The dangers of using a mobile phone while driving are well-established, but researchers have found another potentially fatal distraction: full Moons and supermoons.
A full Moon rises at least once a month, meaning there are at least 12 over the course of a calendar year.
When the full Moon does rise, a team of researchers say there's an increase in fatal motorcycle accidents on the nights in question.
And that number rises even more during so-called supermoons.
Full Moons vs supermoons
A full Moon is a glorious sight. It occurs when the Moon, on its orbital journey round Earth, is located on the other side of our planet from the Sun.
When it's directly opposite the Sun, with Earth in the middle, the Earth-facing side of the Moon is completely illuminated.
And because the Moon's orbit around Earth is elliptical, or slightly egg-shaped, that means sometimes the Moon is closer to Earth than at other times.
The Moon at its closest point to Earth is at 'perigee'; at its farthest from Earth, it's 'apogee'.
When the Moon is full and at its closest point to Earth, this is known informally as a 'supermoon'.
A supermoon can appear up to 14% bigger and 30% brighter than the farthest, 'smallest' full Moon of the year.
The danger of a full Moon?
Back in 2017, researchers published a study that found on nights of a full Moon, fatal motorcycle accidents increased by 5% compared to nights without a full Moon.
And on evenings when the full Moon was at its closest – a 'supermoon' – that number increased to 32%.
The study included data from the United States, the United Kingdom, Canada and Australia.
The team behind the study say the data is 'observational' and cannot be used to reach any firm conclusions, but point to the often distracting effects of the full Moon.
It can be especially distracting when the full Moon is close to the horizon, which makes it look abnormally large; a well-established optical effect known as the Moon illusion.
"Drivers face a number of distractions behind the wheel, from texting, to phone calls to road obstructions and more," says study co-author Eldar Shafir, professor of psychology and public affairs at the Princeton School of Public and International Affairs.
"Our study adds to the literature that small distractions can sometimes lead to life-altering consequences."
"Glancing at the full Moon takes the motorcyclist's gaze off the road, which could result in a loss of control," says the study's lead author Donald Redelmeier, professor in the Department of Medicine at the University of Toronto, Canada.
"The average ride on a motorcycle is more dangerous than a drunk driver with no seatbelt traveling the same distance.
"Because of this, we recommend riders and drivers orient their attention, ignore distractions and continuously monitor their dynamic surroundings."
Gathering the data
The team theorised that motorcycle drivers' attentions might be drawn to a full Moon, thereby potentially contributing to fatal crashes.
They looked at data from the official United States registry of motor vehicle crashes from 1975 to 2014.
During this period, 94 full Moons and 65 supermoons appeared.
For the study, the team defined a full Moon as one night when the Earth-facing side of the Moon was fully lit.
They calculated the number of fatal crashes that occurred on nights of full Moon, compared with nights of a quarter Moon, which refers to when the Earth-facing side of the Moon is half-lit.
And they included occasions when two full Moons appear in the same calendar month, sometimes known as a monthly blue Moon.
They looked at a 16-hour time frame, designated 'nighttime motorcycle activity', between 4pm and 8am, over a period of 40 consecutive years.
The data showed there were 4,994 fatal crashes on the nights with a full Moon, equal to 9.1 crashes per night.
8.64 fatal crashes per night occurred on nights when the Moon wasn't full.
Fatalities increased further when a supermoon was visible, amounting to 10.82 fatal crashes per night.
For every two full moon nights, there was one additional fatal crash. Under a supermoon, this increased to two additional deaths.
73% of the crashes took place before midnight; 27% happened after midnight.
The team say data from the United Kingdom, Canada and Australia showed similar results.
"While these figures might seem low on the surface, they are quite significant," Redelmeier says. "All of these deaths could have been prevented completely by small difference in behaviour."
However, the researchers did note the study's limitations, as other distractions and traffic hazards were not taken into account.
The study also didn't take into account weather or Moon visibility.
"We encourage motorcycle riders to wear a helmet, activate headlights, scan the road surface and be wary of other vehicles," Redelmeier says.
"The best approach to safe driving is reducing as many distractions as possible and keeping alert behind the wheel."
"That's good advice for all the rest of us who don't ride motorcycles, too," Shafir says.
