Why Tides Follow the Moon
the moon does not pull the ocean up on one side only - tides come from the difference in pull across the earth
Two tidal bulges exist because Earth's oceans are being stretched, not simply lifted. The moon's gravity pulls harder on the near side of Earth than on the far side, and this difference in pull across the planet's width creates a stretching force. That stretch produces two bulges: one on the side facing the moon and one directly opposite.
The Mechanics of Differential Gravity
The moon's gravity weakens with distance. Water on Earth's near side is roughly 6,400 kilometers closer to the moon than water on the far side. That might sound like a large distance, but it is small compared to the 384,000 kilometers separating Earth from the moon. Even so, the difference matters. The moon tugs on the near side about 7 percent harder than it tugs on the center of Earth, and about 14 percent harder than it tugs on the far side. That 7 to 14 percent difference is not enough to lift water dramatically on its own, but it is enough to pull the near side oceans slightly ahead of Earth and leave the far side oceans slightly behind. The planet rotates underneath these two frozen bulges, which is why most coastal locations experience two high tides and two low tides each day.
Why This Matters More Than Raw Pull
The common misconception is that the moon simply yanks the ocean upward on the side closest to it. That description captures the near-side bulge but ignores the far side entirely. More importantly, it misidentifies the cause. A single force pulling everything in one direction would accelerate Earth's oceans uniformly and produce no bulge at all. It is the unevenness, the variation across Earth's diameter, that creates the stretching effect. Gravity that is the same everywhere moves you. Gravity that varies from one side of you to the other stretches you.
When the Two-Bulge Model Does Not Apply
Real coastlines rarely see the clean two-bulge pattern the video describes. Continents block the smooth flow of water between ocean basins. The shape of coastlines, the depth of the seafloor, and the resonance frequency of basins all alter when high tide arrives and how extreme it becomes. Some narrow bays experience extreme tidal ranges exceeding ten meters, while some enclosed seas see barely a meter of difference between high and low water. The underlying physics of differential gravity still operates, but geography shapes how dramatically the ocean responds.
Transcript
Cram So the moon pulls the ocean up on the side facing it. That is the tide, right?
Rep That is the usual picture, and it explains only half of what actually happens.
Cram Half? What is the other half?
Rep There are two bulges, not one. One faces the moon and one sits on the exact opposite side.
Cram But nothing is pulling on the far side. Why would water rise there?
Rep Because tides are not caused by the pull itself. They are caused by the difference in pull across the Earth.
Cram Difference between what and what?
Rep The near water is closer to the moon, so it feels a stronger tug than the planet as a whole and moves ahead of it.
Rep The far water is further away, feels a weaker tug, and gets left behind. That lag looks like a bulge too.
Cram So the Earth is basically pulled out from under the far side ocean.
Rep Exactly. Stretch the whole system and you get a bulge at both ends. That is why most coasts see two high tides a day.
Cram Then why are some tides much bigger than others?
Rep The sun does the same thing, weaker. When sun and moon line up the stretches add and you get spring tides.
Rep When they sit at right angles the stretches partly cancel, and you get the smaller neap tides.
Cram So the real rule is not how hard the moon pulls, but how unevenly.
Rep That is the whole idea. Gravity that is the same everywhere moves you. Gravity that varies stretches you.