> He explained it like this which really helped: you can choose all sorts of frames of reference when you think about this stuff, so choose one where the sun and earth are stationary compared to each other, and then only the moon is rotating - less stuff to think about!
Even easier: choose one where the sun and moon are stationary, and place the Earth somewhere near the moon. The side of the moon facing the sun will always be fully brightly lit, and the proportion of that which is seen from the Earth is based on where we place the Earth.
Of course the Earth doesn't orbit around the moon, but for the purposes of this model it's irrelevant.
Even easier: choose one where the sun and moon are stationary, and place the Earth somewhere near the moon. The side of the moon facing the sun will always be fully brightly lit, and the proportion of that which is seen from the Earth is based on where we place the Earth.
Of course the Earth doesn't orbit around the moon, but for the purposes of this model it's irrelevant.