Cosmic Distances:

If  we asked you what the distance between Delhi and Kanyakumari is, you would say that it  is roughly 3000 km. Another way of answering the question would be that a train  takes.about 50 hours to cover the distance. And if  we know the average speed of a train, we would get a fairly good idea of  the distance. The dimensions of  the universe are so large that using familiar units of distance like kilometres would make little sense, Therefore, cosmic distances are measured in'"ight years". One  light year is the distance travelled by  light.  in  one year. Now, light travels,  about 300,000 kilometres in one second, i.e.  its speed is 3  X 10⁵ kms. At this sped  it can travel seven times around the Earth in one second. A year has about 3X  107seconds (i.e.  about 30 million seconds) in it. Therefore, the distance light travelsin one year is about 

3  X 10⁵m/s  X  3 X 10⁷s=9  X 10²km, i.e. about nine trillion kilometres. 

This unit of  length  is called a light year, It measures not time but distances, enormous distances. Space and time are interwoven. We cannot look out into space without looking back  into time. If we see a galaxy a billion  light years away, we are seeing it  theway it was a billion years ago. Thus, the distance  in  units of  light years also tells us how far back  into time we are looking.