What Physics Teachers Get Wrong About Tides! | Space Time | PBS Digital Studios

Tides don’t come from the Moon “stretching” Earth’s oceans like taffy. The familiar gravity-differential picture is partly right in math but wrong in mechanism: the Moon’s gravity does create a differential pull across Earth, yet that differential doesn’t directly lift ocean water into two bulges. Instead, tidal forces act like a planet-sized squeezing and sideways traction system that piles the ocean up along the Earth–Moon line.

In the standard explanation, the Moon’s gravity is stronger on the near side of Earth and weaker on the far side, so oceans bulge outward at opposite points. The transcript acknowledges that this differential exists and that a simplified model would predict two high tides per day at a fixed location if Earth rotated under frictionless bulges. The problem is the interpretation. In Earth’s non-inertial frame—because Earth itself accelerates toward the Moon—the “tidal force” looks like an outward anti-gravity effect along the Earth–Moon line. But that outward effect is far too small to physically raise water. The tidal acceleration from the Moon’s differential gravity along the Earth–Moon line works out to only about 1/10,000,000th of Earth’s surface gravity, meaning it cannot lift water against Earth’s weight.

So what creates the bulges that are undeniably real? The key is where tidal acceleration points. Away from the Earth–Moon line, the tidal acceleration vectors tilt largely tangentially to Earth’s surface—pushing sideways rather than upward. Those sideways pushes are microscopic at any point, but the ocean covers an enormous area. Summed over the planet, the sideways traction squeezes the ocean toward the Earth–Moon line, increasing pressure there and producing the observed high tides. The Moon effectively turns the ocean into a hydraulic pump: the ocean bulges not because it’s being stretched upward, but because it’s being squeezed and piled up.

This mechanism also explains why lakes and other enclosed or small bodies of water show little to no noticeable tides. A lake isn’t a contiguous, planet-sized fluid reservoir, so it lacks the surface area needed for tiny tangential pushes to accumulate into a meaningful pressure change. Very large lakes can generate “mini tides” on the order of centimeters, but winds, boats, and sloshing swamp them. Bathtubs, swimming pools, even a cup of coffee experience tidal effects too—just at microscopic levels. Earth’s own slight deformation further reduces the relative water-level change.

The transcript then ties the model to real-world complications. The Sun’s tidal influence is analogous but about a third as strong, leading to spring tides when the Sun, Moon, and Earth align and neap tides when they’re at right angles. Predicted global swings in a simplified water-world model (roughly 3/4 meter) don’t match everywhere because coastlines and ocean basins redistribute pressure. Places like the Bay of Fundy can see swings over 10 meters, while regions with inlets can experience tidal bores—walls of water moving inland. The overall takeaway is that tides resemble “pimples” more than “taffy”: squeezing and pressure buildup dominate, while stretching is secondary, and local geography determines the details.