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 10^{5} 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 10^{5}m/s X 3 X 10^{7}s=9 X 10^{2}km, 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.