What exactly it *was*, was very subjective. Some cultures/religions forcibly suppressed that knowledge. Assuming universal agreement about any type or nature of object seems pretty ludicrous. They routinely determined this not by observation but by logic. "It is this way because it makes a circle, and circles are perfect.", a rainbow has seven colors because someone thought 7 was a mystic number of supernatural significance, etc. So, yeah, people may agree that there is a big white object in the sky, but what is it? a moon? What's a moon? And why would we call it "the moon?" We should name it after our god's mistress.
Another reason to *not* set this stuff in stone is the way it limits creativity and RP. For example, if you set the celestial objects in canon, then the Bloodstars could never have been formed. No one can posit the existence of a second, hidden moon.