A light year is a unit of length. It is most often used in astronomy to measure distances in outer space. The definition of a light year is exactly that, the distance that light travels in one year. But it is important to realize that a light year is based on light traveling in a vacuum, which means that nothing is affecting its speed. In a vacuum, there is no air movement, which can slow down the speed of light, as well as anything else.
Light moves at a speed of 300,000 km per second. This means that in one year, light travels about 9.5 trillion km (if there are 60 seconds in a minute, 60 minutes in an hour, 24 hours in a day and 365.25 days in a year, you simply multiply these three numbers together. Then multiply your answer by 300,000, the speed per second, and round up to get 9.5 trillion). Therefore, one light year equals 9.5 trillion km/year.
So why do astronomers use light years instead of the miles or kilometers that we are more familiar with? Because distances in outer space are so vast that the kilometer or mile is way too small, and therefore is not practical. For example, the closest star to us, after the sun, is about 24,000,000,000,000 miles away. With all these zeros, errors in calculation could easily be made, which would give astronomers many problems. And this distance is the closest star; imagine how many digits would be in the distance of some stars much farther away!
In fact, sometimes the light year is too small to measure distances. Astronomers usually prefer to use a unit of measure called a parsec, which equals 3.26 light years. However, the general public is more comfortable with light years, so that term is more widely used.
Within our galaxy, measuring in light years is sufficient. The Milky Way Galaxy is roughly 150,000 light years across, our sun is about 8 light seconds away, and Andromeda Galaxy, the next closest galaxy, is about 2.3 million light years away.
Since light years is the distance light travels in one year, then when you gaze up at stars on a clear night, you are not seeing the stars as they are currently, but as they were however many light years ago. For example, if you’re seeing a star that is 3 billion light years away, you are seeing it as it was 3 billion years ago. (because it takes 3 billion years for the light to reach Earth) It’s almost like looking back in time! On a much smaller scale, if the sun, which is 8 light seconds away, blinked out, we wouldn’t even know for 8 seconds.