Tides are the regular rise and fall of the sea, driven by the gravitational pull of the Moon and the Sun on a rotating Earth. The Moon dominates. It is far smaller than the Sun, but far closer, and that proximity gives it roughly twice the tidal influence.
The Moon pulls on the whole planet, but not equally. Its gravity is strongest on the side of Earth nearest it and weakest on the far side, so the planet and its oceans are stretched along the line between them. In the simple model this produces two bulges of higher water: one facing the Moon and one on the opposite side. The far-side bulge is not the Moon pushing water away. It appears because the Moon pulls Earth's center more strongly than it pulls the more distant ocean on that side, leaving water behind relative to the solid Earth. The pull also slightly distorts the solid Earth, though the ocean's response is far more visible at the beach. Earth and the Moon also orbit a shared center of mass, which pushes outward away from the Moon; together with the Moon's stronger pull on the near side, this pulls water toward the Moon on one side and away on the other.
As Earth rotates, each location passes through both bulges in a day, producing two high tides and two low tides. One high-to-high cycle takes a little over 12 hours, so at most locations high tide arrives roughly every 12 hours and 25 minutes. Low tide does not drain the ocean. As water ebbs in one place, it moves across the basin toward wherever high tide is occurring, though its speed is limited by ocean depth, which is one reason real tides lag the ideal model. In shallow coastal waters and estuaries, low tide exposes large areas of seabed, which are flooded again on a dependable schedule; marine life uses these habitats.
The Sun also raises tides, but its effect is smaller in tidal terms because it sits so much farther away. When the Sun, Moon, and Earth line up at new moon and full moon, the two pulls reinforce each other and the tidal range grows: these are spring tides. When the Sun and Moon sit at right angles to each other, near the first and third quarters, their pulls partly cancel and the range shrinks: these are neap tides.
A lunar day, the time for a point on Earth to return to the same position relative to the Moon, is about 24 hours and 50 minutes, so a given high tide typically arrives roughly 50 minutes later the next day. The bulges themselves sit a little ahead of the Moon, because the Moon orbits in the same direction Earth rotates. This offset contributes to the lag. The actual gap varies from place to place.
The astronomical tide is only the starting point. Continents, ocean basins, coastlines, water depth, and seabed shape all modify it, and wind and storms add short-term changes on top. Tides travel as very slow waves along coastlines, distinct from the wind- and density-driven flows that make up ocean circulation. In the North Atlantic, the tide sweeps anticlockwise between Europe and North America twice a day.
Local geography decides which pattern a coast sees. Most places get two high and two low tides of broadly similar size each day, a semidiurnal pattern. Some get two unequal highs, called mixed. A few get only one high and one low, called diurnal. Constricted or funnel-shaped coastal areas can amplify the tidal range. In a funnel-shaped estuary, the same volume of water is forced into a narrowing channel, so it must rise higher. Because these effects differ from place to place, accurate predictions come from local tide tables rather than a single global rule.
In a long estuary, high tide can arrive hours after it reaches the open coast, because the wave moves along the channel at a speed set by water depth. This travel time is one reason two ports a short distance apart can see different times and heights on the same day.
