This is a great question! I especially like the way in which you asked it. The word 'long' can be used to mean both distance between two points in space as well as two points in time. When you ask such a question in relation to a light year both meanings are important!
A light year is the distance between two points in space that it would take light to travel when the distance between the two points in time are one year. So, let's see (pun!) what this would be.
Light travels at 186,000 miles every second! That is a huge number! If you were to write one number per second for eight hours a day without stopping to eat or to rest your hand it would take you six and a half days to get to number 186,000! And to think light travels that many miles in only one second! Another way to think about how large this number is, is to think about how many times you could go back and fourth across the United States. If you go back and fourth across the US 66 times you will have traveled 186,000 miles. If you did this going an average speed of 60 miles per hour you will need one year and three weeks! But light can do this same thing in one second!
In one year there are 365 days of 24 hours. Each hour has 60 minutes and each minute is 60 seconds long. So, 60s/min x 60min/hr x 24hr/day x 365days equals 31,536,000s. This many seconds is an even larger number than the number of miles light goes in one second! It would take you three years to get to number 31,536,000 if you could write one number per second for eight hours per day. This many miles means you could go back and forth across the United States over one thousand times! At 60 miles per hour it would take you 60 years!
So, we take these two very large numbers and multiply them together to see how many miles light can travel in one year. You can tell already that this is going to be huge! That number is 5,865,696,000,000. Working eight hours per day at a rate of one number per second it would take you two hundred thousand years to get to number 5,865,696,000,000! This gets you across the United States two billion times which at our average speed of 60 miles per hour, would take nine trillion years!
Now that you know what the distance is that light travels in one year you can also know the distance between objects in the universe. The closest star to us is about four light years away. This means that it is 23,462,784,000,000 miles away. How about this: the edge of the universe is about 15 billion light years away from us! Can you even imagine how many miles away that is?
Tom Young, B.S., Science Teacher, Whitehouse High School, Texas
'Physics is mathematical not because we know so much about the physical world, but because we know so little; it is only its mathematical properties that we can discover.'