The night sky, explained
Why does the Moon change shape?
It doesn’t. Nothing is covering it either — and that second half of the sentence is where nearly every grown-up gets it wrong. Here is the real answer, in words a child can hold onto, plus a five-minute demonstration you can do tonight with a lamp and an orange.
The short answer
The Sun lights up exactly half of the Moon at all times — the same way it lights half of Earth. The Moon doesn’t change shape, and Earth isn’t casting a shadow on it. What changes is our angle on it: as the Moon travels around us, we see more or less of that permanently lit half, and the whole run of shapes repeats roughly every 29.5 days. Tonight’s “shape” is simply how much of the lit half happens to be facing Earth.
Name it before it sticks
It is not Earth’s shadow. That’s an eclipse.
Ask ten adults why the Moon has phases and most will say Earth’s shadow falls across it. It is the single most common misconception about the night sky, and it is worth saying out loud with your child — a wrong idea nobody names is the one that survives.
Earth’s shadow does land on the Moon sometimes. That event has its own name: a lunar eclipse. It happens two or three times in a typical year, it is over in a few hours, and during a total one the Moon doesn’t vanish — it turns a dim copper red, lit by sunlight bent through the edge of our atmosphere. Phases, by contrast, happen every single night, take a month to come full circle, and would look exactly the same if Earth cast no shadow at all.
There is a proof you can do without any equipment, on a clear afternoon. Find the crescent Moon while the Sun is still up. A crescent sits close to the Sun in the sky — and Earth’s shadow always points directly away from the Sun. A shadow cannot fall on something sitting right next to the light making it. Whatever is trimming the Moon into a crescent, it isn’t us.
What’s actually happening
Half of the Moon is always lit. We just move around it.
The Sun shines from one direction. Anything round it touches gets a lit half and a dark half — the Moon, Earth, an orange on the kitchen counter. The Moon’s lit half never goes away and never gets bigger or smaller. It just points at the Sun, always.
Meanwhile the Moon circles Earth. When it sits between us and the Sun, the lit half is pointed away from us and we see nothing: a New Moon. When it swings round to the far side, opposite the Sun, the whole lit half faces us: a Full Moon. Everywhere in between, we are looking at that same lit half from an angle — and an angle on a lit sphere is exactly what a crescent is.
One lap of shapes, New Moon back to New Moon, takes about 29.5 days. The curious detail for an older child: the Moon actually finishes its circle of Earth in 27.3 days, but by then Earth has moved along its own orbit around the Sun, so the Moon needs roughly two more days of chasing to line up with the Sun again.
The eight phases, in order
The names are just labels for how much of the lit half is turned our way. Waxing means growing, waning means shrinking, and gibbous is the old word for the bulging shape between half and full.
- New MoonThe Moon is between us and the Sun, so its dark side faces Earth. There is nothing to see — it rises and sets with the Sun.
- Waxing CrescentA thin lit edge appears. Look low in the west just after sunset.
- First QuarterHalf the disc is lit, because the Moon is a quarter of the way around its orbit. Up in the evening sky.
- Waxing GibbousMore than half, filling out a little more each night.
- Full MoonThe Moon is opposite the Sun, so the whole lit half faces us. It rises around sunset and sets around sunrise.
- Waning GibbousShrinking now, and rising later and later each night.
- Last QuarterHalf again — but the other half. Best seen in the morning sky.
- Waning CrescentA thin sliver before dawn, heading back toward New Moon to start over.
Which half looks lit depends on where you are standing on Earth. From the northern hemisphere a waxing Moon is lit on the right; from the southern hemisphere, on the left; near the equator it often looks lit along the bottom, like a bowl.
Do it tonight
One lamp, one orange, one child spinning slowly.
This is the part worth forwarding to another parent. A diagram explains phases; holding the Moon in your hand and watching it change is a different thing entirely. It takes about five minutes and needs nothing you don’t already own.
- A lamp, shade off
- A dark room
- A ball or an orange
- Five minutes
- Make the Sun. Take the shade off a lamp and stand it in the middle of an otherwise dark room. It should be the only light.
- Hand them the Moon. An orange, a tennis ball, a ball of foil. Held out at arm’s length. Their own head is Earth.
- Start at New Moon. Face the lamp, ball held up toward it. The side facing them is dark — there is nothing to see, exactly like a New Moon.
- Turn slowly on the spot. Keep the arm out and turn one slow full circle. A sliver appears, grows to half, bulges out, and goes full when the lamp is behind them.
- Now do the eclipse. At full, lower the ball into the shadow of their own head. It goes dark. That — and only that — is what an eclipse is. Lift it back up and the Full Moon returns.
- Ask the question. “Did the ball change shape?” No. “So what changed?” They did. That sentence is the whole of Moon phases.
Step 5 also answers the question the sharp ones ask next: if my head can shadow the ball, why isn’t there an eclipse every month? Because the Moon’s orbit is tilted by about five degrees, so at most Full Moons it passes just above or just below Earth’s shadow rather than through it — the same way the ball misses the shadow of their head unless they aim it there.
Move it yourself
Drag the Moon around Earth and watch the shape follow.
This is the Moon Phase Explorer — one of the interactive activities WeKids builds. Drag the Moon around its orbit in 3D; the inset shows what the Moon looks like from Earth at that exact moment. Find all eight shapes.
Straight answers
The questions that come next
Why does the Moon change shape?
It doesn’t. The Sun always lights up exactly half of the Moon, the same way it always lights half of Earth. As the Moon orbits us we see that lit half from a changing angle — sometimes all of it, sometimes a sliver, sometimes none of it. The shape you see tonight is simply how much of the permanently lit half is facing Earth.
Is the Moon’s shape caused by Earth’s shadow?
No, and this is the most common misconception about the night sky. Earth’s shadow does fall on the Moon occasionally — that is a lunar eclipse, it happens two or three times in a typical year, and it is over in a few hours. Phases happen every night of every month, and they would look exactly the same if Earth cast no shadow at all.
How do I explain Moon phases to a young child?
Use a ball and a lamp. Take the shade off a lamp in a dark room to be the Sun, give your child a ball to hold at arm’s length to be the Moon, and let their own head be Earth. When they turn slowly on the spot, the lit part of the ball grows and shrinks in their hand. Then ask the question that does the work: did the ball change shape, or did you?
How long does the Moon take to go through all its phases?
About 29.5 days from one New Moon to the next. The Moon actually completes its lap around Earth in 27.3 days, but by then Earth has moved along its own orbit around the Sun, so the Moon needs roughly two more days to line up with the Sun again.
Why isn’t there a lunar eclipse every month?
Because the Moon’s orbit is tilted by about five degrees compared with Earth’s orbit around the Sun. At most Full Moons the Moon passes above or below Earth’s shadow instead of through it. Only when a Full Moon lands near one of the two crossing points does the shadow actually reach it.
What is the difference between a waxing and a waning Moon?
Waxing means the lit part is growing, on the way from New Moon to Full. Waning means it is shrinking, on the way from Full back to New. From the northern hemisphere a waxing Moon is lit on the right-hand side and a waning Moon on the left; from the southern hemisphere it is the other way around.
Can you see the Moon during the day?
Yes, often. For much of the month the Moon is above the horizon while the Sun is up, and a daytime Moon is also the easiest way to retire the shadow explanation: a daytime crescent sits close to the Sun in the sky, while Earth’s shadow always points directly away from the Sun.
Who made this page
A question like this one is what WeKids is for.
WeKids is an app for the parent, first. Your child asks something at bedtime. You type it into WeParents in their words and tap their name — and the question comes back as a thing they can actually do: a short video or a hands-on activity built for the age they are, waiting in an app of their own. The explorer above is one of those.
The other half is aimed at you. Alongside the answer you get short, evidence-cited notes from the gentle-parenting research — the studies named, so you can go and read them — on things like how to answer a question without accidentally closing down the next one. The point isn’t a smarter feed. The point is a better equipped parent, and a feed that is the byproduct.
A fact-check pass runs before anything can be sent, and nothing reaches your child until you send it. No ads to children, no recommendation engine deciding what they want next, no behavioural profile being built in the background.
You’re the algorithm. We make you a great one.
More questions, answered properly
- Why is the sky orange?And how to explain it to a five-year-old.
- Why do we have day and night?The Earth spins — that is the whole answer.
- Why does my child melt down when screen time ends?It is the design, not your child. Ages 5–7.
- Why can’t my child put their phone down?Two mechanisms, and what actually changes it. Ages 11–13.
- What is a filter bubble?How to explain it to a teenager without lecturing. Ages 14–18.
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