Unless you're an early 16th century Jamaican, you probably know what an eclipse is. As we just saw recently, the moon eclipsed the sun during a solar eclipse, or when the moon is in perfect alignment between the Earth and sun.
Though a planet’s moon crossing the line of sight it has with its sun is a relatively common occurrence, the chances of the distances being just right for an eclipse are actually really small. In fact, the only reason it happens for Earth is because the size and distance of the sun and moon.
The sun is about 1.392x10^6 km in diameter and about 1.496x10^8 km from Earth. The moon, on the other hand, is 3,474 km in diameter and a distance of 384,400 km from the Earth.
If you take a quick look at the numbers, you’ll notice that means the moon is about 400 times smaller than the sun, but just happens to be 400 times closer to the Earth than the sun as well.
If that weren’t the case, when the moon crossed the sun’s path between the Earth, we would see the moon covering up part of the sun, but a full eclipse wouldn’t be possible.