Why the Moon Has Phases
A recreation in the spirit of Bartosz Ciechanowski — build the idea one step at a time, then drag the Moon yourself. The left view looks down on the orbit; the inset is what you'd see from Earth. Canvas 2D · astronomy.
1 · The Moon orbits the Earth
Once every ~27 days the Moon loops around us. The Sun is far off to the left; its light streams in from that side.
2 · The Sun lights one half
At any moment the Sun lights exactly the half of the Moon facing it — the gold side. That never changes. The dark half is just the half turned away.
3 · A phase is a viewing angle
A "phase" isn't the Moon changing — it's how much of the lit half we happen to see from Earth. The inset shows exactly that slice.
4 · New moon
Line the Moon up between Earth and Sun. The lit half now faces entirely away from us — we see darkness.
5 · Full moon
Swing it to the far side, opposite the Sun, and the entire lit half faces us. Full moon. Everything in between is a crescent, quarter, or gibbous.
6 · Your turn
Drag the Moon around its orbit and watch the phase track its position. That single motion is the whole idea.