Miles and kilometers are effective when it comes to measuring distances in our planet. However, the distances astronomers deal with when it comes to space are enormous. If we were to use miles or kilometers to measure distances between planets and stars, we would be speaking of immensely long numbers, in the billions or trillions for instance. For that reason, it is much more practical to use light years as a measuring unit for distances in space. One light year is 9.46 trillion kilometers.
