The moon also revolves around the earth about once a month, and that orbit is tilted a little. More precisely, the moon’s orbital plane around the earth is inclined approximately five degrees to the earth’s orbital plane around the sun. (Here are a couple of videos to illustrate this.)
Because of this, the moon’s orbit appears to fluctuate over time, completing a full cycle – sometimes called a nodal cycle – every 18.6 years. “It’s happening so slowly,” 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 pulls one of the two tides of the day a little higher at the expense of the other. This does not mean that the moon itself wobbles, nor that its gravity necessarily pulls our oceans more or less than usual.
“The emphasis on the knot cycle is a little different from the message we were trying to convey,” said Dr. Hamlington. But he added that it is worth paying attention to the phenomenon.
Floods related to climate change are expected to break records more frequently over the next decade, and people trying to accurately predict this risk will have to deal with a lot of noisy data, including weather patterns, astronomical events, and regional tidal fluctuations.
The moon wiggling is part of that sound, but it has always kept its own slow, steady rhythm.
“It only plays in the background when the sea level rises,” said Brian McNoldy, a senior research fellow at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science.
“During its fastest upward phase it works to raise the effective sea level and during its fastest downward phase, as we have it now, it works to lower the effective sea level,” said Mr. McNoldy, who wrote on the lunar nodal cycle, but was not part of the Nature study. “It’s not part of the sea level rise projections because it isn’t a sea level rise; it’s just a vibration. “