Tide mechanics Sun. Earth. Moon.
Two tidal bulges. One rotating Earth.
At coast A, each passage through a bulge brings high water in this idealised ocean.
Two high-water regions. One rotating coast.
The surface marker turns with Earth, passing through two high-water regions. The bulges follow the Moon and Sun, not the land. One lunar day takes about 24 hours 50 minutes.
This is an equilibrium picture, not a stationary shell of water. Real ocean water also moves; coastlines, depth and friction change its response. The marked coast is an idealised equatorial observer, not a Solent location.
Water height at Coast A
Relative height · not metresAn idealised equatorial observer and an equilibrium ocean. Orbital paths are circular approximations. The smooth water envelope is an exaggerated illustration of the combined tidal pattern, not the measured shape of an ocean. Sizes and distances are not to scale. Day and Month show a close view of the solar path; Year shows the complete orbit. The lunar orbit is shown in a Sun-relative frame, completing one cycle in about 29.53 days (about 27.32 days relative to the stars). Not a Portsmouth prediction.
Shallow water: why the Solent tide changes shape
Depth, friction and coastline geometry create higher harmonics. Adding M4 to M2 can flatten high water, split a peak and make rising and falling periods unequal.
One constituent changes the shape
Relative height · not metresM4 repeats every 6.21 hours; these amplitudes and phases are freely chosen, not fitted to Solent data.
01 / The force
Gravity varies across Earth.
The near side is pulled more strongly than Earth's centre; the far side less strongly. In Earth's falling frame, that difference stretches the ocean along the Earth-Moon line on both sides.
Tidal forcing ∝ mass / distance³
The Sun's mean tide-generating strength is about 46% of the Moon's. This is a forcing ratio, not the measured amplitude ratio at Portsmouth.
02 / The coast
Height is not current.
A tidal height curve describes the sea surface. Tidal streams respond to pressure gradients, friction and basin geometry. Slack water need not coincide with high or low water.
In the Solent, interacting harmonics help produce extended or double high water. The effect is not simply two tides travelling around the Isle of Wight.
03 / The limits
Real tides contain more.
Lunar declination creates daily inequality. Orbital eccentricity and the 18.6-year nodal cycle alter the forcing. Resonance, Earth's rotation and friction change the local response.
Wind and pressure add weather-driven residuals. None of these teaching curves replaces measured local harmonic constants or the selected-day correction dataset.