Why does the shape of the moon 'change'?

Answer 1

Technically it does not. It is always a round sphere.

We perceive that there is a change because as the Moon orbits the Earth the angle of the Sun's light falling on the Moon changes.

• If the Earth is between the Moon and the Sun - the Moon's lit side is facing us.
  
  
• 14 days later when the Moon is between the Earth and the Sun - The unlit side faces us.
  
  
• In between - when the moon sits beside us - we see equal portions of the unlit and the lit sides of the moon.

If the half-lit Moon is overhead at sunset then the moon is following us in it's orbit and is heading towards fully lit position. It is in this half of the Moon's orbit that we can see an eclipse of the moon by the earth blocking the Sun's rays.

Otherwise if it is overhead as the Sun rises then the Moon is leading us in our orbit around the Sun and is heading towards the full dark position between us and the sun. Its at these times we can see an eclipse of the Sun by The Moon.

Because the Moon rotates at the same rate that it orbits our planet we always see the same face (side of the moon). However all sides of the moon get sunlight through a full orbit. Just like our days, except the Moon's days are 14 days long.