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full moon

In a nutshell: The full moon and other phases of the moon have been linked to all kinds of things, but so far the science hasn't supported folk beliefs about the full moon.

The full moon has been linked to crime, mental illness, disasters, accidents, werewolves, and many other things. Does the scientific evidence support any of these links? Not really. Well, the science does favor one link: when the moon is waning (when the part we can see gets smaller), you would be well advised to stay out of the reach of hungry lions in the jungle. In the dark they can see us better than we can see them.

Why do people believe the full moon makes all kinds of things happen? There are several reasons.

full moon rising

Let's begin with a common belief about the full moon: more people are admitted to hospitals during a full moon than at any other time of month. Is this true? No. Yet, many nurses say it is true because they have seen it happen. But the facts show that there are no more admissions to hospitals during a full moon than at any other time of the month. So why do some nurses believe in the full moon effect? The main reason is that believers rely on memory instead of keeping records.

Memory is tricky. If you believe that more people are admitted to the hospital during a full moon, then you may pay more attention to admissions when the moon is full. You may not pay much attention to the number of admissions on nights when the moon is not full. A scientist doesn't rely just on memory. She records how many people were admitted on each day of the month and she then compares those admissions with the phases of the moon. She also considers such things as weekends and holidays (when more or fewer people might be admitted). When scientists investigate hospital admissions they've found that there's no big difference between nights when there's a full moon and nights when there isn't a full moon. The nurses who believe in the full moon effect have selective memory. They remember the cases that favor their belief and they forget the ones that don't. Don't think too badly of nurses, though, because we all have selective memories when it comes to things we believe.

Another reason people might believe that the moon has all kinds of effects on human behavior is that most people don't know much about physics and astronomy.

The moon is much closer to the Earth than the Sun is, but it still doesn't have that much effect on us. Yes, both the Moon and the Sun affect the ocean tides. But that doesn't mean that either is a very powerful force on us. A mosquito has about as much gravitational pull on your arm as the Moon does. The Sun is much farther away than the Moon, and even though it is much larger than the Moon, it has very little gravitational effect on you. Despite these physical facts, there is still widespread belief that the Moon can cause earthquakes. It doesn't; nor does the Sun, which exerts much less tidal force on the Earth than the Moon.

Some people think that since the moon affects the tides, and the human body is mostly water, the moon has a big influence on us. The human body is about 80% water, but the Earth is not mostly water. Eighty percent of the surface of the Earth is water. Also, the moon only affects unbounded bodies of water, like oceans. The water in the human body is bounded, like the water in a swimming pool.

further reading

Neuroscience for kids: does the moon influence behavior?

Last updated 24-Aug-2011

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