What does a light year measure?

• A. The time it takes light to travel around Earth in a year
• B. The distance light travels in a year
• C. The distance light travels from the Sun to Earth in a year
• D. The speed of light

Answer: (B)

Light year measures distance: it’s the distance light travels in one year. Light moves incredibly fast, so over a year it covers a huge span—about 9.46 trillion kilometers (roughly 5.88 trillion miles). Astronomers use this unit to express vast cosmic distances so that talking about how far away stars or galaxies are becomes practical. It’s not a measure of time or a speed; the speed of light is a constant, and a light year is the distance light covers in a year.