Imagine an Earth-like planet with a moon that is closer to the planet than ours with a lunar cycle of 18-19 days. I am trying to imagine how this different cycle would affect human beings and animals.

I am especially interested in the following question: How would the shorter lunar cycle of 18-19 days affect women's periods?


5 Answers 5


It wouldn't affect it in the slighest. THe processes aren't linked in any way. They just happen to have roughly the same length.

The moon getting closer would have plenty of effects but non directly related to the shorter lunar cycle. Specifically it would increase the Moon's effects on our tides. They would be more extreme so to speak.

  • 1
    $\begingroup$ Playing devil's advocate (not that he needs one): What if it is more than coincidence that they are roughly the same length? $\endgroup$ Mar 17, 2017 at 18:04
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    $\begingroup$ There hasn't been found any evidence for that. Also I raise you women with a variable cycle. $\endgroup$
    – Mormacil
    Mar 17, 2017 at 18:06
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    $\begingroup$ Given that anything from 20-40 days per cycle is considered normal. The majority of women don't match up with the lunar month. $\endgroup$
    – sphennings
    Mar 17, 2017 at 19:05

It couldn't.

The similarity in the menstruation cycle length of the women of our species, and the Moon orbital length (in time), is only a coincide. The reasons behind that:

  1. Other mammal species have quite different menstruation cycle length, for example sheep have only 11 days, while dogs have around a half year.
  2. There is no statistical correlation between the human female menstruation and the moon phases, it is a common misconception.
  3. Most women have not exactly 29 day long menstruation cycle, it is longer and shorter. Also it varies quite often, for example it is quite common in female university students that their period misses (or arrives) before exams.

Although it could be possible in your world, that there is some indirect effect. For example, a closer or larger moon could affect the weather by tidal effects, and the evolution of your human could have adapted to it. It is a common phenomenon also on our world, although not by humans: many mammal speciel (and also a lot of non-mammal) have reproductive era synchronized to the weather. For example, cats are on heat on spring.

We, humans, are evolutionary wired for an "always on" cycle, it is because reproduction by us means also that the male helps the female to help our common descendants. If we would be "on heat" periodically, we wouldn't have the motivation to help our female and children. And thus, also our women have to be always (or nearly always) reproduction-capable.

  • 1
    $\begingroup$ "fire on spring"! whow! do you mean "cats are in heat in spring"! $\endgroup$ Mar 17, 2017 at 19:52
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    $\begingroup$ @EveryBitHelps Thank you very much your helpful bits. :-) I fixed. $\endgroup$
    – Gray Sheep
    Mar 17, 2017 at 19:58

Some species have their reproductive cycle bound to the lunar month: they release sperms and eggs in sea water at full moon.

Homo Sapiens is not among these species, and as far as I know no relation has been proven between the hormonal cycle of women and the moon month.


Bear with me, it's Friday evening, I've already had a glass of wine and I'm hanging around Worldbuilding.SE

According to such websites as yogagoddess, floliving and cycleharmony (yes, those are real websites), there is definite connection, that is never fully explained, between a woman and her cycle. There are even discussions of whether the cycle starts on the Full Red or New White Moon.

Now according to the one somewhat scientific/rational sounding article I could find there is no direct linkage between the two. Pure coincidence. I especially liked the summary at the end where he refutes previous research...

He uses the example of the palolo worm of Samoa and Fiji to show how the phase of the moon influences the reproductive cycle. In this case, the worms, which live inside coral reefs, mate only on the night before the last quarter of the moon in October and November (Lieber, 51). The connection between human women and Fijian worms is left for the reader to make.

with a simple sentence...

Human women are not Fijian worms.

Ok. If we can use this one article in a sample batch of hundreds as statistical support that there is no linkage between the menstrual cycle and the moon cycle we have to fall back onto pseudoscience and the writer's best friend...make it up.

DISCLAIMER There is absolutely no scientific or credible basis for anything else I might write after this...

I agree that Human women are not Fijian worms. But a couple billion years ago, we were...you can take the simple premise that if worms today are linked to the moon cycle, would our primordial ancestors not have been linked as well. Along the way, we liberated ourselves from the moon dependency, most likely after we left the oceans.

So, if your Human-like characters and Earth-like animals on your Earth-like planet evolved there on-planet. I believe you may have the same "coincidental" connections that we have now but with a 18-19 day cycle.

BUT if your Human-like characters and your Earth-like animals evolved on Earth AND THEN were somehow transported to another Earth-like planet. Then I believe you will have a clash and there will no longer be any correlation.

Hope this waffling helps :)


They are not linked at all. Believe me, if they were, that would be so stupendously, fantastically, amazingly USEFUL in organising your life, that women like me would be celebrating in the streets on a daily basis, hugging lunar calendars with joy.

I mean just think about it... if the two things were linked, in January I could look up what the relevant phase of the moon will be in December and say things like "Ooh, I have to make an 9 hour train journey in December and really don't want to do it on a day I'll feel wretched, so let's not book it for the 13th, 14th or 15th..."

In my life I've had times where my cycle was as short as 21 days, and times where it was a long as 8 weeks.

Also, not all women menstruate at the same time, so why is the amount of light which is bouncing off the Moon towards Earth affecting different women in different ways? (Anyone who mentions menstruation being linked to tides needs to be taken to the seaside and slapped with wet fish until they realise the tide is NOT on a monthly cycle. Even the spring tides occur twice a lunar month!).

Basically the Moon has these effects on Earth and the creatures upon it:

  1. Gravitational Pull - this creates the tides
  2. Moonlight - the phases of the moon as seen from Earth. This means a bit of light to see by at night.

Gravitational pull on the surface of the Earth is only affected by the Moon's (and Sun's) mass. The amount of light hitting the Moon and being reflected towards an observer on Earth does not increase or decrease the Moon's mass!

The tides operate on these cycles:

  • In and out roughly twice daily. Approx every 12 hours 50 minutes.
  • Spring tide (highest high tide and lowest low tide) twice a lunar month - every 14 days. This is caused by Moon + Sun's pull adding together.
  • Neap tide (lowest high tide and highest low tide) twice a lunar month - again every 14 days. This is caused by Moon minus Sun's pull.

The phases of the moon are only linked to the state of the tide because the Sun's gravitational pull is involved. There is a good astronomy article on tides and the moon here. For example, spring tides occur a couple of days after the new moon and a couple of days after the full moon.

Sooooo... if you are suggesting that gravity causes menstruation, it should happen on a cycle matching the tides. Twice daily or twice a (lunar) month. And you'll need to explain why women's menstrual cycles don't alter in zero gravity.

If you are suggesting that moonlight causes menstruation, you'll have to explain:

  1. Why not all women have periods on the same phase of the moon?
  2. Why women have periods when the weather is too cloudy to see the moon?
  3. Why other, brighter sources of light - everything from the Sun to camp fires to light bulbs - don't have the same effect?

Animals which time things 'by the phase of the moon' are usually actually doing it by a combination of factors: the state of the tide, water temperature, number of daylight hours at that time of year, a benefit to the event happening at night, and so on.

  • $\begingroup$ It is possible that in a pre-modern (i.e., no buildings, candles, etc) era the amount of light produced by the moon MIGHT have served a purpose in synching periods, perhaps by facilitating the amount of nocturnal activity early humans could do versus being stuck in bed during the new moon. Not sure if anyone knows how long humans have had the menstrual period they have now, but it most certainly pre-dates indoor lighting, A/C, and train schedules. I agree that it is most likely coincidental, but it may not be absolutely coincidental. $\endgroup$
    – Jason K
    Mar 20, 2017 at 14:48
  • $\begingroup$ @JasonK. Moonlight is just too erratic. If it is cloudy you can't see the moon, so that's your trigger mechanism gone. Possibly for weeks in the wet season storms of the tropics. Plus day length (and thus night length) varies with season - so you can stare at the moon for hours longer on a long winter night than a short summer one (sunlight hours affects human fertility a bit). Also, humans sleep at night, as do our closest relatives, chimps & gorillas. If you have rubbish nightvision as most apes do, you hunker down when predators which see well in the dark (e.g. big cats, hyenas) are around. $\endgroup$
    – DrBob
    Mar 21, 2017 at 11:53
  • $\begingroup$ I dunno, since our ancestors evolved hundreds of thousands of years ago in Africa savannahs, I'm not sure concerns about persistent, weeks long, cloudy periods would be valid. Day length doesn't vary much that close to the equator either. Plus the moon cycle may have driven hunting parties (i.e. leave in a new moon, return at a full moon) so even if nocturnal illumination wasn't a factor, the moon as primitive timekeeper is. $\endgroup$
    – Jason K
    Mar 21, 2017 at 13:23
  • $\begingroup$ @JasonK. The savannah has wet seasons. Thunderstorms and clouds all over the place. If you scroll down to the 'weather through the year' list on this website, you'll see Kenya has several cloudy months. expertafrica.com/kenya/info/kenya-weather-and-climate But yes, Moon as calendar/timekeeper is very likely. I'm now wondering what wavelengths of light moonlight has, in comparison to sunlight? And how you stop the sunlight being a trigger during the day time? $\endgroup$
    – DrBob
    Mar 22, 2017 at 12:21

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