU 2012 Resolution B2 (definition of the astronomical unit) · IAU 2015 Resolution B2 (parsec defined as 648,000/π au) · NIST Special Publication 811 (2008), Appendix B