Why does the Moon change shape?

The Moon appears changes shape according to how much of it you can see (as long as it’s not cloudy!) and that depends on how much of the bit you can see is facing the Sun. The Sun’s light reflects off the Moon enabling you to see it.

So, does the Moon actually change shape?

Hang on! Does the Moon actually change shape? Well, before we get into it we’d better establish that the Moon itself stays exactly the same – it doesn’t get smaller or bigger as the month goes by – it’s just the amount of it that you can see that changes and that’s why the Moon appears to change shape.

How can I see the Moon?

You see, the Moon doesn’t actually produce any light at all.  The Sun is a ball of burning gas that provides a constant source of light, zooming away from it in all directions.  Some of that light bounces off objects in space – moons, planets, asteroids, etc., meaning that the same light bounces into our eyes so we can see them.  When you look at the Moon you see the Sun’s light that has reflected off it, to you!  I mean, that works for everything: the screen you’re looking at now produces light but turn the device around (tablet, phone, laptop, etc.) – can you see the back of it?  Of course you can, but it isn’t producing light like the screen. So, if you can see it, where’s the light coming from?  That light, reflecting off the back of the device to your eyes, allows you to see it.

Yeah, but why does the Moon change shape?

OK, OK, here’s why the Moon changes shape. there’s the Moon, it looks like it's hanging up there in the sky, but in fact, it’s travelling around the Earth. Not only that, but the Earth is spinning too. Getting confused? No? Phew! OK then, on a clear night the amount of Moon that you can see is dependent upon two things: where it is on its LUNAR CYCLE (takes about a month … or “moon-th”!) and where the Sun is in relation to the Moon. You see, if the face of the Moon you can see is also facing the Sun (even though it’s night time to us), then it’s fully lit by the Sun’s light. That’s a FULL MOON.

To us, the Sun may have set, but remember, its light is still streaming out in all directions and any that falls on the Moon may be reflected in your eyes. Now, as the Moon undertakes its lunar cycle, different faces of the Moon are lit by the Sun and, depending on where you are, you may be able to see all of the lit side (Full Moon) or only part of the lit side (Crescent moon/Gibbous Moon, etc.). Golly, complicated! Well, some pictures really help to understand this.  So, let’s have a look at the different shapes (or phases) of the Moon:

Why does the moon change shape?

Try to imagine where the Sun is: for the Full Moon, it’s like it’s behind you, shining full on the Moon, so you can see all of it. Now, look at the “Last Quarter” moon in the image above. Can you picture where the Sun must be?  Yes – off to the right (or east!) of us.  That left-hand side of the Moon is fully lit up, but you can only see half of it as the right-hand side is in shadow. Make sense?


Try This: Why the Moon changes shape

This changing shape of the Moon is tough to visualise, so why not try a little experiment?  You’ll need:

  • Ping Pong ball or any other small white ball (if you can attach it to a stick, even better)
  • Small torch with a bright beam of light
  • an adult helper

Ideally, sit in a darkened room with the ball held above your head. Ask your adult helper to shine the beam of light from the torch on to the ball. Next, get the adult helper to move slowly around you - this demonstrates the different parts of your ‘Moon’ being lit up.  Now you've done that, does the moon seemingly changing shape make more sense now?


Looking for more about why the Moon changes shape? Of course!  So, why not check out these EdPlace worksheets to find out more about the phases of the Moon:

Year 5 - The Phases of the Moon

Year 5 - The Earth the Sun and the Moon 1 2 3

Year 5 - Sun and Moon

Want to know more?  Of course you do!  Then check out these wacky websites:

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Why does the Moon change shape