A light-year is the distance that light travels in one year, which equals about 5.88 trillion miles. It's a unit astronomers use to measure the enormous distances between stars and galaxies, because regular miles become impossibly large numbers when discussing space.

Light moves faster than anything else in the universe, traveling at 186,282 miles per second. That means in just one second, a beam of light could circle Earth's equator more than seven times. Yet even at this incredible speed, light from the Sun takes about eight minutes to reach Earth. The next nearest star, Proxima Centauri, is 4.2 light-years away, meaning its light takes over four years to reach us.

When you look at a star four light-years away, you're actually seeing light that left that star four years ago. You're looking backward in time. The Andromeda Galaxy, visible on clear nights, is 2.5 million light-years away, so you're seeing it as it appeared 2.5 million years ago, before humans even existed.

Sometimes people mistakenly think a light-year measures time rather than distance. The confusion makes sense given the word “year,” but remember: a light-year measures how far light travels, not how long it takes. Scientists chose this measurement because it makes cosmic distances easier to grasp than saying “15 trillion miles.”