This image shows the relative sizes of the major moons in our solar system with the size of the Earth and it's moon. Notice that Europa and Ganymede, two of the moons of jupiter are as big as small plantes like mercury.

When we think of distances on Earth, we often use miles or kilometers. But when it comes to the cosmos, these units become practically meaningless. Imagine trying to measure the distance to the nearest star in miles—it would require a number with so many zeroes that it becomes impossible to comprehend. That’s where the term light-year comes in. But what exactly is a light-year, and why is it so useful for astronomers?