The Tide Table Handbook
The tides exist because the Moon's gravity pulls unevenly across the Earth, and the Sun adds a weaker pull of its own.
Every part of the Earth feels the Moon's gravity, but the strength of that pull depends on distance. The ocean on the side facing the Moon is about one Earth-radius closer than the planet's centre, and the ocean on the far side is one radius farther away. The near water is pulled hardest, the solid Earth less, and the far water least of all.
Subtract the pull on the planet's centre from the pull at the surface and a small residual remains: the tide-raising force. It stretches the ocean along the Earth–Moon line, producing a bulge toward the Moon and a matching bulge on the opposite side, where the water is, in effect, left behind.
On narrow screens, swipe or scroll the plate sideways.
Because the force depends on differences across the Earth's diameter, it falls with the cube of distance rather than the square. The Sun is about 27 million times more massive than the Moon but roughly 390 times farther away, so its tide-raising force comes to a little under half the Moon's.
If the oceans covered a smooth planet and responded instantly, high water would arrive whenever the Moon crossed the local meridian. Real oceans are shallow, bounded by continents, and slow to move. Friction, inertia, and the natural oscillation of each basin delay the response, so high water typically lags the Moon's transit by hours.
The lag is stable for any given harbour, which is why it has a name — the establishment of the port — and why tables, not sky-watching, remain the dependable way to know the tide.
Further reading