The answer we have gotten before is a reference to a "celestial sphere," with a vauge argument that when we look up at the sky it is like looking at a curved planetarium surface. But this is assuming that the celestial bodies are painted on a curved sphere of glass around the earth. It doesn't really make sense in geometric space.
If we are standing in space, the moon should always point at the sun, no matter where we stand around the moon.
If, instead of a moon and sun, we have an arrow pointing at a sphere in a digital 3D environment, at what angle would we have to stand where the arrow is NOT pointing at the sphere? It doesn't make sense. The arrow should always be pointing at the sphere, from wherever we look at the scene.