Distances in space are measured in light years. This is the distance that light travels in one year.

How do light years measure distance?

A light year is a measure of distance. A ray of light travels 9.5 trillion kilometres in one year or 9.5 million million kilometers or 9500000000000 km. So a light year is 9500000000000 km.

Scientists use light years to measure distances in the universe because the universe is so immense. They use light years to measure distances between galaxies and between stars.

Light years also tell how long the light has taken to reach Earth. Alpha Centauri is one of the closest stars to Earth. It is 4.3 light years from Earth. This means that the light from this star has taken 4.3 years to reach Earth. We see the star as it was 4.3 years ago.

Our Sun is 150 million kilometers from Earth. Light from the Sun takes eight minutes to reach us. This means that the Sun is eight light minutes away from Earth. It also means that the light we see from the Sun is eight minutes old. We see the Sun as it was eight minutes ago.

Galaxies can extend from a few thousand to a million light years in diameter. Our Milky Way galaxy is 150000 light years across.

In September 2004 the most distant space probe, Voyager 1, was 13 light hours (14.3 billion km) away from Earth. It took Voyager 27 years to cover that distance.

Light travels at a speed of about 300 000 kilometres per second.

Nothing can move faster than light — no object, no matter, no information can directly overtake or catch up with light.