A new study is challenging one of the most familiar explanations in ocean science: the idea that the Moon’s gravity creates two enormous water bulges on opposite sides of Earth. The double-bulge model has appeared for generations in physics, geography, and oceanography textbooks, and is still used in simplified explanations by major scientific and educational institutions. However, researchers from China argue that global observations of high and low tides do not support the physical existence of these predicted bulges at Earth’s surface.
The study was led by Yongfeng Yang of the Water Resources Comprehensive Development Center of Shandong Province, together with Jiajia Yuan of the School of Geomatics at Anhui University of Science and Technology and Mingyuan Fan of the Water Resources Research Institute of Shandong Province. Their analysis examined whether locations that should lie beneath the model’s tidal bulges actually experience more high tides, as the traditional explanation would predict.
The double-bulge concept is derived from the equilibrium theory of tides. In its simplest form, the Moon’s gravitational attraction stretches Earth’s oceans along the Earth–Moon axis, producing one bulge on the side facing the Moon and another on the opposite side. As Earth rotates, coastlines are imagined to pass through these raised regions, producing high tide, and then through the lower regions between them, producing low tide. Although useful as an introductory model, the theory assumes an idealized ocean covering a smooth, rotating planet.
Real oceans are far more complicated. Continents divide the ocean into separate basins, seafloor topography varies dramatically, and the Coriolis force deflects moving water because of Earth’s rotation. Friction along the seabed and coastlines also dissipates energy, while the shapes and depths of individual basins determine how tidal waves propagate and resonate. For this reason, oceanographers generally treat the equilibrium tide as a conceptual approximation rather than a complete description of observed tides.
To test the model against measurements, the researchers analyzed tidal data from 362,370 oceanic locations observed by the Jason-3 satellite during 2021. The data were obtained through AVISO, a satellite-altimetry service that measures variations in sea-surface height. The researchers compared the frequency of high- and low-tide events at each location with its lunar angle, defined as the angle between the observation point and the Moon relative to Earth’s center.
Their results did not show the pattern expected from the double-bulge model. Among 175,402 oceanic locations positioned within the 0-to-60-degree and 120-to-180-degree lunar-angle ranges, which correspond to the regions where the model places its two water bulges, 56.84 percent experienced low tides and 43.16 percent experienced high tides. In the intermediate 60-to-120-degree range, corresponding broadly to the regions between the supposed bulges, 56.38 percent of the 186,968 locations experienced high tides. In other words, the distribution of observations appeared broadly opposite to the simplified prediction.
The researchers then examined records from 166 tide-gauge stations collected in August 2014. According to the study, these independent coastal observations produced a similar pattern: low tides occurred predominantly in the 0-to-60-degree and 120-to-180-degree lunar-angle regions, while high tides occurred more frequently in the 60-to-120-degree range. The authors interpret this consistency between satellite and tide-gauge data as evidence that the predicted bulges are not physically present as global, surface-wide reservoirs of elevated ocean water.
The study proposes a different physical interpretation. Instead of viewing tides primarily as water being pulled into two planetary-scale mounds, the authors suggest that tidal motion may reflect the oscillation of ocean basins driven partly by deformation of the solid Earth. The Moon’s gravitational field does not act only on seawater; it also slightly stretches and compresses Earth’s rocky interior. As the deforming planet rotates, the researchers argue, ocean basins may be continuously uplifted and depressed, changing water depth and driving flows. In this interpretation, areas where the solid Earth rises would become shallower and could display lower sea levels, while compressed regions would become deeper and could show higher sea levels.
The findings are likely to attract attention because they challenge an image deeply embedded in public science communication, but they do not mean that lunar gravity is irrelevant to tides or that ocean tides can be explained by solid-Earth deformation alone. Modern tidal science uses dynamic models that combine gravitational forcing with the motion of water through irregular ocean basins, Earth’s rotation, friction, resonance, and coastal geometry. Solid-Earth tides and the changing shape of the seafloor are real physical effects, yet their relative importance must be evaluated against the much larger and highly variable dynamics of individual ocean basins. The authors’ interpretation therefore represents a proposed alternative perspective, while the observational pattern and its broader implications will require further testing.
Published in Science China Earth Sciences, the study reopens a long-running debate over how tides should be explained outside idealized diagrams. Its central message is that a simple picture of two permanent water bulges may not describe what actually occurs across the global ocean. By comparing satellite altimetry with coastal tide-gauge records, the researchers argue that the real tide is better understood as a dynamic interaction between the Moon, the deformable Earth, and complex ocean basins rather than as the straightforward rotation of shorelines through two fixed masses of water.
Subject of Research: The physical reality of global tidal bulges and the mechanisms governing ocean tides.
Article Title: Testing the physical reality of tidal bulges in the world’s oceans
Web References: https://doi.org/10.1007/s11430-025-1878-3
References: Yang Y, Yuan J, Fan M. 2026. “Testing the physical reality of tidal bulges in the world’s oceans.” Science China Earth Sciences, 69(6): 2015–2021.
Image Credits: © Science China Press
Keywords: tides, tidal bulges, Moon, oceanography, satellite altimetry, Jason-3, AVISO, tide gauges, lunar gravity, Earth deformation, ocean dynamics
