Well, this is bizarre place to enter a conversation. Hi everyone!
I've found this thread pretty interesting, and at the risk of being too late will go ahead and give my personal answer to the above questions.
If gravity was needed for menstruation, they wouldn't sell huge all-night pads, right?
And, 28 days -- not 29+, which is the lunar cycle -- is the AVERAGE length of the menstrual cycle. It's considered "normal" to be anywhere from 21-35 or so. Also, women mestruate for as little as 2 days or as long as a week, and can mestruate at any time of the lunar month. I haven't yet met a woman whose cycle correspopnds with the moon, but i'll bet it's possible. Mine doesn't -- it's 31.5 days, give or take a few hours. Oh, and I have no electricity in my house, or streetlights nearby. Not that one person means anything, statistically...