Would you measure the distance between New York and New Delhi in inches? For the same reason, astronomers don't use miles to measure vast distances in space. Instead, they use light-years: the distance that light travels in a year. That makes Proxima Centauri, the closest known star to the Sun, 4.28 light-years away; that's a lot easier than saying 24,000,000,000,000,000 miles.

What: distance light can travel in one year
Equal to: 5,900,000,000,000 miles
Significance: used by astronomers to measure large distances in the Universe outside of our solar system
Characteristics: a measure of distance, not time
First measured by: Ole Roemer

Image credits: main image: Eric Hamilton; Mordecai-Mark MacLow: AMNH.