If you're talking about either Jonas or other astrology-based stuff, that is nonsense and totally unscientific. Because the boy days and girl days are separated by only a couple days either way, people (99.99% of whom have no idea when they ovulated and when they had sex in relation to their ovulation, let alone the time of their own birth let alone when they were conceived) fudge the results until they get the day they "must have" conceived to make the moon phase come out right. So many of the people who say it is accurate for them really are mistaken.
It simply makes NO EARTHLY sense based on the substantial evidence we have about diet/exercise and other lifestyle factors swaying that you could conceive a different gender kid on Thursday than you would have on Wednesday. Either swaying works for a reason, because some things like diet and lifestyle favor the survival of one gender over another, or else it's just random BS based on mysticism and magic, and I've seen too much scientific evidence in favor of real-world explanation to ever believe in magical hooh-hah.
Now, if you are asking about moon phases in terms of "ions" which is a faux-scientific explanation that some sway sites delve into, ions for swaying make no sense whatsoever and I have a really long explanation of why they cannot possibly work in the ways that some people claim here:
https://www.genderdreaming.com/forum...t=ion+insanity
Finally, if you are referring to moon phase in terms of moonlight and when you ovulate, this is the most plausible (but still not particularly plausible) and I have an essay on that here:
https://www.genderdreaming.com/forum...t=ion+insanity