So, one light year is the distance light travels in one year and this is roughly equal to six trillion miles. And a galaxy is 100,000 light years. I don’t fully understand this; so light is traveling six trillion miles in one year? Thought I understood this, but was watching a snippet of news this morning where they were discussing billions of light years, and maybe I don’t understand. Why do astronomers use light years to measure how far away other galaxies may be?