UPSC Prelims 2021 Question:

Which one of the following is a reason why astronomical distances are measured in light-years?

Explanation:

• A light-year is a measurement of distance and not time. A light-year is the distance a beam of light travels in a single Earth year, which equates to approximately 6 trillion miles (9.7 trillion kilometres).
  
  
• The reason for using light years to measure distance in space is because the speed of light is constant throughout the universe and is known to high precision. Measuring in light-years allows astronomers to determine how far back in time they are viewing.
  
  

Therefore, option (4) is the correct answer.

Subject: Science & Technology | Space