A Light-Year Is About 6 Trillion Miles

Despite the name, a light-year measures distance, not time. NASA defines it as the distance light travels in one Earth year, and puts the figure at about 6 trillion miles, or 9 trillion kilometers. Light can cover that much ground because it is so fast. It travels at a speed of 186,000 miles, or 300,000 kilometers, per second. Astronomers use light-years because the distances in space are too large for ordinary units. NASA gives some examples. The next closest star to us after the Sun is about 4.3 light-years away. The nearest large galaxy, Andromeda, is 2.5 million light-years away. Looking that far means looking back in time. NASA notes that an object 13.4 billion light-years away is seen today as it was 13.4 billion years ago.