PHYSICS & ASTRONOMY — Identify the quantity measured by the given unit. Question: What physical quantity does a light-year measure?

Introduction / Context:
A light-year is one of the most commonly used astronomical units. Despite the word “year,” it does not measure time. This question checks whether you can distinguish among speed, time, and distance units that often get confused in popular science.

Given Data / Assumptions:

• The context is astronomy and interstellar scales.
• Light travels at a constant speed in vacuum, approximately 3 * 10^8 m/s.
• We are comparing different physical quantities: speed vs distance vs time.

Concept / Approach:
A light-year is defined as the distance that light travels in vacuum in one Julian year. Using speed * time = distance, the quantity must be distance. Astronomers prefer such large units because kilometres or metres become unwieldy across interstellar space.

Step-by-Step Solution:
Use relation: distance = speed * time.Speed of light c ≈ 3 * 10^8 m/s.Time for one year t ≈ 365.25 days = 365.25 * 24 * 3600 s.Distance d = c * t ≈ 9.46 * 10^15 m, which is by definition 1 light-year.Therefore, a light-year measures interstellar (stellar) distances.

Verification / Alternative check:
Convert to astronomical units: 1 ly ≈ 63,241 AU, again a distance figure. No speed or time appears as the final dimension, confirming that the unit represents length.

Why Other Options Are Wrong:
A) “speed of aeroplanes” — speed is length/time; light-year is length only.B) “speed of light” — that is measured in m/s; not light-years.D) “speed of rockets” — also speed, not distance.E) “time taken by light in one year” — that is simply one year, a time unit; light-year is distance.

Common Pitfalls:
Assuming the word “year” implies time. In compound units like “man-hours” or “light-years,” the second word does not always dictate the dimension. Always analyze via speed * time = distance.

Final Answer:
stellar distances