Satellite Data From 362,000 Ocean Points Contradict the Textbook Two-Bulge Model of Tides

For generations, students have been taught that the Moon's gravity pulls Earth's oceans into two broad bulges on opposite sides of the planet, and that high tide arrives as a location rotates through them. A research team led by Yongfeng Yang of the Water Resources Comprehensive Development Center of Shandong Province, with Jiajia Yuan of Anhui University of Science and Technology and Mingyuan Fan of the Water Resources Research Institute of Shandong Province, has now tested whether those bulges actually appear on the ocean surface. Their study is published in Science China Earth Sciences.

The team analysed tidal observations from 362,370 ocean locations recorded through 2021 by the Jason-3 altimetry satellite, comparing each high and low tide with the lunar angle — the angle between a given location and the Moon, measured from Earth's centre. Among the 175,402 locations at angles where the model places its raised water, 56.84 percent experienced low tides. The reverse held in the 60-to-120-degree band the model marks as depressed: 56.38 percent of the 186,968 locations there recorded high tides.

"The findings directly contradict the physical existence of two water bulges on the Earth's surface," the authors state. Readings from 166 tide-gauge stations in August 2014 showed the same overall pattern. Researchers have long doubted that two physical bulges could form on a planet whose real oceans are shaped by landmasses, the geometry of ocean basins, the Coriolis force and friction with the seafloor — but that doubt had remained largely theoretical, without direct observational evidence of the discrepancy.

The study also examines an alternative explanation in which tides arise from oscillations of ocean basins rather than from water being pulled into bulges: the Moon's gravity deforms the solid Earth, and as the elongated planet rotates, its basins are repeatedly raised and lowered, moving the water above them. The new observations match that mechanism — low tides occur more often where the solid Earth rises and the water becomes shallower, high tides where it is compressed and the water deepens.