That seems relevant. I have a feeling that it's not that people hadn't noticed the pattern followed by the moon (people knew a lot about the sky back then, apparently, probably because people looked at it more and more carefully since there were no wristwatches, weather reports, or televisions). I have a feeling that the reason has more to do with the moon being a symbol of night. I know very little about art history, but I've made the observation that symbols and artistic precedent are extremely important, probably much more than naturalism.