The Moon is always half-lit by the sun, like any ball in sunshine. As it circles us, you see that bright half from different angles — sometimes face-on (full moon), sometimes edge-on (half), sometimes from behind (new moon).
Most people think the Moon's phases are Earth's shadow falling on it. In fact the Moon is always exactly half-lit by the sun, and a phase is just how much of that lit half you can see from your angle.
What's actually happening
Hold any ball in sunlight and half of it is lit — always exactly half, the half facing the sun. The Moon is no different. It has no light of its own, no shape-shifting, and despite the most common playground theory, its phases are not Earth's shadow falling on it.
The phases are a tour around that half-lit ball. When the Moon sits between us and the sun, its floodlit side faces away from us and we stare at the dark half: new moon. A week later it has swung a quarter of the way around, and we see it side-on — half bright, half dark. Another week and it's on the far side of us from the sun: now we face its lit half square-on, the full moon. The "shape" is just the slice of daylight visible from your angle, growing and shrinking on a 29.5-day lap.
Two bonus mysteries fall out of the same picture. The crescent moon's "dark part" is often faintly visible — that's earthshine, sunlight bouncing off Earth and dimly lighting the lunar night. And we only ever see one face of the Moon because it spins exactly once per orbit, locked by tides — the far side gets just as much sun; it just never gets an audience.
The Moon never changes shape — phases are your changing view of a permanently half-lit ball, and the shadow theory belongs to eclipses.
- 1Put a lamp (the sun) across a dark room. You are Earth. Hold an orange (the Moon) at arm's length.
- 2Turn slowly on the spot, keeping the orange in front of you. Watch its lit portion: dark when you face the lamp, half when you're side-on, full when the lamp is at your back.
- 3You've just run a month in ten seconds — and proved no shadow was needed.
Common questions
At new moon the Moon's lit side simply faces away from us, so we see its dark half. An eclipse is a rarer, different alignment where Earth's shadow actually falls on the Moon.
That is earthshine: sunlight bouncing off Earth and dimly lighting the lunar night. It is often called "the old moon in the new moon's arms".
The Moon spins exactly once per orbit, locked by tides, so the same side always faces us. The far side gets just as much sunlight; it simply never faces Earth.