hairston%utdssa%utadnx%utspan.span@VLSI.JPL.NASA.GOV (03/29/89)
In an earlier issue John Roberts asks the right question: >>I mean, maybe the moon has some effect on noctournal animals, but if you're >>trying to defend the age-old astrologer's "the moon effects the tides, and >>they're water, and since the human body is 98% water, the moon must affect >>people" argument, then I would really love to see some data on that. to which Jim Merritt replies >Ask your wife or girl friend... >(hint: ask for PERIODic events with a CYCLE of appox a lunar month.) The idea that the human female menstrual cycle is somehow connected to the lunar cycle has been circulating for years (millenia?), but age doesn't make it any more correct. For starters the lunar cycle (full moon to full moon) is 29.53 days while the human menstrual cycle is (on average) 28, so the two AREN'T even the same period. And if you're female (or are male and have a wife or significant other you can ask about this) then you know that 28 days is a nice average, but rarely do you hit exactly 28 days (or 29.53 days for that matter). There is quite a range of lengths of menstrual cycles both between individuals and at different times within the life of an individual. The length of an individual's cycle is determined by environmental and biological factors (overall health, nutrition, exercise, stress, illness, age, medication, etc.) but not by the gravity of the moon. The gravitational pull of the moon on a human body is roughly equal to the gravitational pull of a couch you're sitting on. (If you calculate the tidal difference, the couch overwhelms the moon by a factor of a million or so...) What is happening is that people are confusing correlation with causation, the lunar cycle is a handy rule of thumb for timing menstrual cycles, but that doesn't mean that one is causing the other. (This fallacy shows up far too often in all sorts of arguments, such as "Lack of prayer in public schools has caused the downfall of the American education. Since prayer was been banned in the early 60's, the national average SAT scores have dropped by x points..." Both statements are true, prayer was banned and SAT scores have fallen, but that doesn't mean one is the cause of the other.) Another version of the argument runs something like "it's part of evolutional history of us. Our ancestors were amphibians/fish/whatever that had reproduc- tive cycles tied to the tides, much like the spawning of comtemporary grunions." This argument at least tries to give us a causative agent, but if it were true then most (if not all) descendents of those early amphibians or whatever would have fertility cycles tied to the lunar cycle. Instead, examining the cycle for various mammals shows that the lengths of their periods are all over the place ranging from mice and rats with a cycle of 5 days to 37 days for chimp- anzees. The only other mammal that matches the human cycle of 28 days is the opossum. Suppose for a minute that there WAS a causative agency between the lunar cycle and the menstrual cycle. That would mean somehow the phase of the moon is triggering the menstrual period of human females. That would mean that at some point in the cycle (say first quarter) ALL the women on Earth would begin having their period and the sanitary products on supermarket shelves would all disap- pear at once. Instead, what really happens is that the number of women having their period at any one time is uniformly spread throughout the lunar cycle, which is the same thing as saying there is no causative connection between the two. In fact, (taking this to its absurd extreme) if there REALLY was a tidal cause affecting the menstrual cycle, the 29.53 day lunar cycle is the wrong one to look at. Instead we should see women following the earthly tides and having two menstrual cycles per day! (I will refrain from making any sexist jokes in poor taste here.) Then Gabriel Velasco pointed out: >When my wife was pregnant, a doctor told her that sometimes they pre- >fer to think of a full term pregnancy as 10 lunar months. So what? Human pregnancies run for roughly 10 menstrual cycles for biological reasons that have nothing to do with any astronomical influences. They can just as easily think of the pregnancy lasting 280 solar days, three quarters of one orbit of the earth around the sun, one third of the time between oppositions with Mars, the length of time between the first and last appearance of Venus as either an evening or morning star [now THAT'S too much of a coincidence, there MUST be a connection! :^) ], one fifth of a presidential term of office, etc. If you want to dig further in these and other lunar cycle myths check out "Moon Madness" by George Abell in the book "Science and the Paranormal" edited by George Abell and Barry Singer (Scribner, 1981) or the article "The Moon Was Full, and Nothing Happened" in the Winter 85-86 issue of "Skeptical Inquirer". Marc Hairston--Center for Space Sciences--Univ of Texas at Dallas SPAN address UTSPAN::UTADNX::UTD750::HAIRSTON ______________________________________________ I have ABSOLUTELY NO IDEA what the University of Texas System's opinion about menstrual cycles is, but I sure they must have one....somewhere.....and I'll bet we disagree....