Question

Why does the shape of the lighted part of the moon change when viewed from earth?

Answer 1

Because the earth is always rotating and we see what part of the moon that isn’t shawdowed

Answer 2

The half of the Moon that points toward the Sun looks bright because it is lit by sunlight. The Moon appears to change shape because we see different amounts of the lit part as the Moon orbits Earth. ... As it moves around Earth, more and more of the lit side comes into view.