Human menstrual cycles are tied to the moon, right? Do we know why?
And what happens to astronauts in space who aren’t subject to the normal lunar orbit/gravity/whatever it is that times the second?
Periods…in…spaaaaace!
I’m sorry, I know I’m not supposed to do that before someone chimes in with a real answer. I just couldn’t help myself!
:: slaps own hand ::
They are not tied to the moon. The fact that the average female cycle matches the lunar cycle is a coincidence, and in any case, that 28 days is a very rough average: lots of women have longer or shorter cycles, and lots of women have cycles that are irregular, some by as little as a day or two, some by weeks or months.
I do suspect that female astronauts are likely to use hormonal birth control to avoid their menses on flights, just because it seems like it would be a PITA to deal with and easy to avoid.
R Mike Mullane, in his book Do Your Ears Pop in Space, says that female astronauts have had periods in space, and that there was no difference in the timing or flow for them. Taking along pads or tampons was an option for female Space Shuttle astronauts. He doesn’t say whether pads would work in zero gravity or not (obviously, he never got to try them out himself).
If they would, then the diapers that female astronauts use during spacewalks should work for that as well, if they were willing to wear them all the time.
If the menstrual cycle is influenced by the moon, it’s a subtle effect. 28 days doesn’t actually match the lunar cycle. The moon has two cycles, a sidereal cycle (from lining up with one star to lining up with the same star again) of 27.3 days, and a synodic cycle (from new moon to new moon) of 29.5 days. Neither of those cycles is 28 days, and a 28-day cycle is going to get out of sync with either one of those pretty quickly.
There are other species that have estrus cycles. It’s not the case that most species that have estrus cycles have cycles around the length of the moon’s cycle. Chimpanzees have menstrual periods, with an average period of 35 days. Cats have a heat cycle of 14-21 days. If the moon were a strong influence on the length of human menstrual cycles, we’d expect to see other species having about the same cycle length as humans. We don’t.
This sentence makes little sense.
It sounds like maybe you think the length of the time period we call a “second” is somehow connected to some aspect of the Moon.
If you think that you’ve either been listening to people who have no idea what they’re talking about, or you’ve managed to mix up several different ideas into a mental hash.
Please explain a bit more what you think & what you’re asking about and maybe we can get your knowledge straightened out.
If you take one thing away from this conversation, please make it this! There is nothing out of the ordinary about a menstrual cycle that is longer or shorter than 28 days. There is usually nothing out of the ordinary about menstrual cycles that vary in length, particularly when we’re talking about young teenagers and women approaching menopause. The whole Moon thing is nonsense.
As for how an astronaut would deal with it, in addition to bringing along pads or tampons, she could choose to use hormonal birth control that stops menstruation for the duration of the mission.
Oops, I completely forgot to reply to this. Thank you all for the responses! Ignorance fought 
I believe our OP wasn’t talking about “the time unit called ‘second’” but about “the second one of the aforementioned cycles,” where actually he should have said “the first” (since he ended up talking of menses first and appears to have perceived those as being triggered by lunar cycles)
IOW, OP needed more coffee.
I’d guess that an evolutionary endocrinologist might be able to speculate on the relationship between the length of the menstrual cycle and the monthly orbit of the moon, but the only real connection between these things is linguistic: the words moon, month, and menstrual all come from the same root, and for obvious reasons.
NASA puts women on pretty strong birth control so that this question isn’t something that they have to worry about.
It’s hard enough going to the bathroom, I cannot imagine how much of a headache female astronauts having their periods would be!