Full Moon effect debunked again

By Phil Plait | July 31, 2007 4:08 pm

An article just came out over the wire discussing a study which shows — yet again — that the phase of the Moon has no effect on humanly efforts.

Nurses, cops, doctors — they all swear that things get crazy around a full Moon. However, study after study shows that is simply not the case… yet the swearing continues.

I was interviewed this morning by Bill Kole, the reporter who wrote that article, but he didn’t use what I wrote. Oh well. Update: Well, this online version does have me quoted in it! Weird.
Just for grins, though, here is what I wrote him:

Never underestimate the power of anecdote! People read about this idea, and it makes some sort of sense — we’re raised with stories of werewolves, and the power of the full Moon on romance — and they believe it. After all, nurses, police, doctors, they can’t all be wrong, can they? Of course they can. Like anyone else, they remember the time the emergency room was a madhouse during a full Moon, and forget the dozens of times it wasn’t, or the far more common times it was crazy but there *wasn’t* a full Moon. That’s human nature.

The Moon does have a profound affect on Earth; it powers our tides, changes the Earth’s spin, and provides light at night (many animals have cycles based on the Moon), but there are no physical effects on humans themselves. The tides from the Moon are far, *far* too weak to affect us physically, and the gravity from it is incredibly weak as well.

Also, the gravity of the Moon does not depend on its phase at all; the orbit of the Moon is elliptical, so sometimes it’s closer, and its gravity stronger, than other times. But that’s different from its phase, which only depends on the geometry of the Earth, Moon, and Sun.

The tides we feel on Earth are greatest at full and new Moon, because the Moon and Sun align in the sky, and their tidal force combines. But again, that’s far too weak to affect humans directly… and if it did, we’d expect to see the same craziness at new Moon as we do when it’s full. We don’t.

Next time I’ll talk about sex instead of romance. That’ll up the chances of me getting in.


Comments (55)

  1. Phil, a look at the statistics clearly shows that nearly 50% of all police calls and emergency room visits happen within a week (before or after) of the full moon. If we move out to two weeks, the rate jumps to nearly 100%! Deny that!

    (Interesting related statistic: according to the results of an analysis published in Dilbert, about 40% of all work absenteeism takes place on either a Monday or a Friday.)

  2. Ms Viesha Lewand

    I want to share some information given to me several years ago. If you leave a glass of water on your table…and mark where the water level is, that it will actually rise above this level during the full moon – I haven’t tried this experiment yet.

    The information makes sense though due to the rise of our tides and if our bodies are almost all liquid, wouldn’t the full moon, rising of water, etc have an effect on the entire liquid in our bodies?

  3. Ms. Lewand, I’m afraid not; tides only work across large distances. Read my essay on tides.

  4. Haa haa… nice one, Harold… I just got it.

    Yeah, Phil you got us with the moon. But how can you deny that the frequency vibration of Venus is the cause of the manifestation of the divine feminine?

  5. If the full moon did effect people’s behavior I would think it would be more along the lines of: full moon means more light which makes it easier for crazy people to do crazy stuff.

  6. But Phil, what about animals? :)

    A local meteorologist here posted this a few weeks ago:

    Pet injuries higher when there’s a full moon!

    Of course, he links to a FOX News item (so we all know how valid that is)..

    Vets: Pet Injuries Peak During Full Moons

    Phil, the study was done in Colorado at Colorado State.

    I was going to post debunking this crap on the meteorologists blog, but the people there don’t like to hear about science.

  7. A couple of weeks ago I needed to talk with a cop because my car had been broken-into. He was lamenting the fact that it was already shaping up to be a crazy night (there’d also been an armed robbery not so far away). Knowing the moon was not quite first quarter, I asked him if it was true what people said about the full moon. He said, “To be honest, I’ve never really noticed.”

  8. Jonas

    For once, I do not agree with the BA.

    I agree that humans tend to remember the hits and forget the misses, and gravity and tides are of course not strong enough to have any effect.

    But you disregard psychological effects completely. After all, if people think the full moon is more romantic, they might be more inclined to do romantic stuff. I do not know if romantic actions tend to cause more injuries, but the full moon have an effect. Maybe only in the heads of people, but it is still an effect.

    In short, I agree that there are no astronomical reasons, but there might be psychological ones. (Yeah, I know. This is an astronomy website).

    Hmm, it is getting late here, this is not as well-written as I would like it to be. Hope you catch my drift anyway.

  9. i may be a lunatic, but i’ve counted lots of hits with migraines!
    (don’t remember the misses though.)

  10. Jonas does make a compelling point and we do need to visit psychology here. However, a compelling point does not an effect make. There’s no statistical evidence of any correlation to the phase of the moon.

  11. John Krehbiel

    How many ordinary people actually know what phase the moon is in anyway? We have a local meteorologist who sometimes mentions the phase of the moon and gets it wrong more often than not.

    So somebody is having a rough night, and they say “Gee, it must be a full moon tonight.”

    Does anybody check? Could it be that the “full” moon they see is really a waxing or waning gibbous moon? If they don’t check the actual moon (or even if they do) people hearing the comment are left with the impression that there really is a full moon, and that it has caused (or at least is correlated with) a crazy night in the emergency room.

    That’s why it’s so hard to get people to understand the scientific viewpoint. They don’t care what the statistics say; to them, their personal experiences are more real.

  12. Of course, in some animals (not humans), there are real tidal and lunar rhythms.

  13. I believe the “effect” was in light of the fact that “once upon a time” (before electricity was extremely common) the full moon was the most economical evening light. So most of the activity you ever did at night occurred when there was a full moon. This may still be true to an extent for some agricultural communities. As I recall a few more famous burglaries were timed around phases of the moons for more than tidal purposes.

    So it’s not an direct “effect”, but a correlation, likely limited to certain groups and time periods.

    Many in the medical field find patients tend to pass away more frequently after holidays/birthdays. Some believe this is because of the desire to be around for “one last [event]”. It would be incorrect to blame this on the event(s) itself since the event doesn’t cause the death. But that doesn’t mean there can’t be a correlation. This also doesn’t mean it’s a universal correlation. For example in places where Christmas isn’t celebrated, it wouldn’t be likely to occur in early January.

    Interesting stuff.

  14. Jim A

    I would just like to say the only thing I could ever think of was that people might be looking at the moon while it’s full and become distracted–e.g. crash their car into a tree while staring out the side window. However, I don’t think that would be measurable.

    In short, I think that this Jonas guy might have something, but your conjecture about people remembering it when it happens and vice versa explains things pretty well.


  15. Allan

    Harold, if you extend your period of human lunar influenced events in question to 2 weeks either way, you have taken in virtually the entire 29.5 day lunar synodic period. Even one week either way takes in nearly half (50%) of the lunar synodic rotation period. That 50% of such events take place in about 50% of the synodic lunar period is only a simple statement of the obvious. What kind of a point are you trying to make?

  16. autumn

    The light of a full moon would, I conjecture, actually be detrimental to those in the act of crime, as it would reveal their activities to a higher degree than, say, a new moon or overcast night sky.

  17. To Jonas and Jim A:
    BA does not claim anything, he just reports that study after study done by people who.. eh.. study, show no rise in numbers of incidents during full moon.

  18. Arthur Maruyama

    There was a Skeptical Inquirer article (printed in the mid-1980’s, I think) which explained why some short-term studies showed some relationship between some police records with the phases of the moon, but nearly all long-term studies fail to do so. If you check a calendar which shows the lunar phases you will see that 2 or 3 consecutive months out of every 5 to 7 months the full moon will roughly coincide with the weekend (Friday through Sunday for the US). If a study on, say, police being called out to break up drunken brawls were done and the study period happened to include June and July 2007, then there would be a statistically significant rise of such incidences with the full moons of those months, but only because those full moons also coincided with weekends. If you normalize the data for the day of the week (and making a long-term study which uses several years of data can have this effect), then the “weekend effect” almost always disappears.

    Add to this selective memory (where only the hits are recalled) and you get a compelling story but no science.

  19. Troy

    One thing that I’m curious about is how did menstruation get in sync with the moon? This is a physiological phenomena of course but there has to be a connection.

  20. Marlayna

    Uh, menstruation *isn’t* in sync with the moon. If it were so, all women would have their period at the same time. Besides, menstruation cycles almost never last exactly as long as moon cycles.

    @Allan: It was a joke 😛

  21. Mark

    I’ve not talked about this with many women, but once long ago read about it in a book by Hungarian doctor and geneticist (is that how you call it?) Czeizel Endre. In this book (about human sexuality, written in the 80’s I think) he presented some statistics which showed that though women’s cycles are spread throughout the month, and they really can last for less or more than a lunar month, there’s a visible peak at a lunar month’s length and at the time of full moon. He couldn’t provide an explanation, of course, he didn’t even try to, being scientist. Unfortunately I’m in another part of the world now and don’t have access to this book, but will try to get some statistics if I can.
    With my astronomical and electrical training, but not in biology, I admit I can’t find any logical explanation to this (as it was already pointed out that gravity is not connected with the moonphases). I agree with others that generally we should consider psychological effects, but I don’t think it applies here.. I have to note though that the statistics were made decades ago – today the various contraception pills modify the cycles and might distort statistics, so it might be better to collect data from ‘less-developed’ countries..

  22. Freddy the Pig

    I recall a long time ago the author of “Saturday Night in America” explaining that this myth arose in part from a study that showed an apparent correlation between emergency room acitivity and a full moon, because during the period studied, the full moon was occuring on a Saturday when emergency rooms are normally busier – a classic example of correlation without causation.

  23. Nigel Depledge

    I think also that, on the two nights either side of the full moon, the moon still looks full to the inexperienced eye. This could increase the remembered hit rate.

    Or something…

  24. ART, still a Browncoat

    The full moon certainly does have an effect on human beings. We notice it, then we attribute all kinds of phenomena to it.

    When working with the nurses who would swear by the full moon phenomenon, I always wondered if the kinda of things that happened weren’t caused by patients, but the nurses and assistents themselves, who were anticipating catastrophes due to the full moon and thus changing their behavior accordingly.

  25. Zoot

    To say that moonlight has no physical effect on humans is self-contradictory. It must by definition have some kind of neuro-chemical effect or we would not see it.

  26. Andyo

    Hi there, I always enjoy your comments, I have been a long (lay-)fan of space science since watching Cosmos and reading Hawking years ago.

    So I always tend to get ticked when friends and others talk about astrology and this stuff about the moon. A co-worker whose friend is a cop used to tell me that at his workplace things did get crazy over a full moon night. The way he told me, it seems most cops there believed it too. I did tell him the same (obvious) thing, that people just remember certain instances while forgetting others just as significant. He wasn’t too sure still.

    People seem to think if the moon is powerful enough to affect the whole ocean (gravity), or other animals’ behavior (light?) then why not humans? I have found somewhat useful, after telling them that thing about people forgetting, to make some comparisons with ordinary objects.

    Hopefully these are right (you tell me), but it seems to me that the moon will have the same gravitational effect on you as any 80kg-or-so person standing very close to you, like 1cm or closer. Or, a car parked a little less close. When you translate gravity to these ordinary standards, people will tend to believe you more.

  27. Tony

    Well why don’t we just ask the people who went to the moon, surely being that close would have had an affect on them – oh wait, we can’t because the moon landings were all a HOAX

    I’ll get my coat…

  28. (Interesting related statistic: according to the results of an analysis published in Dilbert, about 40% of all work absenteeism takes place on either a Monday or a Friday.)

    Some years ago, I sent a notice out to various managers at my company that it had come to my attention that 40% of absenteeism was happening on Mondays and Fridays.

    This caused a huge uproar, and there was an investigation for a couple weeks as this issue was looked into.

    So I sent a response that said “it’s worse that 60% of absenteeism is tuesday/wednesday/thursday”. HR called to ask how I knew this information because absentee records were sensitive.

    I finally had to send out a message to all that “100% of senior managers and HR types have poor arithmetic skills”. Fortunately, I was leaving the company of my own volition anyway.

  29. Marlayna

    @Mark: That’s very interesting, if true. Has the book been translated to english? Can you remember the title?

    I too would like to see a bit more extensive research on the matter. (BTW, no reason to go to underdeveloped countries; just choose women who don’t use contraceptive pills)

    @Freddy: Not to sound nitpicky, but a single instance doesn’t indicate correlation.

  30. Marlayna

    Isaac Asimov had something to say about the “full moon effect”:


    @Evolving Squid: Looool ;D 😀

  31. Allan, please see Evolving Squid’s comment for further clarification. You may also want to see Phil’s earlier post regarding exploding irony glands.

  32. I work in an emergency room. It drives me nuts how often people who work there say, “It must be a full moon.”

    So whenever it isn’t a full moon (which is a lot of the time) and it’s crazy (which is a lot of the time) I say, ‘Come with me. See that? No full moon.’ Sometimes you can see the lightbulb come on. Sometimes I have to explain it to ’em.

  33. Next time I’ll talk about sex instead of romance.

    Yeah, baby! (I suppose photographic illustrations are probably out of the question though, right?)

    @ EvolvingSquid

    😆 too funny…


  34. Next time I’ll talk about sex instead of romance.

    Yeah, baby! (I suppose photographic illustrations are probably out of the question though, right?)

    @ EvolvingSquid

    😆 too funny…


  35. I’m surprised it’s not true for two reasons:

    1. There is more light at night during a full moon than a moonless night. A lot more light. More light means more opportunity for the mischievous to engage in mischief. I suppose that was a bigger deal in ye olden times, because there is so much nighttime lighting everywhere now.

    2. Self-fulfilling prophecy. If people believe that a full moon leads to crazy behavior, they might use it as a subconscious (or conscious) excuse to act crazy. Kinda like how a lot of your misbehavior while drunk can be attributed to you expecting to misbehave while drunk moreso than the inebriation itself. But I guess it’s a pretty tiny fraction of people that really pay attention to the phase of the moon.

  36. ABR

    Harold and Evolving Squid,

    I am in awe of your amazing grasp of statistics. It may interest you to note that I myself have demonstrated 100% accuracy guessing a person’s birthday to within +/- six months!

  37. Allright, so these are anecdotes, but…

    In general, I don’t believe that the full moon really influences anything (beyond the amount of light at night when the sky is clear).

    However, I gave birth a few days before the full moon in October, 1980. The nursery was about 3/4ths full. My daughter was a few weeks early, so she was in the hospital about a week. When the full moon hit a few days later, every single bassinett was full. The nurses said this was common.

    I recently had surgery and stayed on the ob/gyn floor. There was only one newborn during the days I was there. It was so quiet, I asked the nurse if they normally had more babies during a full moon. For whatever reason, she said they did.

  38. Mike

    To say that the moon being full has absolutely no effect on human behavior is Bad Psychology. People can behave differently depending on lighting conditions. The brighter the moon, the greater the potential for brightness-based behavior change. Sorry Phil, but the moon can change a person’s behavior!

  39. Egghead

    Why does everybody focus on the moon’s gravity? The gravity doesn’t change very much during the lunar cycle. They should take a look at what DOES change during the lunar cycle: the amount of light reflected back to the earth. There is a great deal more ambient light at night when there is a full moon than when there is a new moon. Presumably, diurnal creatures (such as humans) would be more likely to be active at night during full moons than during new moons.

    That’s what they should be looking for.

  40. ART, still a Browncoat

    Cafeenman, now that you mention it, I did that once, only I was a bit surreptitious about it. Things were going crazy in the ward and I said wow, is it a full moon tonight? They looked at their calandars and: no, it wasn’t–

  41. Sorry ’bout the double post…


  42. Lyle Gaulding

    Yes, when I was doing security at a hospital, the nurses said the full moon effect was true – but how many people working indoors, or as far as that goes, outdoors in an urban area, are that aware of the moon anyway? ‘Things are hectic tonight, must be a full moon.”

  43. Mark

    Unfortunately the book’s in hungarian and I have no access to it from here.

    The light of the moon comes naturally as an idea – for animals (incl.humans) living in ‘natural’ environment. But as someone pointed out, most of us don’t see the moon, or don’t notice it. (Especially in a country like singapore – it’s almost always cloudy… so bad for an astronomer..finally a chance to see southern sky and i can’t :(

    And imho evolution is not the answer here either – that is, saying that man adopted to lunar cycles long ago and kept it. We can’t even keep the 24 hr cycle for long when in artificial environment (if I remember well, it changed to something like 25 or so hours (there were some experiments long ago)… and Earth’s rotation was actually faster long ago)

  44. Mixelle

    Menstrual cycles aren’t in sync with the moon, no, but there’s been some evidence that does point to nocturnal light exposure possibly playing a role in ovulation regulation & the menstrual cycle. Look up Dewan’s 1967 study & related keywords if you’re interested – there has supposedly been some replication of his original results. (biological rhythms, menstrual phase regulation, melatonin suppression, light regulation, etc.)

    I’m thinking, purely as a fanciful speculation, that perhaps in some distant four-footed past, we did have a reproductive clock with light-based regulation…

  45. Mark

    “I’m thinking, purely as a fanciful speculation, that perhaps in some distant four-footed past, we did have a reproductive clock with light-based regulation…”

    I think, too; the period being very close to the lunar cycle is probably not by chance. Seems there’s still many areas for evolutionary biology to research :)

  46. KaiYeves

    This piece of BA is everywhere! Once, I was stargazing with a friend and pointing out the Southern Cross. They said, with a straight face “You know, my dad’s a cop, and he says that there’s more crime when the moon is full. That’s where the world ‘lunatic’ comes from.” I didn’t have the heart to tell him that it was false or that I’ve known that since I was six. (I also knew what SETI stood for when I was six, but that’s another story) Besides, when I try to debunk something for my friends, they tell me that I’m wrong or crazy or need to do more research. Don’t ever tell Valley/shopping Girls that astrology is fake. I need some new friends.

  47. al

    the moon,artemis, those who undermine her do so at thier own risk, and ignorance.

  48. veronica

    I don’t know about everyone else…. but EVERY full moon something bad happens to or around me. And I mean EVERY one!!!! I call it to! For instance, yesterday I was riding with some friends discussing our plans for the weekend. I noticed the moon was almost full and I stated that I was staying in because of my experiences when the moon is full. Well, last night my friend was nearly shot at a nightclub and today my close girlfriend was rushed to the hospital!!!! So for theses reasons I always try to stay in and be extra cautious when there is a full moon…. I almost dread it even though it is beautiful! I am almost certain the full moon has an effect on human behavior, it just hasn’t been proven yet!


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