Tom Turner, of Amuri Area School, asks :-

What is a light year?

Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.

A light year is the distance light travels in a year, about nine million million kilometres. It's a convenient way to express the awesome distances between the stars and galaxies, just as we frequently think of distances in terms of the time it takes to get there.

Light travels at 300,000 km per second. So light, or a radio signal, takes a little over a second to reach the moon. Earth-to-moon chats with astronauts had a two-second delay. You get something of the same effect on a satellite phone call.

Earth is 150 million km from the sun, or eight light minutes. Neptune, the most distant of the large planets, is four light hours from the sun. It took the Voyager spacecraft 12 years to cover the same distance. Some comets orbit out as far as one light year.

The nearest star, Alpha Centauri, is 4.3 light years away. That's a 100,000 year journey for Voyager, the reason why travel to the stars is in the realm of science fiction.

The stars seen by eye are within a few hundred light years of us. They are our near neighbours in the Galaxy, the giant pancake of stars 100,000 light years across that we see as the Milky Way. The sun is 30,000 light years from the centre of the Galaxy.

The Clouds of Magellan, the two faintly glowing patches in the southern sky, are small galaxies about 170,000 light years away. Light from the most distant galaxies detectable with giant telescopes takes billions of years to reach us.