A tide begins with a difference in gravity
Stand beside the sea for several hours and the shoreline may move noticeably, even on a calm day. That change is a tide: a rise or fall in sea level that is driven mainly by the gravitational effects of the Moon and Sun. Wind and weather can also move water, but they are separate influences layered on top of the regular tidal cycle.
Gravity acts on all of Earth, not just its oceans. The Moon pulls slightly more strongly on the side of Earth nearest to it than on Earth’s center, and slightly less strongly on the far side. These differences, rather than the Moon’s overall pull, are what matter for tides. They stretch Earth and its oceans very slightly along the Earth–Moon line. The solid ground responds too, although the movement of ocean water is usually easier to notice.
This explains a point that can seem puzzling: in a simplified picture, there is a tidal bulge on the side facing the Moon and another on the opposite side. Water on the near side is pulled more strongly than Earth’s center; water on the far side is pulled less strongly. The far-side bulge is not caused by the Moon pushing water away. Both arise from comparing the Moon’s pull at different places on Earth.
Why the Sun matters, but the Moon matters more
The Sun’s gravity also produces tides. Although the Sun pulls on Earth more strongly overall than the Moon does, it is much farther away. Its pull changes less from one side of Earth to the other. Because tides depend on that difference, the Moon has the larger tidal effect, while the Sun adds an important second influence.
When the Sun, Moon and Earth are roughly in line, their tidal effects reinforce each other. This happens around new moon and full moon. The resulting spring tides have a larger-than-usual difference between high and low water. “Spring” here means a greater rise and fall; it does not refer to the season.
Around first and third quarter moon, the Sun and Moon lie roughly at right angles as seen from Earth. Their tidal effects partly counteract each other, producing neap tides with a smaller difference between high and low water. Neither arrangement switches tides on or off: it changes the size of their range.
Why high tide does not arrive at the same time every day
As Earth rotates, a place on its surface changes position relative to the Moon. In the simplest two-bulge picture, many locations would pass through two high tides and two low tides during one lunar day. But the Moon is also moving along its orbit in the same direction that Earth rotates. Earth must turn a little farther for a location to face the Moon again, so a lunar day is about 24 hours and 50 minutes. Where there are two high tides, they are often roughly 12 hours and 25 minutes apart.
That simple schedule is a starting point, not a timetable for every beach. Some coasts have one prominent high tide per lunar day, while others have two highs of unequal height. A high tide at one harbor need not occur when the Moon is directly overhead. The water takes time to move, and real oceans do not behave like a smooth shell of water circling an unobstructed Earth.
Why the coastline changes the result
Continents interrupt the movement of water. Ocean basins have different sizes and depths, and the tide travels through them as a long wave shaped by the seafloor and Earth’s rotation. Bays, channels and estuaries can further alter that wave. A narrowing inlet may concentrate the incoming water, while the shape of another coast may produce a much smaller change in level.
Local weather adds variation. Strong winds can push water toward or away from shore, and changes in air pressure can affect sea level. For that reason, a predicted astronomical tide and the water level someone actually sees may differ. Tide tables for a particular coast account for its usual tidal patterns, but they cannot make weather disappear.
The useful mental model is therefore a combination of causes. The Moon supplies the strongest tidal influence, the Sun changes its strength through the lunar month, and Earth’s rotation carries places through the changing pattern. Ocean basins and coastlines determine how that pattern appears at any particular shore.
