What does a light-year measure?

• A. Time
• B. Mass
• C. Distance
• D. Temperature

Answer: (D)

A light-year measures distance. It describes how far light travels in one year, linking the constant speed of light to a practical length.

Light moves about 299,792 kilometers every second. Over a year, that adds up to roughly 9.46 trillion kilometers (about 5.88 trillion miles). So a light-year is the distance light covers in a year, not a measure of time, mass, or temperature. This unit is handy in astronomy because interstellar and intergalactic distances are so vast that ordinary kilometers or miles become unwieldy. For example, nearby stars are a few light-years away, while galaxies lie tens of thousands of light-years distant.