An independent tide reference
The sea rises and falls twice a day on most coasts, and the pattern is not weather but astronomy. This handbook explains the forces behind the tides, the monthly rhythm of springs and neaps, and the layout of the printed tables that turn that astronomy into times and heights.
The tides are the slow rise and fall of sea level driven mainly by the Moon's gravity and, to a lesser degree, the Sun's.
Gravity from the Moon pulls on the whole Earth, but not evenly. The ocean on the near side is pulled harder than the solid planet, and the planet is pulled harder than the ocean on the far side. The difference between these pulls — not the pull itself — is the tide-raising force.
Because this force depends on the difference in gravity across the Earth's diameter, it falls off with the cube of distance. That is why the Moon, though far less massive than the Sun, raises a tide roughly 2.2 times stronger: it is vastly closer.
The result is an idealised ocean with two bulges, one facing the Moon and one facing away, while the solid Earth rotates beneath them. Real coastlines, basin depths, and friction turn this simple picture into the complicated local tides that tables record.
Most coasts pass through both tidal bulges each day, so the water rises twice and falls twice in a semidiurnal rhythm.
As the Earth spins once relative to the Moon — a lunar day of about 24 hours 50 minutes — a given shoreline passes through the near-side bulge, a low-water belt, the far-side bulge, and a second low-water belt. High waters therefore arrive about 12 hours 25 minutes apart.
Not every coast follows this pattern. When the Moon stands far north or south of the equator, the two bulges sit at different latitudes, and some places receive one high and one low per day (a diurnal tide) or two clearly unequal pairs (a mixed tide).
The shape of the basin decides which regime prevails. The Atlantic is dominated by semidiurnal tides, much of the Pacific by mixed tides, and parts of the Gulf of Mexico by diurnal tides.
Twice a month the Sun and Moon pull together and the range grows; twice a month they pull at right angles and it shrinks.
Near new moon and full moon, the Sun, Earth, and Moon line up. The solar tide stacks on top of the lunar tide, producing higher high waters and lower low waters — the spring tides, named from the idea of the water springing up, not from the season.
Near the first and last quarter, the solar and lunar bulges sit at right angles. The Sun's high water partly fills the Moon's low-water belt, and the range shrinks to its minimum — the neap tides.
The full cycle from spring to spring takes about 14.8 days, half a lunar month. The largest springs often lag the exact new or full moon by a day or two, a delay known as the age of the tide.
These dates mark the main steps from explaining the tides to predicting them by machine.
Each figure sketches one idea from the text; none is drawn to scale.
The list below points to the kinds of public sources — oceanographic textbooks and hydrographic datum standards — against which the terminology used here can be checked.