Why does the moon have phases and how are they observed?

The moon changes shape in the sky because it moves around Earth, and we see different parts of it from our spot on Earth.

Imagine your friend is holding a round ball (like a moon) and walking around you (like Earth). Sometimes you can see the whole ball, sometimes just part of it, depending on where they are. That’s exactly what happens with the moon!

The sunlight shines on the moon, and we see parts of that light from Earth. When the moon is between Earth and the sun, we can’t see any of its lit side, that’s a new moon. When the moon is on the other side of Earth from the sun, we see all the lit part, that’s a full moon.

The moon takes about 28 days to go all the way around Earth. As it moves, we get to see more or less of its lit side each night. That change in shape is called a phase. So the moon isn’t changing, we’re just seeing different parts of it as it orbits Earth!

Sometimes the moon looks like a crescent, like a smile; other times it’s like a half-moon, or even a quarter-moon. It's like watching your friend walk around you and see more of their ball each time!