He elaborated on it. The Sun is force, the Moon is cunning. The Sun is more Lawful; the Moon, Chaotic. The Sun is female, the Moon male (think Athena and Hermes).
And then he said that people who are born by day are under Sun, and by night, Moon. And it's one or the other, not both. I was starting to ponder what if you're born at dawn or sunset when it struck me --
Ain't no way that the moon can rule everyone born by night. It's as likely to be up by day as by night, even if you can't see it. Why would someone born on midnight during the new moon be ruled by the Moon? The metaphor connecting them does not work.
Sun and Stars would fit in if you want to have them divided as day and night. You can exploit the fact of one and many to make them Unity and Multiplicity -- or even Simplicity and Complexity. The Moon could be demoted to the role of celestial messenger; it moved between the night and day so that the Sun and the Stars, which could never meet, could ensure that their realms never destructively collide. Or perhaps it could be the Sun, the Moon, and the Stars, with the Moon ruling those in odd corners. But the sun by day and the moon by night is improbable. Especially since characters in his Dark-Age-tech level would be much more aware of the heavens than we are. Closest thing they've got to a clock or calendar.