Tukuk-OSA Difference in Gravity, Not a Pull
The usual explanation — "the Moon pulls the water up on the near side" — is only half right, and the half it misses is the interesting one. The Moon's gravity does raise a bulge of ocean on the side facing it, but there is a second bulge on the far side of the planet, too. The reason is that gravity weakens with distance: the Moon tugs hardest on the near side, less on the solid Earth in the middle, and least on the far side. The whole planet is subtly stretched along the Earth–Moon line, and water — being free to flow — piles up at both ends while sinking in the middle. As the Earth spins inside this pair of bulges, most coastlines sweep through two bulges and two troughs each day: two high tides and two low tides.
The Lunar Day: Why Tides Drift Later Each Day
The Moon orbits the Earth in the same direction the planet spins, so after one full rotation (24 hours) the Moon has moved on, and the Earth must rotate a little further to bring the same place back under the same bulge. That takes about 24 hours and 50 minutes — a "lunar day." So high tide at a given beach arrives roughly 50 minutes later every day, which is why tide tables are keyed to the Moon, not the clock. This is a direct consequence of the Moon's slow journey around us, the same orbital mechanics behind moon phases.
Spring Tides and Neap Tides
The Sun raises its own, weaker tides — about half the Moon's — and the two add up or cancel depending on alignment. At new and full Moon, Sun and Moon pull along the same line, and their bulges reinforce: you get the biggest highs and lowest lows, called spring tides (nothing to do with the season). At the first and third quarter, the Sun pulls at right angles to the Moon and partially flattens the bulge, giving the smallest range — neap tides. Every fortnight the ocean swings between the two.
Why Some Places See Huge Tides and Others Almost None
If tides were only about gravity, every coast would behave the same. They don't, because local geography dominates the range. A wide, shallow bay can funnel the incoming water and make it pile up dramatically through resonance and funnelling — which is why the Bay of Fundy in Canada sees tides over 15 metres, while the Mediterranean, almost landlocked with only a narrow connection to the Atlantic, barely notices any tide at all. Estuaries, seabed shape and the Coriolis effect all reshape how the bulge actually moves along your coast.
Tides Beyond the Sea
The solid Earth flexes too — "earth tides" lift the ground by tens of centimetres — and large lakes show faint tides. Long before satellites, sailors used tides to time departures through shallow channels, and tidal barrages today harvest the rising and falling water for electricity. The energy ultimately comes from the Earth's rotation being very slowly drained by the Moon's tidal drag — which is also why the Moon creeps about 3.8 centimetres farther away every year.