- A light year measures distance, not time
- How far one light year is in miles and kilometres
- The official IAU definition of a light year
- Light year distances to nearby stars and galaxies
- Why a light year lets you look back in time
- Light years, astronomical units and parsecs
- Common light year mix-ups to avoid
- Frequently asked questions
A light year is a unit of distance, not time: it's how far light travels in one year. NASA puts it at about 5.88 trillion miles, or 9.46 trillion kilometres. The International Astronomical Union defines it using a Julian year of 365.25 days. Proxima Centauri, the nearest star after the Sun, is about 4.25 light years away.
The phrase sounds like a stretch of time, and that's exactly why it trips so many people up. A light year is a yardstick for space, and once you know how it's built, star distances and those headlines about ancient galaxies start to make a lot more sense.
I've gathered the facts here from NASA, NASA's Space Place and the International Astronomical Union's definition as summarised on Wikipedia, so every number below comes from those pages. Here's how I'd explain it, step by step.
A light year measures distance, not time
Despite the word year in its name, a light year is a unit of distance. NASA defines it simply as the distance light travels in one year. The year tells you how long the light has been travelling; the answer you get is a length.
Think of it like saying a town is two hours' drive away. You're describing a distance by how long a journey takes at a known speed. With light years, the speed is the speed of light, which is the fastest thing we know of, so the distances are enormous.
How far one light year is in miles and kilometres
NASA says light moves at about 186,000 miles (300,000 kilometres) per second. Keep that up for a year and you've covered about 5.88 trillion miles, or 9.46 trillion kilometres. That's one light year.
NASA's own examples help shrink it down to something easier to picture:
- In one minute, light travels about 11,160,000 miles.
- In one hour, it covers about 671 million miles.
- Earth is about eight light minutes from the Sun.
- Sunlight takes about 43.2 minutes to reach Jupiter.
Those smaller units follow the same logic as the light year. A light minute is how far light travels in a minute, and a light hour is how far it goes in an hour. Scale that up and the numbers get dizzying fast: NASA notes that light from Kepler-443 b, one of the more distant known planets in our galaxy, would take about 3,000 years to reach us. That planet is roughly 3,000 light years away.
The official IAU definition of a light year
For scientific work, the length needs to be exact. The International Astronomical Union defines a light year as the distance light travels in a vacuum in one Julian year, which is 365.25 days. Using the defined speed of light, that comes to 9,460,730,472,580.8 kilometres.
The Julian year is used because it's a fixed, tidy length, so everyone calculating a light year gets the same answer. You don't need that level of detail day to day; rounding to about 9.46 trillion kilometres is fine for almost any conversation.
Light year distances to nearby stars and galaxies
Here's a sense of scale, using the distances NASA and NASA's Space Place give. Different NASA pages round these slightly differently, so treat them as approximate.
| Object | Approximate distance |
|---|---|
| The Sun | About 8.3 light minutes |
| Proxima Centauri, the nearest star after the Sun | About 4.25 light years |
| The TRAPPIST-1 planetary system | About 40 light years |
| Polaris, the North Star | About 320 light years |
| The centre of the Milky Way | About 26,000 light years |
| The Andromeda Galaxy | About 2.5 million light years |
NASA adds that the Milky Way itself is about 100,000 light years across. It also points out that even our nearest neighbouring star is out of reach for now: a jet flight to Proxima Centauri would take around 5 million years.
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Why a light year lets you look back in time
Because light takes time to travel, you never see a distant object in its present state. You see it as it was when that light set off. NASA's Space Place explains that the farther away an object is, the further back in time you're seeing it.
The Andromeda Galaxy is the classic example. It's about 2.5 million light years away, so the light reaching your eyes left it about 2.5 million years ago. Space Place also describes the galaxy GN-z11, observed by Hubble in 2016: it's about 13.4 billion light years away, so we see it as it was roughly 400 million years after the Big Bang. Telescopes, in that sense, are time machines.
Light years, astronomical units and parsecs
Astronomers use a few different distance units, depending on the scale:
- Astronomical unit (au). NASA bases it on the mean distance from Earth to the Sun, 149,597,870 kilometres. It's handy inside our solar system. NASA puts one light year at about 63,197 au.
- Light year. The everyday unit for distances to stars and galaxies.
- Parsec. The distance at which an object shows a parallax of one arcsecond. It's about 3.26 light years, roughly 30.9 trillion kilometres, and professional astronomers usually prefer it. The name blends parallax and second, and it was proposed by the British astronomer Herbert Hall Turner in 1913.
Common light year mix-ups to avoid
- Using it as a time. Saying something took light years to happen is a turn of phrase, but in science it's always a distance.
- Assuming you see stars as they are today. You're seeing old light, sometimes very old.
- Expecting exact matches between sources. Rounded distances differ a little from page to page, so check which source a number comes from.
Light also explains plenty closer to home. Once you're thinking about how it travels, you might enjoy why the sky is blue, which comes down to sunlight being scattered by the air.
Astronomy writers and teachers might enjoy my list of science websites open to contributor articles.
Frequently asked questions
Is a light year a measure of time?
No. It's a distance: how far light travels in one year. The word year describes how long the light travels, not what's being measured.
How many kilometres are in one light year?
About 9.46 trillion kilometres, or about 5.88 trillion miles, according to NASA. The IAU's exact value is 9,460,730,472,580.8 kilometres.
How far away is the nearest star?
Apart from the Sun, it's Proxima Centauri, which NASA puts at about 4.25 light years away.
How long does light from the Sun take to reach Earth?
About eight minutes. NASA's Space Place gives about 8.3 minutes, which is why astronomers say Earth is around eight light minutes from the Sun.
What's the difference between a light year and a parsec?
Both are distances. A parsec is longer, about 3.26 light years, and it's based on parallax rather than on how far light travels.
