The Moon Changes Shape Night by Night
Lumi the night sky guide floats in a soft blue night sky, pointing happily at a row of moons that grow from a thin curve to a big round circle and back again to a thin curve.
- Name three Moon shapes: thin sliver, half, and full round.
- Describe that the Moon looks different on different nights.
- Order Moon pictures from a thin sliver all the way to full and back to thin again.
- Predict that the Moon shapes repeat again and again in the same order.
Key terms
- phase
- One of the changing shapes the lit-up Moon shows us on different nights.
- sliver
- A thin curved shape of the Moon, like a small smile in the sky.
- full Moon
- The big round shape we see when the whole lit-up side faces Earth.
- repeating pattern
- A set of shapes that happens again and again in the same order.
- lit-up side
- The half of the Moon that the Sun is shining on at any time.
Why the Moon Seems to Change
The Moon never truly changes its shape; it stays a round ball all the time. The Sun lights up only one side of it, just like a lamp shining on a ball lights up only the side facing the lamp. As the Moon travels around Earth, we see different amounts of that bright side. Some nights we see just a thin slice, and other nights we see the whole round lit-up face.
A Pattern You Can Count On
The Moon's shapes follow a steady, repeating pattern. The lit part slowly grows from a thin sliver, to a half Moon, to a full round Moon. Then it slowly shrinks back, from full, to half, to sliver again. This whole cycle takes about a month and then begins all over. Because it repeats, you can even predict what shape might come next.
Worked examples
Predict the shape that comes after a full Moon.
- Recall the pattern grows sliver, half, full, then shrinks back.
- After full, the lit-up part starts to shrink.
- The next shape is a half Moon, then later a sliver.
Answer: After a full Moon, the lit-up part shrinks to a half Moon, then a sliver.
Decide if the Moon really changes its shape.
- Remember the Moon is always a round ball.
- The Sun lights only one side, and we see different amounts of it.
- So the Moon only looks like it changes; its real shape stays the same.
Answer: No — the Moon stays round; we just see different amounts of its lit-up side.
Activity
Put the Moon pictures in order from a thin sliver, all the way to full, and back to a thin sliver again.
Practice
Look at the Moon tonight and name which shape you can see.
Put these shapes in order: full Moon, sliver, and half Moon.
Common mistakes to avoid
- The Moon really grows and shrinks.The Moon stays the same round ball; we only see different amounts of its lit side.
- The Moon moves closer to look bigger.The shapes change because of which lit-up side we see, not from getting nearer.
Check your understanding
Does the Moon look the same shape every single night?
Which one is a full Moon?
Why does the Moon look like a different shape on a new night?
After a full Moon, what shape will you see next as the nights go on?
Recap
The Moon looks like different shapes on different nights, from a thin sliver to a half Moon to a full round Moon and back again. It never really changes shape; the Sun lights one side, and as the Moon orbits Earth we see more or less of that lit-up side in a repeating monthly pattern.
Reflect
Which Moon shape is your favorite to spot in the night sky, and why?