Why does the Moon show phases during the month?

The Moon does not change shape; instead, its visible brightness changes as its position relative to Earth and the Sun changes. Half of the Moon is always illuminated by sunlight, but from Earth we see more or less of that lit side during the Moon’s 29.5-day synodic cycle. New Moon occurs when the Moon is between Earth and the Sun, while Full Moon occurs when Earth lies between them. The cycle is slightly longer than the Moon’s 27.3-day orbit around Earth because Earth is moving around the Sun at the same time. Lunar phases have guided calendars for thousands of years.