Q. Which one of the following is a reason why astronomical distances are measured in light-years? (UPSC Prelims 2021)
Answer: Speed of light is always same.
Notes: The correct answer is [D] Speed of light is always same. In the vastness of the universe, conventional units like kilometers or miles become impractical due to the immense scales involved.

• Speed of Light (Statement D – Correct): According to the laws of physics, the speed of light in a vacuum is a universal physical constant, denoted as c. Its value is exactly 299,792,458 meters per second (approximately 300,000 km/s). Because this speed remains constant throughout the universe, it provides a stable and absolute "ruler" for measuring vast distances.
• Definition of a Light-year: A light-year is the distance that light travels in a vacuum in one Julian year (365.25 days). This equates to approximately 9.46 trillion kilometers.
• Stellar Distances (Statement A – Incorrect): Celestial bodies are in constant motion. Galaxies are receding from each other due to the expansion of the universe, and stars orbit galactic centers. Therefore, distances are not static.
• Gravity (Statement B – Incorrect): Gravity varies significantly depending on the mass of the stellar bodies and the distance between them. It is not a constant factor used for defining a unit of distance.
• Propagation of Light (Statement C – Incorrect): While light travels in a straight line in a uniform medium, it can be bent by gravity (gravitational lensing) according to General Relativity. However, the speed at which it travels remains constant regardless of the path.

Using light-years allows astronomers to describe the distance to the nearest star (Proxima Centauri) as 4.24 light-years rather than roughly 40 trillion kilometers, making the data much more manageable.