Cosmic distances

How far is it to the nearest star? The honest answer in kilometres would be a number with 14 digits, and that is where all imagination stops. Space is so vast that our everyday measures fail. Astronomers therefore use their own units and one trick: they measure distances by the travel time of light.

The astronomical unit

For distances within the solar system there is the astronomical unit, or AU for short. One AU is the average distance between Earth and Sun, about 150 million kilometres, more precisely 149.6 million. That keeps the numbers manageable: Mars is about 1.5 AU from the Sun, Jupiter 5.2 AU and Neptune 30 AU. Instead of huge kilometre figures, you simply compare everything with the Earth to Sun distance.

Travelling at the speed of light

Nothing is faster than light: in empty space it covers about 300,000 kilometres per second. Even so, it needs noticeable time for cosmic distances. From the Moon to the Earth it travels about 1.3 seconds, from the Sun 8 minutes and 20 seconds, from Neptune already about 4 hours. And to the next star after the Sun, Proxima Centauri, even light takes about 4.2 years. You always work out the travel time the same way: distance divided by the speed of light. These travel times are the best measuring tape for the universe.

The light year

A light year sounds like time but is a distance: the stretch that light covers in one year, about 9.5 trillion kilometres. So Proxima Centauri is 4.2 light years away. Our Milky Way measures about 100,000 light years across, and the Andromeda galaxy, our large neighbour galaxy, lies about 2.5 million light years away. Without the light year we would have to juggle numbers nobody could read any more.

A model you can touch

Scale models make the distances tangible. Shrink the solar system a billion times: the Sun becomes a ball of 1.4 metres, about the size of a large exercise ball. The Earth shrinks to a good centimetre, a marble circling at a distance of 150 metres. Neptune draws its orbit 4.5 kilometres away. And Proxima Centauri? In this model the neighbouring star would sit about 40,000 kilometres away, which is once around the entire real Earth. So there is already plenty of room between the planets, but from Neptune the way to the nearest star is about nine thousand times longer again.

Looking into the past

Because light needs time, you never see the now, always the before. You see the Moon as it was 1.3 seconds ago, the Sun as it was a good 8 minutes ago, Proxima Centauri as it was 4.2 years ago. The light of the Andromeda galaxy has even been travelling for 2.5 million years, it set off long before modern humans existed. Telescopes are therefore time machines: the farther they look out, the deeper they look into the universe's past.