The Tide Table Handbook
The range of the tide swells and shrinks on a roughly two-week rhythm as the Sun and Moon take turns pulling together and pulling crosswise.
The solar tide is a smaller copy of the lunar tide. Where the two line up, their bulges coincide and reinforce each other; where they sit at right angles, the solar bulge partly cancels the lunar one. The changing geometry of the Earth–Moon–Sun system therefore modulates how high and how low the water goes.
Near new moon and full moon — the alignments called syzygy — Sun and Moon pull along the same line. High waters rise higher and low waters fall lower than average. These are spring tides; despite the name, they occur in every month of the year.
On narrow screens, swipe or scroll the plate sideways.
Near the first and last quarter — the quadratures — the solar tide is offset by 90 degrees from the lunar tide. The Sun's high water falls in the Moon's low-water belt, trimming the range to its minimum. These are the neap tides, from an old word for scant or lacking.
The interval from one spring tide to the next is about 14.8 days, half a synodic month. The very largest ranges usually arrive a day or two after the exact new or full moon, because the oceans need time to respond; this delay is the age of the tide.
Range also varies with distance. When perigee — the Moon's closest approach to Earth — coincides with a spring tide, the range can exceed the average spring by a noticeable margin, and such coincidences produce the largest tides of the year.
Further reading