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

Correct Answer: Speed of light is always the same.

Explanation:

Astronomical distances are immensely vast. Measuring these distances using everyday units like kilometers or miles would result in extraordinarily large, unwieldy numbers. To simplify these measurements, astronomers use a unit called a light-year, which is defined as the distance that light travels in a vacuum in one Julian year.

The primary reason the light-year is a reliable standard unit of measurement across the universe is that the speed of light in a vacuum is a universal fundamental constant (approximately $3\times {10}^8\text{m/s}$). Because the speed of light never changes regardless of the motion of the source or the observer, the distance light travels in a given timeframe remains completely constant and predictable.

Let's evaluate the given options:

• Distances among stellar bodies do not change: Incorrect. Celestial bodies like stars, galaxies, and planets are constantly moving relative to one another due to orbital mechanics and the expansion of the universe.
• Gravity of stellar bodies does not change: Incorrect. Gravitational forces change as stars evolve, lose mass, or change distance relative to other objects.
• Light always travels in a straight line: Incorrect. According to General Relativity, light rays bend when passing through strong gravitational fields (gravitational lensing).
• Speed of light is always the same: Correct. Because the speed of light is constant everywhere in space, it serves as a precise baseline for measuring astronomical distances.