Light year is a measure of

A light-year is a fundamental unit of distance used in astronomy to measure the vast spans between celestial objects [1]. Despite containing the word 'year', it is not a measure of time [2]. It is defined as the total distance that light travels in a vacuum during one Earth year [1]. Given that light travels at a constant speed of approximately 300,000 kilometers per second, one light-year equates to roughly 9.46 trillion kilometers (or about 5.88 trillion miles) [1]. This unit is essential because standard units like kilometers or miles are too small to practically describe the scale of the universe, such as the diameter of the Milky Way galaxy, which spans between 150,000 and 200,000 light-years [2]. It provides a more manageable numerical scale for expressing distances to stars and distant galaxies.

Sources

1. [1] FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 2: The Origin and Evolution of the Earth > The Star Formation > p. 14
2. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 1: The Universe, The Big Bang Theory, Galaxies & Stellar Evolution > Our Galaxy (The Milky Way) > p. 8