Why do ocean tides actually rise and fall?

Ocean tides rise and fall because the Moon's gravity is not the same everywhere on Earth. It pulls a little harder on the side facing it and a little more weakly on the far side, which stretches the planet slightly along the line between them. Solid rock barely gives, but the ocean can flow, so water piles up in two bulges: one facing the Moon and one on the opposite side. As Earth spins, your beach passes through both bulges, which is why most coasts get two high tides and two low tides a day.

The rhythm also slips later each day, because the Moon moves along its orbit and takes about 24 hours and 50 minutes to return to the same place in the sky. Bede noticed this as early as 725: the tides rise and fall about four-fifths of an hour later each day, just as the Moon rises and sets later. Even Galileo got tides wrong. He blamed Earth's own motions and dismissed the idea that the Moon moves the seas as an appeal to occult qualities. It took Isaac Newton's theory of gravity, published in 1687, to explain them.

The Sun joins in, with a twist. It pulls on Earth about 179 times harder than the Moon does, yet its tidal force is only 46% of the Moon's. What matters for tides is how much the pull changes from one side of Earth to the other, and the Sun is so far away that its pull barely changes. When Sun and Moon line up at new and full moon, their effects add up into big spring tides; at quarter moons they partly cancel into weak neap tides. Coastlines do the rest: in Canada's Bay of Fundy, the sea has been measured rising and falling by 16.3 metres.