The Moon pulls — but not evenly
The Moon pulls on Earth, and Earth pulls back. But gravity weakens with distance: the side of Earth facing the Moon is tugged a little harder than the centre, and the centre is tugged a little harder than the far side. That difference (the tidal force) falls off not with the square of the distance, as ordinary gravity does, but with its cube. Which is why the Moon, though 400 times smaller than the Sun, lifts our oceans about twice as effectively.
The water answers by stretching into two bulges, one toward the Moon and one on the far side. The solid ground, stiffer, barely shifts, a few tens of centimetres, but the ocean follows.
Two tides a day, slightly late
If Earth simply rotated beneath two motionless bulges, every coast would pass under one every 12 hours, two highs, two lows a day. Almost true. But the Moon is also moving along its orbit while Earth turns. The lunar day runs 24 h 50 min, and the tides slip by about 50 minutes each day. Which is why a high tide at 9 a.m. today shows up around 9
a.m. tomorrow.When the Sun joins in
The Sun adds its own tidal pull, weaker, but real. When the Moon, Earth and Sun line up (full or new moon), the two effects stack: these are the spring tides. When they sit at right angles (first and last quarter), they work against each other: these are the neap tides.
Geography amplifies or smothers all this. In the Bay of Fundy, in Canada, the funnel shape lifts the water as much as 16 metres, the world record. The Mediterranean, by contrast, is a near-closed basin that holds the tide to a few tens of centimetres. The Atlantic breathes; the Mediterranean barely stirs.