In a rare crossover of superstitious circumstances, September 2019’s full moon will happen to coincide with the most dreaded date on every calendar: Friday the 13th.

The September full moon, known as a “Harvest Moon” due to its appearance closest to the autumnal equinox, will appear in the final hours of Friday the 13th over many time zones. However, those living in American’s Eastern time zone will be spared any misfortune from the overlap of these two events, as the moon won’t “officially” reach its peak full stage until 12:33 AM on September 14th in this time zone, according to the Farmer’s Almanac.

As noted above, there are a number of beliefs and superstitions that pertain to the appearances of the full moon, as well as when Friday falls on the 13th day of the month. But what are they, what are their origins, and why do many of them remain today?

Moon Madness

The idea of “moon madness” has been a reoccurring theme over the ages. One of the most common superstitions pertaining to the full moon actually involves its appearance on a Friday; in many European traditions, sleeping beneath a full moon on this day of the week is one way to transform into a werewolf.

The belief today that violent crime tends to increase during a full moon could be seen as a sort of modern variant of the lycanthropic “moon madness” motif. A range of other purported full moon phenomena exist, which include general sleeplessness, to spikes in hospital admission rates during a full moon. However, scientific studies often argue that these perceived “peaks” of activity during a full moon are questionable.

According to a 2015 NCBI study, there are no scientifically proven links between a full moon and higher hospital admission rates. “Data collection and analysis shortcomings, as well as powerful cognitive biases, can lead to erroneous conclusions about the purported lunar effect on human affairs,” wrote JT Margot, the study’s author.

Nonetheless, an earlier study, published in 2011, found that up to 40% of professionals feel that there actually is a correlation between hospital visits and the full moon. Dr. John W. Becher, an emergency medical professional and former chairman of emergency services at AtlantiCare Regional Medical Center in Atlantic City, New Jersey, is among those proponents of such a link.

“You could almost tell the phase of the moon by how crowded that area of the ED was. Anytime the moon was full, that area was overflowing,” Becher said during an interview with an osteopathic publication in 2015.

Fertility Traditions

Along with hospital visits, many have also argued that pregnant women seem to be more likely to go into labor coinciding with a full moon. While this is another phenomenon some clinical professionals actually do believe to exist, there are undeniable connections to ancient mythology that may also help explain this.

Dating all the way back to Babylonian times, ancient people believed that a full moon was linked to fertility. Similar ideas carried over to the ancient Greeks, who also saw connections between the full moon and fertility, as evidenced by the goddess Artemis (or Diana, as she was called by the Romans), who was the goddess of the hunt, but had also been associated with both the moon and childbirth. In Mesoamerica, Ixchel, the Mayan goddess of fertility, was also associated with the moon.

Today, echoes of these ancient traditions continue, with many believing that childbirth is more likely to occur during a full moon. However, according to R. Phillips Heine, MD, a maternal-fetal medicine specialist with Duke University, these connections are more likely to stem from past associations made by ancient cultures, rather than any connections recognized by science.

“People have thought a full moon is responsible for many things — bad or good — through time. So when babies were born in a full moon, it naturally became the reason, rather than just the normal time… There are lots of belief systems and cultures around the world linking the cycle of the moon and women’s fertility.”

Full Moon, Poor Sleep

While the verdict on whether the full moon is really linked to births remains undetermined, the idea that a link exists between sleeplessness and a full moon has received some support in recent years from the scientific community. A study that appeared in the journal Current Biology in 2013, titled “Evidence that the Lunar Cycle Influences Human Sleep,” found that lunar stages actually do appear to influence the quality of some people’s sleep.

According to the study:

“We found that around full moon, electroencephalogram (EEG) delta activity during NREM sleep, an indicator of deep sleep, decreased by 30%, time to fall asleep increased by 5 min, and EEG-assessed total sleep duration was reduced by 20 min. These changes were associated with a decrease in subjective sleep quality and diminished endogenous melatonin levels.”

“This is the first reliable evidence that a lunar rhythm can modulate sleep structure in humans when measured under the highly controlled conditions of a circadian laboratory study protocol without time cues,” the study’s authors concluded.

Friday the 13th

The idea of bad luck associated with Friday the 13th has everything to do with the negative stigmas associated with the number 13. Many ancient traditions see the number 12 as being one associated with completion; we see this reflected in the twelve months of the year, or concepts like the 12 days of Christmas. The fact that 13 follows an even and harmonious number in this way is likely to be one reason for the negative attitudes people traditionally have toward it. Even into modern times, certain buildings and hotels around the world omit 13 from the numbering of their floors due to such superstitions.

As to why Friday is perceived as an unlucky day, here again we have influences from ancient times. According to Christian beliefs, Jesus was crucified on a Friday. However, a possible literary source for the unluckiest day of the week may have to do with Chaucer’s The Canterbury Tales, in which the characters mention their aversion to traveling on a Friday.

In the essay “The Plan of the Canterbury Tales” by Dr. Michael Delahoyde of Washington State University, he noted the interest scholars have shown in the dating of the fictional events related in this story:

Dating the (fictional) pilgrimage has absorbed other critics. W.W. Skeat fits it with the other works, figuring the earliest possible year would have been 1385. The April 18th reference in The Man of Law’s Introduction seems to indicate the second day of the trip. 1389 is eliminated because it would be a Sunday, Easter Sunday, and no travel would have taken place. 1390 is eliminated because April 17th, the first day of the pilgrimage, was a Sunday. 1391 is too late in reference to the other works. In 1386, April 20th was Good Friday and it’s unlikely the pilgrims, especially the Monk and the Prioress, would be travelling during Holy Week. In 1388, April 18th was a Saturday, making it impossible to reach Canterbury in time. But in 1387, April 18th fell on a Thursday. Easter was early that year (April 7th), and the group could have started out on Wednesday and reached Canterbury by Saturday — five days being reasonable. This date works well with the other writings too and coincides with the loss of two jobs in Chaucer’s official life.

The fact that the Monk and the Prioress would likely not have traveled during Holy Week gives us another clue here, as this might indicate their particular reason for avoiding travel on a Friday: or in this case, Good Friday, the day traditionally observed by Christians as the day Christ was crucified.

So for those in North America this week who will be able to enjoy seeing the appearance of a full moon on Friday the 13th, you now also have a few ideas about the mythic history–and the science–behind this crossover of superstitious themes.