why do we have a full moon

Dear Science,
Does the moon actually make people act crazy, or change human behavior in any way? I have a teacher who insists this is true. б Your teacher is mistaken, but they're in pretty good company. Because of historical beliefs and a lot of inconclusive and misleading research, plenty of folks assume that the moon has some sway over human behavior. First, let's get the historical context out of the way. You probably already knowб that the word "lunatic" comes from the Roman moon goddess "Luna". In ancient Greece and Rome, creating a wave of strange behavior whenever the moon was particularly full or large in the sky. Some version or another of this belief has survived throughб hundreds of years and countless cultural shifts. Even today, some psychiatrists continue to hold up this watery brain theory. But while humans are indeed made of mostly water, this theory doesn't hold any. As the late astronomer George Abell of the University of California, Los Angeles, noted, a mosquito sitting on our arm exerts a more powerful gravitational pull on us than the moon does. Yet to the best of our knowledge, there have been no reports of a Бmosquito lunacy effect. Б Second, the moonБs gravitational force affects only open bodies of water, such as oceans and lakes, but not contained sources of water, such as the human brain. Third, the gravitational effect of the moon is just as potent during new moons Б when the moon is invisible to us Б as it is during full moons. But even without a plausible physical cause, some still point to anecdotal evidence of higher rates of crime, emergency room admissions and surgical mistakes, just to name a few loony examples. One-off studies have occasionally shown one or another of these things to be "true," but remember: A single study means nothing.


No one has ever been able to show consistently, with multiple studies, that the full moon has any effect on behavior. [ ] In fact, when researchers Ivan Kelly, James Rotton, and Roger Culver, they found none of them to show a significant relationship between the moon and human behavior. Much of the research had been poorly conducted, they found, or ignored obvious variables (one study concluded that full moons increase the incidence of car accidents, ignoring the fact that nearly all the full moon nights used for data had occurred on the weekend Б when car accidents are more likely no matter what). Kelly, Rotton and Culver suggested that many of the study participants wereб swayed by certain cognitive biases. They might recall the full moon nights when strange things happened and forget all the gloriously mundane full moon nights they'd seen, or even be more likely to see an occurrence as being spooky or unusual because they knew a full moon was hanging in the sky at the time. [ ] "It is important to note that there are two hurdles to overcome before any findings on lunar variables and human behavior are deserving of public attention,". "The first hurdle is that reliable (i. e. , replicable) findings need to be reported by independent investigators. The second hurdle is that the relationship should not be a trivial one. The lunar hypothesis fails on both counts. " But full moons might make certain behaviors more likely even without any sort of freaky mind control. б One of the more convincing studies on lunar madnessб with injuries after a full moon. [ ] Let's think about that one for a second.


If someone told you thatб the consumption of ice cream puts you at a greater risk of drowning, would you believe it? Good, because it doesn't. But the consumption of ice cream does go up and down in the same patterns as death by drowning does, becauseб both of those things happen when it's warm. When the moon is full, pet owners might be more likely to take their animals out at night. The bright light of an inviting moon could also explain upticks in crime (though there's no strong evidence that this actually occurs). Just as warm weather puts more people outside, interacting with others and sometimes getting up to no good, a well-lit night might help some folks end up inб the wrong place at the wrong time. The brightness of the full moon brings us to a possible explanation for all of the historical work on lunar lunacy. б did create a kind of madness at one time Б back before humans controlled their light exposure with indoor electricity and street lights. Those living outside or in shelters that did little to block the sky might have been kept awake by the full moon's intense light, making them act strangely or exacerbating any mental illnesses they may have suffered from. So the next time your teacher tries to blame the rambunctiousness of the classroom on an upcoming full moon, feel free to correct them: It's not the moon. You guysб are just a handful. Have a question for Dear Science? Ask it here. One of the reasons people often have bad intuitions like yours about the relationship between the Earth and the Moon is because they've never seen an accurate picture. The distance from the Earth to the Moon is often pictured something like this: The relative sizes of the Earth and the Moon are accurate but the distance is not.


Given this picture it looks like the Moon ought to be almost always in the shadow of the Earth. A picture that accurately shows the relative sizes and distances is more like this: And now it should be pretty clear that it would be really hard to get the Moon exactly in the shadow of the Earth from that far away. And if that's not clear, try it. Get a light bulb, a big grapefruit, a small orange, and a dark room and see if you can get the orange in the shadow of the grapefruit from twenty grapefruit-diameters away. A fact that is missing from this diagram is: where exactly is the shadow of the Earth, and how large is it compared to the size and position of the moon? I've edited the diagram above to give a rough idea of it. The white lines on the left of the Earth, when extended, go to the "north" and "south" poles of the Sun, 150 million km away. The Sun is about 1. 4 million km in diameter. The white lines that continue on the right of the Earth indicate where the shadow of the Earth is; inside this region you can see neither the top nor the bottom of the sun. That region is about 1. 5 thousand km long, or about four times the distance from the Earth to the Moon. Imagine those lines meet three or four screen widths to the right of your screen. The Moon's orbit takes it both "north" and "south" of that shadow region; I've marked the approximate maximum positions of the Moon on the diagram. So you can see, there's a pretty small region that the Moon has to hit in order to be in shadow on a full Moon. Most of the time the full Moon will be too far north or south of the shadowed region.

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