Short · Space
Why does the Moon change shape?
Sunlight always illuminates half the Moon. As the Moon moves around Earth, the fraction of that bright half facing us changes through a 29.5-day cycle.
Try it yourself
Opens Moon Phase Lab. Change the same thing yourself.
Transcript
Why does the Moon change shape? It doesn't. Sunlight always lights half of it. What changes is how much of that bright half faces us. Now move the Moon around Earth. Between us and the Sun, we see only its dark side, a new moon. A quarter of the way around, half the face is lit. Opposite the Sun, the whole face shines. That dark part isn't Earth's shadow. One full cycle takes about twenty-nine and a half days. Try it yourself at Explainer Tools dot com.