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

• (a) Distances among stellar bodies do not change — False. Stellar bodies are in motion; their distances can change, so this is not the reason for using light-year.
• (b) Gravity of stellar bodies does not change — False. Gravitational effects vary with mass and distance; this has nothing to do with the unit light-year.
• (c) Light always travels in straight line — Not the reason. Even if approximated as straight in uniform media, the choice of light-year is about using the distance light covers in a fixed time.
• (d) Speed of light is always same — Correct in this context. A light-year is defined as the distance light travels in one year, and this is useful because the speed of light in vacuum is constant.

Answer: (d)