Which one of the following is a reason why astronomical distances are

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

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Distances among stellar bodies do not change.

Gravity of stellar bodies does not change.

Light always travels in straight line.

Speed of light is always same.

Answer is Right!

Answer is Wrong!

This question was previously asked in

UPSC IAS – 2021

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The correct answer is D) Speed of light is always same.

Astronomical distances are vast, making standard units like kilometres or miles impractical. A light-year is defined as the distance light travels in one year in a vacuum. The use of light-years as a unit of distance in astronomy is based on the fact that the speed of light in a vacuum is a constant (approximately 299,792,458 meters per second). This constant speed provides a reliable basis for measuring extremely large distances across space based on time.

Options A, B, and C are incorrect. Distances between celestial bodies do change (due to cosmic expansion, orbital motion, etc.). Gravity of stellar bodies also changes depending on mass distribution and distance. While light travels in approximately straight lines in uniform media, it can be bent by gravity (gravitational lensing). The constancy of the speed of light is the fundamental principle behind using light-years to measure distance.

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