Astronomical Distance Converter

What this converter does

Space is too big for kilometres. The distance to the nearest star, written out in km, runs to fourteen digits, so astronomers reach for bigger rulers: astronomical units for the solar system, light-years and parsecs for the stars. This converter moves a distance across all of them, from kilometres and miles up to parsecs.

How to use it

Type the distance, set From to the unit you have and To to the unit you want, and press Calculate. It opens on kilometres to astronomical units. Swap reverses the pair and Reset clears the field. Because these figures get enormous, the result is shown to nine significant figures.

How it works

Every unit is pinned to the metre, the same base the everyday distance tools use, just stretched to cosmic scale. An astronomical unit is fixed at 149,597,870,700 metres, and the light-based units are built from the speed of light: a light-second is how far light travels in one second, a light-year how far it travels in one year. The tool converts your value to metres and then out to the unit you want.

Common astronomical conversions

The ones that come up most:

A light-year is a distance, not a time

This trips people up constantly, so it is worth saying plainly: a light-year is a distance. It is how far light travels in a year, about 9.46 trillion kilometres, and the word "year" in the name is the source of the confusion. The same holds for light-seconds, light-minutes, and light-days.

Each unit suits a different scale:

• The astronomical unit is the average Earth-to-Sun distance, about 150 million km. Light crosses it in roughly 8.3 minutes, so the sunlight you see set off about eight minutes ago. It is the natural ruler for the solar system, where Neptune orbits at about 30 AU.
• The light-second through light-day fill the gap in between. The Moon, for instance, is about 1.28 light-seconds away.
• The light-year and parsec are for the stars. The nearest star beyond the Sun, Proxima Centauri, is about 4.24 light-years, or 1.3 parsecs, away. The parsec (about 3.26 light-years) is the professional astronomer's unit, defined by a quirk of geometry: the distance at which the Earth-Sun gap would appear one arcsecond wide.

Questions people ask

References

1. National Institute of Standards and Technology (NIST), Office of Weights and Measures. The International System of Units (SI), the metre, and units used with it. https://www.nist.gov/pml/owm/metric-si/si-units