Have you ever rounded a corner in the early evening, or crested a hill, to find a full moon that you did not expect? It is off-white, or perhaps a bit yellow. Impossibly large. Perhaps there is a crater or a ridge on it, visible to the naked eye. When this happens to me, I am stopped in my tracks. It fills me with awe.
As I post this, the moon is full. It is also the equinox—or nearly so.
On the equinox, the days and nights are of equal length, but things are changing fast. For the northern half of the Earth, it is the Fall equinox, the transition between the long hot days of Summer and the shorter, cooling days of Fall. The southern half of the planet is experiencing the Spring equinox, coming out of Winter, blinking into the early days of Spring. The days are the same length everywhere, but where we are emerging from, and where we are going, depends quite literally on where we are standing.
Everywhere on Earth, though, the moon is full.
Humans have made meaning of the moon in myriad ways. To focus on just one country, in Lithuania in times past, children born on the waxing moon were understood to be timid, but joyful; children born on the waning moon, on the other hand, were thought to be angry, but strong. Furthermore, Lithuanian folklore holds, many activities are better done at some moon phases rather than others. As the moon waxes towards full, catching fish, weaving, and planting vegetables are all auspicious. In contrast, the final days before a new moon suggest acts of preservation instead: cutting wood, or salting meat, or smoking it.
One thing that we can glean from the lunar folklore out of Lithuania is that the moon can act as a synchronizer of human activity. It is a giant sky clock, or at least, a sky calendar.
Another tale of the moon’s influence comes from farmers in the highlands of Guatemala, and their use of the moon phase to synchronize their farming activities. (Bret Weinstein and I also relate this story in A Hunter-Gatherer’s Guide to the 21st Century; the research was done by people we knew while in graduate school, Helda Morales and Ivette Perfecto.) These farmers have a long-standing tradition to both plant and harvest crops when the moon is full. What the farmers say, when asked about the tradition, is that doing so allows the plants to grow stronger, and resist insect damage. With our modern scientific lens, we might ask what possible protective capacity the moon’s phase could have on crop health. And we might easily answer: none.
But what the moon’s phase can do is synchronize the farmers and their activities. The giant sky clock keeps time in a way that everyone in the region can see.
So, the farmers tell a story that the full moon has salutary effects on their individual crops. That the story is literally false does not matter, if the behavior that the false story produces allows those farmers to thrive. This is an example of what Bret and I call “literally false, metaphorically true.” Farmers who believe the story restrict their planting and harvesting to the full moon, and this does in fact benefit the crops of those who do so, just not for the reason they state. By synchronizing both the planting and harvesting of crops, Guatemalan farmers are satiating the predators who would destroy their crops. That is: crop predators are always hungry. If crops are always available to eat, the crop predators will find them, and the number of crop predators will grow. If, however, the predators can be satiated, by concentrating all available crops to one moment in time, the crop predators will not be able to eat all of the crop, leaving some of it for the humans who planted it to begin with. Predator satiation is a well-known ecological tactic; by telling stories that synchronize their behaviors, farmers employ this tactic without being conscious that this is what they are doing. This should not be seen as mystical, but rather both brilliant, and evolutionary.
The story of the Guatemalan farmers reveals how myth can prove adaptive in driving human behavior, even when the stories being told aren’t literally true. But the moon has been with us since long before we were human, and has physical reality—and likely literal physical effects on us—beyond what we have yet been able to fully understand.
One oft noted truth is that the length of a woman’s menstrual cycle is very close to that of the cycle of the moon. Is this a coincidence? The evidence is mixed, and most modern research finds little to support the idea that the human menstrual cycle is synced with that of the moon. But it may also be true that, as in so much else, modernity is obscuring or even altering an ancient pattern. The near ubiquity of electric lights in the nighttime for WEIRD people, and the fact that we sleep indoors, are both likely to affect any relationship between menses and moonlight, a relationship that would have existed long before we could obscure our sky in the modern way.
In order to assess whether there is an ancient pattern linking menses with moon phase, we might hope to find data on moon phases and the reproductive cycles of women who live far away from city lights, and entirely outdoors. But such women are rather difficult to find in modernity. That said, there is another approach to discovering whether reproductive cycles of humans are in some way tied to the moon’s cycle. We might ask this: Do the females of any other animals show synchrony between moon phase and their reproductive cycles?
Why yes, they do. Several species of reef fish, for instance, are known to synchronize their spawning to particular phases of the moon. Similarly, honeybees have activity rhythms—when they tend to come and go from their hive—that are tied to the phases of the moon. Indeed, several aspects of honeybee’s anatomy and physiology vary with moon phase as well, including their body weight, which peaks at the new moon.
And in Lake Tanganyika, one of the African Great Lakes, a cichlid (fish) by the scientific name of Cyprichromis leptosoma has an unusual reproductive strategy: it is a mouth brooder, meaning that a parent holds and protects their young in its mouth until they are ready to become independent. In this species, it is mothers who brood their young this way, and they synchronize their brooding with other mothers to begin around the first-quarter moon. The author of this research further hypothesizes that the lunar timing of mouth brooding provides safety in numbers (which is possibly another example of predator satiation), as this species and another with whom it shares territory, are both reproductively cueing off of a certain phase of the moon, such that they are both most at risk, and most protected by their parents, at the same moment—thus spreading that risk across a larger population.
Given that many other animals sync some of their reproductive functions to the phase of the moon, it is not so far-fetched to imagine that humans might as well.
I used to ask my students, when I had students, to tell me what phase the moon was in. Usually only a small handful of students knew without checking. Sometimes nobody in the room did. These were classrooms full of outdoors- men and women, people who had taken extensive backpacking trips domestically, or explored foreign lands on foot. While in those environments, they would surely have known the phase of the moon. But put us back in our normal milieu, and we often forget the lessons from other places.
One of the lessons that we should keep track of, I am arguing, is the astronomical realities of our lives, and their manifestations here on Earth: What phase is the moon in? How long are our days right now, and are they lengthening, or shortening? Given where you are when you answer that question, what can you infer about whether to take a jacket when you leave the house, or whether the produce at the market is likely to be local, or shipped from far, far away? (And we might then ask what effect, in turn, will the length of the supply chain that put that peach or potato in front of you, have on its flavor, its nutrient load, its overall quality?)
When I was a professor, I always requested classrooms with large windows, in part because I know that in any hours-long class, everyone’s attention will wander at some point. When we are together, and I am the professor, or in some other way the person directing the action, I want to make available a place for your wandering attention that is itself healthy. I wanted students’ attention to be drawn out the window when a gust of wind made the leaves move in unison, or a pileated woodpecker flew by, its red crest and undulating flight instantly recognizable to those who had seen it before. And on rare occasion I was in a classroom high enough up, with few enough trees just outside, that we would see the full moon as it rose, a bit yellow, impossibly large, a dark crater visible even to the naked eye. Such a moon will draw the attention of everyone in the room. Such a moon should draw the attention of everyone in the room.
I will end with this, the final words from the chapter on sleep, in A Hunter-Gatherer’s Guide to the 21st Century: “Humans deserve a night sky, a sky full of possibilities—sometimes of clouds, often the moon, occasionally planets, nearly always stars and the Milky Way in which we live. Besides sleep, which we need, what else might we lose when we disappear our own night sky?”
 I have adopted the pattern of hot-linking references in these posts, rather than providing in-line citations and full references at the end, as is standard scientific practice. I admit that it is difficult for me not to include citations, but I also recognize that they are distracting for the vast majority of people. But the hot links go to stubs, more often than not, whereas citations are stable, so it’s not a perfect solution. I am wondering if I should include the complete references somewhere; perhaps in the text accompanying the audio reads of these posts, for paying subscribers? If you have an opinion on this, do let me know.