Why Does the Moon Change Shape?

Ages 3-5 Simple Explanation

You know how if you hold a ball up to a lamp, one side of the ball is bright and the other side is dark? That is just like the Moon! The Sun shines on the Moon, and one half is always lit up, but we can only see part of that bright side depending on where the Moon is.

The Moon is like a friend walking in a big circle around you. Sometimes your friend is facing you and you see their whole face. Sometimes they are turned sideways and you only see part of their face. The Moon does the same thing as it moves around Earth!

When you can see the whole bright side of the Moon, it is called a full moon. It looks like a big, round, glowing circle in the sky. When you can only see a little sliver, that is called a crescent moon, and it looks like a banana!

The Moon makes this trip around Earth about once a month. So every month, you can watch it slowly go from a tiny sliver to a big full circle and then back to a tiny sliver again. Next time it is bedtime, look out the window and see what shape the Moon looks like tonight!

Ages 6-8 More Detail

The Moon does not actually change shape. It is always a round ball, just like Earth. What changes is how much of the Moon's bright side we can see from where we are standing on Earth.

Here is how it works. The Moon does not make its own light. It is lit up by the Sun, just like a ball is lit up when you hold it under a lamp. At any moment, half the Moon is bright because the Sun is shining on it, and the other half is dark.

The Moon travels around Earth in a big circle, and this trip takes about 29 days, roughly one month. As it moves, we see different amounts of its sunlit half. Sometimes we see the whole bright side, which is a full moon. Sometimes we see just a sliver, which is a crescent. And sometimes the bright side is facing completely away from us, so the Moon seems to disappear. That is called a new moon.

The different shapes the Moon appears to have are called phases. They go in this order: new moon, crescent, first quarter where it looks like a half circle, gibbous which is more than half but not quite full, and then full moon. After the full moon, it goes back through those shapes in reverse.

This cycle repeats every single month, which is actually where the word month comes from! If you start watching the Moon each night, you can track the phases yourself and even predict what it will look like tomorrow.

Ages 9-12 Full Explanation

The Moon does not actually change shape at all. It is always a sphere. What changes is how much of the Moon's sunlit surface we can see from Earth as the Moon orbits around us. This cycle of changing appearances is called the lunar phases, and it takes about 29.5 days to complete, which is close to one calendar month.

At any given moment, the Sun is lighting up exactly one half of the Moon, just like it lights up one half of Earth creating day and night. As the Moon moves in its orbit around Earth, the angle between the Sun, Moon, and Earth changes. This angle determines how much of the lit-up half is facing toward us.

The cycle starts with the new moon, when the Moon is between the Earth and the Sun. The lit side is facing away from us, so the Moon appears invisible in the sky. Then over the next two weeks, we see more and more of the lit side. It goes from a thin crescent to a first quarter or half moon, then to a gibbous shape, and finally to a full moon when we can see the entire sunlit face.

After the full moon, the process reverses. We see less and less of the lit side over the next two weeks, going from gibbous to last quarter to a thin crescent, and then back to a new moon again. The whole thing is as regular as clockwork and has been used by civilizations throughout history to track time. Many calendars around the world, including Islamic and Chinese calendars, are still based on the Moon's phases.

One common mix-up is confusing moon phases with eclipses. A lunar eclipse happens when Earth passes directly between the Sun and the Moon, casting Earth's shadow on the Moon. That is a special event that only happens a few times a year. Regular moon phases are just about the changing viewing angle and happen continuously every single month.

Here is a cool trick to remember whether the Moon is growing or shrinking. In the Northern Hemisphere, if the lit part of the Moon looks like the curve of the letter D, it is waxing, meaning growing toward full. If it looks like the curve of the letter C, it is waning, meaning shrinking back toward new. Once you learn this, you can always tell which part of the cycle the Moon is in.