If you want to use it as birth control try to get hold of "natural fertility" by Francesca Naish. Basically she says that in addition to charting for mid-cycle ovulation you have to avoid the period when the moon is at the same angle it was when you were born. You can have a chart drawn up to show this by an astrologer or by the Jocelyn Centre. Not sure exactly how foolproof this is as contraception but if you are planning to use it check out that book.