The moon wobble

Humans have long charted the phases of the moon, observed and predicted eclipses, and defined its elliptical orbit. Now NASA scientists have released a report studying the effects of one of the moon’s lesser-known characteristics: its wobble.

The subtle changes in distance, tilt, and orbit of the moon may not look like much to the naked eye. But NASA scientists who studied the phenomenon say the lunar wobble could significantly alter tidal patterns on Earth. According to the researchers, who published their work in Nature Climate Change in June, it will likely lead to increased risk of flooding during the 2030s.

“This is eye-opening for a lot of people,” NASA scientist and study co-author Ben Hamlington told Reuters. “It’s really critical information for planners. And I think there’s a great amount of interest in trying to get this information from science and scientists into the hands of planners.”

Because the moon makes one full revolution on its axis per orbit around the Earth, we only see one of its faces. But sometimes that face shifts as the moon’s tilt or the shape of its orbit changes. Scientists call the subtle changes libration. When time-lapse photography speeds up the process, the human eye perceives these changes as a wobble.

According to NASA scientists, the moon’s wobble falls into predictable 18.6-year cycles that alter its gravitational effect on tides. During the first half of that nearly two-decade cycle, the distinction between high tide and low tide weakens—high tides shrink and low tides grow. During the second half of the cycle, the pattern reverses, with higher high tides and lower low tides.

Scientists pulled data from 89 tide gauges that cover almost every U.S. coastline. According to the researchers, the moon will wobble into the second half of its libration cycle in the 2030s, heightening the risk of tidal flooding in low-lying areas.

Scientists first noticed the moon’s libration cycle in 1728, according to NASA. The researchers expressed concern that rising sea levels from climate change will exacerbate the upcoming cycle’s increased tidal intensity, amplifying flooding and forcing governments of coastal populations into difficult decisions.

Though flooding likely won’t compare to the storm surge from a hurricane, high-tide floods could become routine. “It’s the accumulated effect over time that will have an impact,” University of Hawaii oceanographer and study co-author Phil Thompson said. “But if it floods 10 or 15 times a month, a business can’t keep operating with its parking lot under water. People lose their jobs because they can’t get to work. Seeping cesspools become a public health issue.”