What is a light – year?

A light-year is how astronomers measure distance in space. It’s defined by how far a beam of light travels in one year – a distance of six trillion miles. Think of it as the bigger, more interesting cousin of the inch, the mile and the kilometre!

Why use this type of measurement at all? First, light is convenient. Throughout the universe, all light travels at exactly the same speed: about 670 million miles per hour. We don’t usually think about light traveling anywhere because when we flick on a light switch – there it is! We don’t have to wait for the room to light up. It happens instantaneously. Except that it’s not instantaneous, just insanely fast. In fact, let’s pause for a second and reflect on how quick the speed of light is

Traveling at that speed, you would encircle the globe of Earth almost eight times in one second. Impressive! Certainly faster than traveling by airplane. So if you were to travel off the Earth in a straight line at light speed, you’d get pretty far in the same amount of time, right? Actually, since space is so vast, you wouldn’t have even made it to the moon. Travel to the moon takes about a second-and-a-half, at light speed. Travel to the sun at light speed takes about eight minutes. Can you even imagine? Traveling at a speed where you cross nearly 200,000 miles every second for eight full minutes would only get you to the centre of our solar system.

The fact that light takes time to get anywhere has an interesting side effect. If light from the sun takes eight minutes to get to us, then we’re actually seeing the sun as it was eight minutes ago. We’re looking into the past! While this sounds a little crazy, it’s actually something with which you’re already familiar. If you’ve ever seen fireworks, for example, you know that you see the explosion and then a few seconds later you hear it. If you close your eyes during the fireworks show, you’d only have your ears to know when things were happening. Since it takes some time for the sound to get to you, you’d always be hearing things a few seconds after they happened. The same happens with light: we only see something once the light from that event actually gets to our eyes.

The main reason for using light years, however, is because the distances we deal with in space are immense. If we stick to miles or kilometres we quickly run into unwieldy numbers just measuring the distance to the nearest star. Moving beyond our galaxy, it’s just over two million light years to our nearest galactic neighbour, the Andromeda galaxy. The light we currently see from that galaxy left there about the same time the ancestors of modern humans were first discovering stone tools.