Science / Astronomy
Light-Year Converter (Light-Years, Parsecs, AU, km)
Convert astronomical distances between light-years, parsecs, astronomical units, light-minutes, light-seconds, and kilometres, and see how long light takes to cross the distance.
Light-Year Converter (Light-Years, Parsecs, AU, km): Every unit is converted through metres using exact definitions: the speed of light is 299,792,458 m/s, a light-year is the distance light travels in 365.25 days, the astronomical unit is 149,597,870,700 m, and a parsec is 648,000 ÷ π AU. Proxima Centauri, 4.24 light-years away, is about 40 trillion km from us. Runs 100% locally in your browser with zero server file uploads.
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| Kilometres | 4.0113 × 10^13 |
|---|---|
| Astronomical units | 268,142.1668 |
| Parsecs | 1.3 |
| Light-seconds | 1.3380 × 10^8 |
| Light-minutes | 2.2301 × 10^6 |
A light-year is the distance light travels in a Julian year of 365.25 days: 9,460,730,472,580.8 km. The astronomical unit, the average Earth–Sun distance, is defined as exactly 149,597,870,700 m, and a parsec, the distance at which 1 AU looks 1 arcsecond wide, is 3.26 light-years. Proxima Centauri, the nearest star after the Sun, is about 4.24 light-years away.
Scale of the universe
The Moon is 1.3 light-seconds away, the Sun 8.3 light-minutes, Neptune about 4 light-hours, the nearest star 4.24 light-years, the centre of the Milky Way about 26,000 light-years, and the Andromeda galaxy about 2.5 million light-years.
Big numbers
Distances in kilometres quickly need powers of ten; the scientific notation converter helps to read and write them.
How to use it
- Choose the unit you have.
- Enter the distance.
- Read it in every other unit, and the light travel time.
Privacy & limitations
Everything is calculated in your browser.
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Frequently asked questions
Is a light-year a unit of time?
No, of distance: how far light goes in a year, about 9.46 trillion km.
Why do astronomers use parsecs?
A parsec comes straight from parallax measurements: a star 1 parsec away appears to shift by 1 arcsecond as Earth goes round the Sun.
How long does sunlight take to reach Earth?
About 8 minutes 19 seconds, since 1 AU is 499 light-seconds.
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