Prelims 2021

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

a) Distances among stellar bodies do not change.
b) Gravity of stellar bodies does not change.
c) Light always travels in a straight line.
d) Speed of light is always the same.

Correct Answer: d) Speed of light is always the same.

Question from UPSC Prelims 2021 GS Paper

Explanation : 

Why Light-Years Measure Astronomical Distances Effectively

A light-year is a unit of distance that measures how far light travels in one year. Since light travels at a constant speed of approximately 299,792 kilometers per second (or about 186,282 miles per second) in a vacuum, it provides a consistent measure for astronomical distances.

Key Reasons for Using Light-Years

Consistency: The speed of light in a vacuum is one of the fundamental constants of nature and does not change. This constancy allows astronomers to use light-years as a reliable unit of measurement, knowing that it will always represent the same distance.

Universal applicability: The speed of light is the same throughout the universe (as far as we know), making light-years a universal unit that can be used by all astronomers, regardless of their location in the universe.

Comprehensible scale: The distances between stars and galaxies are so vast that using conventional units like kilometers or miles would result in impractically large numbers. A light-year provides a scale that is more manageable when discussing the vastness of space.

Relates to observation: When we look at the stars and other celestial objects, we are seeing them as they were in the past, because the light has taken time to travel to us. Measuring distances in light-years directly relates to the time it takes for that light to reach us, providing a sense of both distance and time.

Common Misconceptions

  • Distances among stellar bodies do not change: This is not true; distances between stars and galaxies can change due to their motion through space and the expansion of the universe.
  • Gravity of stellar bodies does not change: While the gravitational constant is indeed a fundamental constant, the gravity of individual stellar bodies can vary widely and does not provide a fixed reference for measuring distances.
  • Light always travels in a straight line: While light generally travels in a straight line, it can be bent by gravity (gravitational lensing). However, this does not directly relate to why light-years are used as a unit of measurement.
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