A light year is the distance that light travels in a year.

There is an official figure for the speed of light: 299,792, 458 metres per second.

You already know something about this: you know that you can use the time between when you see lightning until you hear the thunderclap to calculate how far away the lightning was. For every five seconds between seeing the flash and hearing the bang, the lightning was one mile away. Why? It's because the speed of sound is 343 metres per second.

True, these figures assume ideal conditions and there are factors that can change them but let's be sensible: the changes are playing around the margins and in the real world they don't make a material difference. Except that some weather conditions can make things seem much nearer, or further away, than they are. Prevailing light is actually very important in some agriculture and even cinematography which is one reason why Hollywood became the centre of the US film industry. Today's tech makes that much less important, though.

So, back to the point: with a speed of around 300 million metres per second, light travels a very long way in a very short time.

In astronomy (that's the study of planets, etc., not horoscopes) distances are huge. Our sun, Sol, is 93 million miles away. It takes about 8 minutes for its light to reach us on earth.

The stars are suns. Many of them are already dead but they are so far away that the light they emitted has been en route for so long that it is still reaching us. It is a literal truth that when we look at a star, we are looking back in time. It's like the cosmos is sharing its memories with us even though reality is, now, very different.

So, a star is light-years away because the distance from us to that is measured with reference to the length of time its light takes to reach earth.

True, in cities where traffic is bad we often say "how far away are you?" and get the answer "about an hour." But that is a non sequitur. Just because it's common doesn't mean it's right.

Our sun is at the centre of our Solar System. What's the next closest star? According to NASA, there are two stars, twins if you like, at the centre of the Alpha Centauri system. They are Alpha Centauri One and Two and they are about 4.24 light years away.

If you want to know how far away that is, you can work it out using this formula:

x=a*(b*c*d*e)*4.25

a= 299,792,458 being the number of metres per second light travels
b=60 seconds in a minute
c=60 minutes in an hour
d=24 hours in a day
e=365.25 days in a year
x=the distance from earth to the nearest star in metres.
Divide by 1,000 to get the distance in kilometres.

It's not like popping round to Granny's for tea, is it?