Which one of the following is a reason why astronomical distances are measured in light-years?
Answer & explanation
Answer: (d) Speed of light is always same.
A light-year is the distance light covers in one year, and it works as a fixed yardstick because the speed of light in vacuum is constant. Space distances are so large that a unit with a fixed, known value is far more convenient than kilometres.
- ✓ (d) Light travels at about 3 × 10^8 m/s in vacuum, always the same value. So the distance it covers in one year is a fixed length, roughly 9 trillion km.
- ✗ (a) Distances between stars and galaxies do change, because they move; the light-year is chosen for another reason.
- ✗ (c) Light does not always go straight: it bends when it passes from one medium to another, so straight-line travel is not the basis of the unit.
- ✗ (b) The strength of gravity of a star has no role in defining a unit of length.
Remember · A light-year is a distance (about 9.46 trillion km), not a time. It is usable because the speed of light in vacuum is constant.
📘 Read it in NCERT: Class 10 Science, Ch 9 (practise this chapter)
Sources
- NASA Space Place, What Is a Light-Year? ↗ “A light-year is the distance light travels in one Earth year. One light-year is about 6 trillion miles (9 trillion km).”
- NCERT Class 10 · Science, Chapter 9 “Light travels fastest in vacuum with speed of 3×108 m s–1.”
Question and answer: UPSC's official GS Paper I (2021, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). ·