Why does looking far into space mean looking back in time?

A light-year sounds like a measure of time, but it is a measure of distance: the distance that light travels in one Earth year. Light is the fastest thing there is, moving at about 300,000 kilometres every second, so in a year it covers about 9 trillion kilometres.

Astronomers need such a large unit because the distances in space are so enormous. Sunlight takes about 8.3 minutes to reach us from the Sun, which is about 150 million kilometres away. The nearest star beyond the Sun, Proxima Centauri, is 4.3 light-years away. The Pole Star, Polaris, is about 320 light-years away, and the centre of our own galaxy, the Milky Way, is about 26,000 light-years away.

Because light takes time to travel, looking far into space means looking back in time. The Andromeda galaxy, our nearest large neighbouring galaxy, is 2.5 million light-years away, so we see it as it was 2.5 million years ago. One very distant galaxy, called GN-z11, is about 13.4 billion light-years away; its light shows us the universe as it was only about 400 million years after the Big Bang.