Up in the Air

Are more babies born during a full moon?

June 15, 2014 

On our Friday the 13th full moon last week, I suspected the myth about more lunacy occurring at full moon was certainly making the rounds. There is also the notion that there are more births at full moon than at other times.

I actually used to include mention of the birth myth in my astronomy class until I asked myself one day, “How do I know that?”

I decided to investigate. A couple of small studies had been done. but nothing with a really large data set. I contacted Joyce Martin at the National Center for Health Statistics and described what I wanted to study. When she stopped laughing, she agreed to provide the data on 50 million live births spanning a decade.

The data were supplied in printed tables of the number of births on each date for 10 years. I had a student research assistant enter all of the numbers into computer data files; we then developed programs that would sort the data into bins for each day of the lunar phase cycle.

The result? No statistically significant peak in the birthrate occurs at full moon or at any other particular phase, just random variations about the average.

A common but incorrect explanation for the purported but absent peak is the gravitational effect of the moon. After all, we need a physical connection, and gravitation (think of tides) is all that we have.

But if a gravitational effect existed, it would not depend on the phase of the moon but the variation in the distance to the moon. The moon’s distance varies about 10 percent during its elliptical orbit around Earth, and this variation is not tied to the phases: It is not closest every full moon, for example.

Therefore, we plotted the birthrate vs. the distance to the moon and, again, we saw no dependence in the results. But, we would not expect one anyway, since the gravitational force of the moon on a person is too small to have any biophysical effect.

So, why does the full-moon birth myth live on? Probably for the simple reason that you remember what you see and not what you don’t. The most likely moon to be seen up in the air at night is the full moon – it rises at sunset and sets at sunrise. The delivery nurse, changing shifts or taking a break outside during a hectic night, is most likely to see and remember a full moon. All the other nights will not be remembered.

The myth will outlive me and my science. Anecdotal evidence will trump my analysis. Still, believing a myth will not make it true.

Daniel B. Caton is a physics and astronomy professor and director of observatories at Appalachian State University. Email: catondb@appstate.edu. More on this month’s column: www.upintheair.info.

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