A guide to lightyears, the unit used by astronomers to measure vast distances in the cosmos

Using lightyears to measure distance in the Universe and how long it would take to travel one lightyear by foot, car, plane and rocket.

By Jenny Winder

Published: Saturday, 17 February 2024 at 08:34 AM

The numbers we use in astronomy are, literally, astronomical. It can be hard to get your head around so many zeros.

If we were to use kilometres and miles it would be like measuring your commute in millimetres.

To try to simplify things, when we discuss objects within our Solar System, we use the Astronomical Unit (AU) to measure distance.

One AU is the average distance between the Earth and the Sun or 150 million kilometres (93 million miles). Our Solar System has a diameter of just 1,921 AUs. So far so good.

To measure vast distances across space, scientists use the Parsec, the distance 1AU subtends an angle of 1 arc-second (1/3600 of a degree) which is 206,265 AUs, or 30.9 trillion km (19.2 trillion miles) and difficult for most of us to comprehend.

So the lightyear is the standard measure of distance for anything outside the Solar System.

Earth’s distance from the Sun is 1AU, but to describe much larger distances across the cosmos we need much bigger values. This is where the lightyear comes in. Credit: NASA

A simple definition of lightyear

Put simply, a lightyear is the distance light travels in space in a year, 9.46 trillion km (5.88 trillion miles) or 63,241 AU, 0.30 parsecs.

Nothing travels faster than light. It travels nearly one million times faster than sound. A lightsecond equals 300,000 km (186,000 miles).

A lightminute is about 18 million km ( 11 million miles) and a lighthour is 1.1 billion km.

One AU equals 8.3 light minutes and a Parsec equals 3.26 lightyears.

A diagram showing parallax. A and B show how a nearby star appears to move against its background when Earth is at different positions. C is equal to 1 AU. D is a parallax angle of one arcsecond. E is a parsec

Lightyears and looking back in time

The further we look into space, the farther back in time we see.

Proxima Centauri, the nearest star is 4.25 lightyears away, so the light we see from it today, started its journey four years and three months ago.

If Proxima Centauri exploded today it would take 4 years and 3 months before we saw it happen.

The radius of the observable Universe and so the farthest we can see into space is 46.6 billion lightyears.

Galaxy NGC 1097 is 45 million lightyears from Earth, meaning we see it as it existed 45 million years ago. Credit: ESO/TIMER Survey

Travelling a lightyear

Our crewed spaceships, like Apollo, reach speeds of around 39,400 km/h (24,500 mph). It would still take around 27 thousand years to travel one lightyear.

A plane travelling at 965 km/h (600 mp/h) would take 1 million years to travel one lightyear.

A car with an average speed of 90 km/h (56 mph) would take 12 million years, and if you fancied a walk, at 5 km/h (3 mph) it would take you a whopping 216 million years to travel one lightyear, with no comfort breaks!

Earth orbits the Sun at 107 thousand km/h or 67 thousand mph, so it would take 10 thousand years for Earth travel one lightyear.

But our Solar system is also travelling through the Galaxy at 720 thousand km/h (448 thousand mph) which takes just 1,500 years to travel one lightyear.

A car with an average speed of 90 km/h (56 mph) would take 12 million years to travel a lightyear. Credit: Jeffrey Davis / Getty Images

Travelling at the speed of light

Currently, faster-than-light travel seems an unreachable goal, despite movies showing us using wormholes, warp drives and spore drives in the future.

The closest proposition is to use the energy and momentum of light itself to propel a spacecraft.

A city-sized arrangement of synchronised lasers, firing photons to push a small hand-sized spacecraft to 25 per cent the speed of light.

That would get us 4.25 light years to Proxima Centauri in under 20 years.