UPSC Previous Year Question | Prelims GS | 2021

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

  1. Distances among stellar bodies do not change.
  2. Gravity of stellar bodies does not change.
  3. Light always travels in straight line.
  4. Speed of light is always same.

Answer

Correct Answer

Speed of light is always same.

Explanation

A light-year is the distance light travels in one year in vacuum. It is a convenient and reliable unit for vast astronomical distances precisely because the speed of light is constant. Distances between stars do change over time and gravity varies, and light bends near massive bodies, so the other options are not the reason.

Topics: astronomy, light-year as a unit of astronomical distance, physics, science & technology