What is a light year?

Last updated 4 November 2026

A light year is a unit of distance, not time: how far light travels in one year, about 9.46 trillion kilometres (roughly 5.88 trillion miles). Astronomers use it because the distances to stars are far too large for kilometres to be practical. The nearest star beyond the Sun, Proxima Centauri, is about 4.2 light years away.

The exact figure

Light crosses space at 299,792.458 kilometres per second, a figure fixed by definition, and for the light year astronomers use a year of exactly 365.25 days. Multiply the two and you get:

• 9,460,730,472,580.8 kilometres, about 9.46 trillion km;
• about 5.88 trillion miles;
• about 63,000 times the distance from the Earth to the Sun (NASA's space flight glossary gives 63,197 astronomical units).

Distance, not time

The name makes it sound like a length of time, but "year" describes how long the light has been travelling, and the light year measures how far it went, just as a town can be "an hour's drive away". A star 25 light years away is one whose light has taken 25 years to reach us.

How far familiar things are

Light could circle the Earth more than seven times in one second. Even so:

For scale: a car driven at a steady 70 miles an hour, without stopping, would take more than nine million years to cover a single light year.

Looking at stars is looking back in time

Because light takes time to travel, you never see a star as it is now, only as it was when its light set off. Sirius on a winter night is light that left it nearly nine years ago. The light from Polaris set out more than four hundred years ago. The faint glow of the Andromeda Galaxy, visible to the naked eye from a dark site, left it around 2.5 million years ago, long before modern humans existed. It is also why astronomers can study the history of the universe: the further away they look, the further back they see.

How star distances are measured

For nearby stars, astronomers use parallax. As the Earth orbits the Sun, a nearby star seems to shift very slightly against more distant stars behind it, just as your thumb jumps when you look at it with one eye and then the other. The smaller the shift, the further away the star. Measuring those tiny angles from space was the main job of the European Space Agency's Hipparcos satellite, which measured parallaxes for over 100,000 stars; its successor, Gaia, has since measured far more. Further out, astronomers use a chain of other methods, such as stars and exploding stars of known true brightness, calibrated by those parallax measurements.

Light years, parsecs and astronomical units

• Astronomical unit (au): the average distance from the Earth to the Sun, about 150 million km. Used within the Solar System.
• Light year: about 9.46 trillion km. The unit most used in popular astronomy.
• Parsec: about 3.26 light years. Professional astronomers prefer it because it comes straight from parallax measurements; a kiloparsec is a thousand parsecs and a megaparsec a million.

The brightest stars in the UK sky range from under 9 light years to thousands of light years away; our list of the brightest stars in the UK sky gives their distances.

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