A light-year is the distance light travels in one year. Which option correctly describes this distance?

• A. The distance light travels in one year
• B. The distance from the Sun to Pluto
• C. The distance light travels in one month
• D. The distance Earth travels around the Sun in a year

Answer: (D)

A light-year represents how far light travels in one year. Light moves incredibly fast—about 299,792 kilometers every second—so in a year it covers roughly 9.46 trillion kilometers. That distance is the unit we use to describe vast separations to stars and galaxies.

The Sun–Pluto distance, while enormous, is only a few hours’ worth of light travel—roughly 5 hours—far less than a full year. Light traveling in one month would cover about 0.08 of a light-year. The Earth's orbit around the Sun in a year is about 940 million kilometers, which is roughly 0.0001 of a light-year. So the only description that matches a light-year is the distance light travels in one year.