How big is the universe? For a long time, people believed the universe was infinitely big: it went on forever in every direction. Today, we know that is not true. The universe does end somewhere very far way. But the end is so far away that our regular units of distance are no longer useful to measure it.
The distance from New York City to Los Angeles is about 4,000 kilometers (2,500 miles). The distance from the Earth to the moon is 356,000 kilometers (221,000 miles). The distance from the Earth to the Sun is 150,000,000 kilometers (93,000,000 miles). The distance to even the nearest star is more than 40,000,000,000,000 kilometers (25,000,000,000,000). You can see that our regular units of distance don’t work for the size of the universe.
Instead, astronomers use a unit of distance called the light-year. One light-year is the distance that light travels in one year: one light-year is equal to 9,458,000,000,000 kilometers (5,877,000,000,000 miles). Using this new unit makes astronomical distances easier to express: the nearest star is 4.3 light-years away. (Remember that the star is still the same distance away, whether we express it as 40,000,000,000,000 kilometers or 4.3 light-years).