Final answer:
Astronomers use light-years instead of kilometers to measure distances in the universe because it provides a more convenient and appropriate unit of measurement due to the vastness of cosmic distances. Light-years help emphasize the concept of looking back in time.
Step-by-step explanation:
Astronomers use light-years instead of kilometers to measure distances in the universe because light-years provide a more convenient and appropriate unit of measurement due to the vastness of cosmic distances. A light-year is the distance that light travels in one year, which is about 9.5 trillion kilometers. Since light travels at a speed of 300,000 kilometers per second, using kilometers would make the numbers very large and cumbersome to work with.
Using light-years also helps to emphasize the concept that as we observe objects in space, we are also looking back in time. For example, when we see a star that is 100 light-years away, it means that the light from that star took 100 years to reach us. Therefore, we are seeing the star as it was 100 years ago. This time aspect adds to our understanding of the universe.