Question 12 from UPSC CSE 2021 - S&T

Question

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

IASCODEX · Accepted Answer

Correct Option: D. Speed of light is always the same.

Strategy: Focus on the constancy of the speed of light as the defining feature for a universal astronomical unit. Disregard options based on non-absolute truths like straight-line paths or unchanging interstellar distances. The correct choice is the light-year, which is derived from this constant.

Key Info: **Definition**
A light-year is the distance light travels in one year in a vacuum, equivalent to about 9.46 trillion kilometers or 5.88 trillion miles.

**Key Principles**
- The speed of light in a vacuum is a fundamental physical constant, approximately 299,792 kilometers per second.
- This constancy, established by relativity and confirmed by experiments, ensures the unit is consistent across the universe.

**Practical Use**
Astronomical distances are so vast that units like kilometers or miles become impractical. The light-year provides a manageable and meaningful scale for these distances.

**Clarifications**
While light's path can bend due to gravitational lensing and distances between objects change, these factors do not affect the constancy of light's speed. The light-year remains a standardized unit for expressing cosmic distances and the scale of the universe.

IASCODEX · Answer

Option A: Distances among stellar bodies do not change.

IASCODEX · Answer

Option B: Gravity of stellar bodies does not change.

IASCODEX · Answer

Option C: Light always travels in a straight line.