Comment by Joker_vD

> Also it's hard to reason about up and down

The moon is above the horizon. The sun is below the horizon. Seems pretty straightforward, no?

The trouble is that shouldn't the Earth block the sun's light from reaching the moon then? And the answer is no, because the space is three-dimensional, so the Earth isn't on the straight line(s) connecting the sun and the moon (sometimes it is, but that's what causes lunar eclipses, not the lunar phases).