Light travels, it's not instant. Shine a tourch to the moon and it takes 1.3 seconds for the light to reach it. The reverse is also true...

So a Light Year is simply the distance that light travels in one year - 9,461,000,000,000 km.  

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what is a light year?

1) If you stand on a beach, the horizon is about 11 miles away (17 km). 

2) The moon is 238,000 miles away (380,000 km). Its light takes around 1.3 seconds to reach us. We say that the moon is 1.3 light seconds away.

3) The sun is 93 million miles away (150 million km). Its light takes around 8.3 minutes to reach us. We are seeing the sun as it was 8 minutes 20 seconds ago; around 8 light minutes.

4) Saturn is the furthest planet that can be seen clearly without telescopes, its average distance is 800 million miles (1.5billion km). Its light takes around 72 minutes to reach us. 

5) The brightest star (apart from the sun) is Sirius-the Dog Star. Its light takes around 9 years to reach us. We say that it is 9 light years away. 

6) The North Star is 323 light years away. So we are seeing it as it was 323 years ago. 

7) The bottom right hand star in Orion is called Rigel. This is around 1300 light years away. Betelgeuse on the top left of Orion is 640 light years away. So if it exploded today, we would continue to see it as normal for 640 years.

8) The furthest object that we can see with our own eyes without telescopes is a galaxy called The Andromeda Galaxy (sometimes depicted as M31 from the Messier catalogue of distant objects). It contains around 600 billion stars and is a spiral like the Milky Way. It can be viewed between September & January – find the Square of Pegasus first high in the south and the 'W' of Cassiopiea the Queen. A smudge can be seen half way between on a very dark night. This is 2.5 million light years away. You are looking 2.5 million years into the past.