What causes the Moon to have phases?

The Moon has phases because of its orbit around Earth and the changing angles of sunlight hitting its surface. As the Moon revolves, we see varying portions of its illuminated half. The phases progress from new moon (when the Moon is between Earth and the Sun) to full moon (when Earth is between the Sun and the Moon), passing through waxing and waning crescents and gibbous phases. This cycle takes approximately 29.5 days to complete.