What causes our perception of the moon's shape to change ?

The shape of the moon does not actually "change". The reason we see the moon in different ways is because of the angle and where the moon is placed at the time you look at it. Depending on where the moon is, the SHADOW OF EARTH blocks part of the moon and makes it look like the moon has changed its shape, but in reality it has not. So basically you are seeing the moon with the sun's light upon it and the Earth is blocking part or all of the light being distributed to the moon

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