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Some nights the Moon is a fat circle. Some nights it's a thin fingernail. The Moon never actually changes shape — so what are you really looking at?
Watch the Moon every night for a month and it does something strange.
It starts as nothing at all. Then a thin sliver. Then half. Then a full round circle. Then it shrinks back down to nothing.
And then it does the whole thing again. It takes about 29 and a half days — which is roughly where the word month comes from.
The Moon is a ball of rock the whole time. It never grows. It never shrinks. So what is going on?
Ask around and someone will tell you: Earth's shadow falls on the Moon and covers part of it.
That sounds sensible. It is not what is happening.
Here's how you can tell. Earth's shadow really can land on the Moon — that's called a lunar eclipse, and it's a rare treat. It happens a couple of times a year, it lasts a few hours, and the Moon goes a dark coppery red while it's inside the shadow.
But the Moon changes shape every single night, all year, forever. A shadow that only shows up twice a year cannot be doing something that happens every night.
Earth's shadow on the Moon is an eclipse — rare, red, and over in hours. It has nothing to do with the shape you see on an ordinary night.
Hold an orange up in front of a lamp.
One side of the orange is bright. The other side is dark. That never changes, no matter where you stand — the lamp lights half the orange and only half.
The Sun does exactly this to the Moon. Half the Moon is lit up all the time. Not sometimes. Always.
Now, still holding the orange, walk slowly around it in a circle. The orange didn't move. The lamp didn't move. But your view changed. Sometimes you see the whole bright side. Sometimes you see mostly the dark side with a bright edge. Sometimes half and half.
That walk is the Moon's month. Over 29 and a half days the Moon travels all the way around us, so we look at that same lit half from a new angle every night.
The Moon's lit half never changes. What changes is how much of it we can see from where we are standing.
Here is the same circle you just walked with the orange, written out. Watch the last column — that's the shape you'd see in the sky that night.
Tonight the Moon is a thin crescent. The Sun is lighting up half of it right now, like it always does. So why can you only see a sliver?
These two get mixed up constantly. Sort each clue into the one it belongs to, and read what it tells you.
Tap an item, then tap where it belongs
It's a full moon tonight — a complete bright circle. So where must the Moon be?
You’re previewing as a parent — nothing here is recorded.
The synodic month — new moon to new moon — is about 29.5 days; the Moon's orbit around Earth takes 27.3 days, and the extra two days are Earth's own movement around the Sun (NASA Science, 'Moon Phases'). Lunar phases are caused by the changing Sun-Earth-Moon geometry: the Sun always illuminates one half of the Moon, and what varies is how much of that lit hemisphere is visible from Earth (NASA Science; Lunar and Planetary Institute). Phases are NOT caused by Earth's shadow. Most months the Moon passes above or below Earth's shadow; when it does pass through, that is a lunar eclipse, which happens roughly twice a year and can turn the Moon a coppery red as sunlight refracts through Earth's atmosphere (NASA). The Moon is tidally locked to Earth — it rotates once per orbit — so we always see the same face; the phases are not that face turning away.