What is a light-year? A unit of distance with "year" in its name

A news story says, “This star is 8 light-years away from us.” Because the word “year” appears in the sentence, many people assume it is talking about a length of time. But a light-year is a unit of length, just like the kilometre. Its name comes from the distance light travels in a year. In this article we work it out from scratch and look at how it changes the way you see the sky.

The sum: how far does light travel in a year?

The speed of light in a vacuum is defined as exactly 299,792,458 metres per second; we can say roughly 300,000 km/s. In astronomy a year is taken as the 365.25-day Julian year:

• 365.25 days × 24 hours × 3,600 seconds = 31,557,600 seconds
• 299,792.458 km/s × 31,557,600 s ≈ 9.46 trillion kilometres (9.46 × 10¹² km)

You do not need to memorise this number; what matters is knowing that it is a distance, not a time.

Using light as a ruler

The speed of light is enormous, but it is finite. So we can also express distances by how long light takes to cover them:

• The Moon: On average 384,400 km away. Light reaches it in about 1.3 seconds.
• The Sun: Its average distance from Earth is 149,597,870.7 km (1 astronomical unit). Light covers this in about 499 seconds, that is 8 minutes 19 seconds. The Sun you see right now is the Sun of more than 8 minutes ago.
• Sirius: The brightest star in the sky, about 8.6 light-years from Earth. If you look at it tonight, the light you see set off about 8.6 years ago.
• Proxima Centauri: The nearest star to the Sun, about 4.2 light-years away; it cannot be seen with the naked eye.
• The Andromeda Galaxy: About 2.5 million light-years away. In a dark sky it can be picked out with the naked eye as a faint smudge; the light you see set off about 2.5 million years ago.
• The Milky Way: Our galaxy is of the order of 100,000 light-years across.

Why light-years instead of kilometres?

Inside the Solar System the astronomical unit is enough to describe distances: Neptune is about 30 astronomical units from the Sun. When it comes to stars, figures in kilometres run to thirteen or fourteen digits and become impossible to hold in your head. The light-year makes these numbers manageable: we can talk in small numbers such as “4.2” or “8.6”. It also has a pleasant side effect: when you state a distance, you also state how old that light is.

How long would it take to get there in a spacecraft?

Voyager 1, which is moving away from the Solar System, travels at about 17 km/s; that is very fast for a spacecraft, but very slow next to light. The roughly 4.2 light-years to Proxima Centauri is close to 40 trillion km. At that speed the journey would take about 40 × 10¹² ÷ 17 ≈ 2.4 × 10¹² seconds, that is about 75,000 years. For comparison, about five thousand years have passed since writing was invented. This is why astronomy advances by analysing the light that reaches us rather than by travelling to the stars.

Looking at the sky means looking into the past

Because the speed of light is finite, when you look at a distant object you see it not as it is today, but as it was when its light set off. This is not an illusion but a physical fact. It lets astronomers observe the past directly: looking at very distant galaxies means looking at a younger stage of the universe.

This also clarifies the sentence “the star has long since died but we still see it”. In theory it is possible: a very distant object may have changed in the time its light takes to reach us. But stars usually live for millions of years, and the light of the stars you see with the naked eye mostly set off somewhere between a few years and a few thousand years ago. So there is no reason to think the overall look of the sky has long since changed.

Common mistakes

• “A light-year is a unit of time.” No, it is a unit of length. The answer to “how many years would it take to get there?” only makes sense together with a speed.
• “Light arrives instantly.” No; it takes 1.3 seconds to reach the Moon and more than 8 minutes to reach us from the Sun.
• “Stars in the same constellation are close to one another.” No; appearing side by side in the sky is a result of our line of sight. Stars in the same constellation can be at very different distances from Earth.

Try it yourself

If the distance to the Moon is 384,400 km and the speed of light is 300,000 km/s, how many seconds does light take to reach the Moon? (384,400 ÷ 300,000 ≈ 1.28 seconds.) Now try the same for the Sun: 149,600,000 ÷ 300,000 ≈ 499 seconds, or about 8.3 minutes.

In Kuazara’s How small are we in the universe? journey you can try out the scale of the planets and the light-year interactively. For more, see our astronomy articles and the Science topic.

Summary

• A light-year is the distance light travels in one Julian year: about 9.46 trillion kilometres.
• The Sun’s light reaches us in about 8 minutes 19 seconds, and Sirius’s light in about 8.6 years.
• Looking at a distant object means looking at its past.
• At Voyager 1’s speed, reaching the nearest star would take about 75,000 years.