About the Astronomical Converter
Distances in astronomy jump across twenty orders of magnitude, so no single unit is comfortable for all of them. Solar system work is quoted in astronomical units, nearby stars in light years or parsecs, and galaxy surveys in megaparsecs. This converter carries all of those alongside metres, kilometres and miles so a figure from a press release can be turned into something you can picture.
The definitions used are the modern exact ones. Since 2012 the astronomical unit has been fixed at exactly 149597870700 metres rather than being tied to a measurement of the Earth orbit. A light year follows from the exact speed of light and the Julian year of 365.25 days, giving 9460730472580800 metres. A parsec is the distance at which one au subtends one arcsecond, about 3.26 light years, which is why professional papers prefer it over the light year.
Light second, light minute, light hour and light day are included because they double as travel times for a radio signal. Sunlight reaches Earth in roughly 8.3 light minutes, so entering 1 au with light minutes as the target shows that directly. Earth radius and solar radius are there for planet and star size comparisons. For distances that stay on the ground, the Length Converter is the better fit.
How to use
- Enter one distance per line, without the unit suffix if you can.
- Choose the source unit, for example astronomical unit.
- Choose the target unit, for example light year or kilometre.
- Read the converted values and the reference table underneath.
Common questions
- How far is one astronomical unit in kilometres?
- Exactly 149597870.7 kilometres, since the au is defined as 149597870700 metres.
- Why do astronomers use parsecs instead of light years?
- A parsec falls straight out of parallax measurements, which is how stellar distances are actually determined. One parsec is about 3.26 light years.
- How many astronomical units is a light year?
- About 63241 au. Enter 1 with light year as the source and astronomical unit as the target to see the full figure.
- Can I convert galaxy scale distances?
- Yes. Kiloparsecs and megaparsecs are both listed, so a 20 Mpc figure converts cleanly into light years.