What Is a Light-Year? Making Sense of Space’s Enormous Distances

Some of the stars you see in the night sky are a few light-years away; others are thousands of light-years away. The phrase sounds familiar, but most people do not really know how vast a light-year is or why we need such a unit. Many even assume it is a unit of time.

A light-year is a distance

Light is the fastest thing in the universe. In a vacuum it travels about 299,792 kilometres per second. At that speed, light:

• could circle Earth 7.5 times in one second;
• reaches the Moon in 1.3 seconds;
• travels from the Sun to Earth in 8 minutes and 20 seconds.

A light-year is the distance light covers at that dizzying speed in one year. Let’s work it out:

299,792 km/s × 60 seconds × 60 minutes × 24 hours × 365.25 days ≈ 9.46 trillion kilometres

That is about 9,460,000,000,000 kilometres — a number so huge that kilometres stop being practical, rather like measuring the distance between London and Edinburgh in millimetres.

How big is a light-year?

It is very hard to grasp. Here is a comparison:

• A car driving non-stop at 100 kilometres per hour would take about 10.8 million years to cover one light-year.
• An airliner flying at 900 kilometres per hour would need about 1.2 million years.
• Voyager 1, humanity’s most distant spacecraft, travelling at roughly 61,000 kilometres per hour, would still need about 17,700 years.

After almost 50 years of travel since its launch in 1977, Voyager 1 is only reaching one light-day from Earth in autumn 2026 — just 1/365th of a light-year.

The scale of the universe

Light-years make distances in space understandable. Here is how long light takes to reach various objects:

Even if the Solar System were the size of a football pitch, the nearest star would be in another country.

Looking at stars is looking into the past

The most fascinating consequence of the light-year is this: when we look into space, we look into the past.

Light takes time to reach us, so we see objects not as they are now but as they were when their light set off:

• Looking at the Sun, you see it as it was 8 minutes ago.
• Seeing Proxima Centauri through a telescope, you see it as it was about 4 years ago.
• Looking at the Andromeda Galaxy, you see it as it was 2.5 million years ago — when its light set off, modern humans had not yet appeared on Earth.

That also means some stars may no longer exist. Even if a distant star has exploded, its light keeps arriving, so we continue to see it in the sky.

Astronomers use this like a time machine. By observing very distant galaxies, they can study the universe as it was billions of years ago. The James Webb Space Telescope has imaged galaxies whose light set off more than 13 billion years ago, from the early days of the universe.

How big is the universe?

The universe is about 13.8 billion years old. You might think, then, that we can see at most 13.8 billion light-years away. The truth is even stranger.

The universe is expanding. While light travelled towards us, the galaxies that emitted it kept moving away because of that expansion. So the observable universe — the part we can see today — is calculated to be about 93 billion light-years across. How much universe lies beyond it is unknown; it might even be infinite.

Astronomers’ other units

The light-year is the best known, but astronomers use other units too:

• Astronomical unit (AU): the average distance between Earth and the Sun, about 150 million kilometres — used within the Solar System.
• Parsec (pc): about 3.26 light-years, from the parallax method, which measures how much a star’s position shifts in the sky as Earth orbits the Sun.
• Megaparsec (Mpc): a million parsecs, used for distances between galaxies.

Why can’t we go faster than light?

The light-year also shows why space travel is so hard. According to Einstein’s special theory of relativity, no object with mass can reach the speed of light in a vacuum. As an object approaches it, the energy needed to accelerate it further heads towards infinity.

That is why reaching even the nearest star would take tens of thousands of years with today’s technology. Scientists are working on new ideas such as tiny laser-pushed light sails, which might let very small probes reach the nearest star within a few decades.

In short

Despite its name, a light-year is not a time but a distance: about 9.46 trillion kilometres, the distance light travels in a year. It makes the mind-boggling distances between stars and galaxies understandable. Because light’s journey takes time, looking into space means looking into the past — we see the Sun as it was 8 minutes ago and Andromeda as it was 2.5 million years ago. The scale of the universe stretches the limits of human imagination.