The moon also revolves around the Earth about once a month, and that orbit is a little bit tilted. To be more precise, the moon’s orbital plane around the Earth is at an approximate five-degree incline to the Earth’s orbital plane around the sun. (Here are a couple videos to illustrate this.)
Because of that, the path of the moon’s orbit seems to fluctuate over time, completing a full cycle — sometimes referred to as a nodal cycle — every 18.6 years. “It happens on such a slow scale,” said Benjamin D. Hamlington, a co-author of the paper who leads the Sea Level Change Team at NASA. “I think ‘precession’ is a more specific word than wobble.”
At certain points along the cycle, the moon’s gravitational pull comes from such an angle that it yanks one of the day’s two high tides a little bit higher, at the expense of the other. This does not mean that the moon itself is wobbling, nor that its gravity is necessarily pulling at our oceans any more or less than usual.
“The emphasis on the nodal cycle is a little bit different from the message we were trying to convey,” Dr. Hamlington said. But he added that the phenomenon was worth paying attention to.
High-tide flooding related to climate change is expected to break records with increasing frequency over the next decade, and people who want to accurately forecast that risk have to work with a lot of noisy data, including weather patterns, astronomical events and regional tidal variation.
The moon wobble is part of that noise, but it has always maintained its own slow, steady rhythm.
“It’s just acting in the background as sea levels rise,” said Brian McNoldy, a senior research associate at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science.
“During its most rapid upward phase, it acts to enhance the effective sea level, and during its most rapid downward phase, like we’re in now, it acts to suppress the effective sea level,” said Mr. McNoldy, who has written about the lunar nodal cycle but was not a part of the Nature study. “It is not part of sea level rise projections, because it’s not sea level rise; it’s just an oscillation.”