The apparent shape of the Moon changes throughout the month due to its orbital position relative to the Sun and Earth. This phenomenon is known as lunar phases.

Here's a breakdown:

1. The Moon's Orbit: The Moon orbits the Earth in roughly 27.3 days.

2. Sunlight Reflection: The Moon itself doesn't emit light. We see it because sunlight reflects off its surface.

3. Changing Angles: As the Moon orbits Earth, the angle at which sunlight hits the Moon changes. This causes the illuminated portion of the Moon visible from Earth to change in shape.

4. Phases: We see these different illuminated portions as the phases of the Moon:

* New Moon: The Moon is between the Earth and the Sun, so we can't see it.

* Waxing Crescent: A sliver of the Moon becomes visible on the right side.

* First Quarter: Half of the Moon is illuminated on the right.

* Waxing Gibbous: More than half of the Moon is illuminated, appearing "fuller."

* Full Moon: The entire Moon is illuminated, facing the Earth.

* Waning Gibbous: The Moon is still mostly illuminated, but the illuminated portion shrinks on the left.

* Last Quarter: Half of the Moon is illuminated on the left.

* Waning Crescent: A sliver of the Moon is visible on the left.

In essence, the Moon's shape changes because we see only the illuminated part of the Moon as it orbits the Earth and its position relative to the Sun changes.