Board Thread:General Discussion/@comment-25266931-20161228230634/@comment-26397825-20161229032305

I believe Monty explained that it's been like that for ages. So in universe, some great cataclysmic event happened that shattered the moon. Out of universe, it was literally Monty just saying 'Wouldn't it be cool if the moon was broken?'

The different stages of it being broken is the phases of the moon - as the moon spins, more of the break is shown to Remnant, before being hidden again. Instead of having cresents and half moons like us, they have stages of brokenness.