🌙 Earth Tide Clock

Two tidal bulges, not one. The Moon (and a weaker Sun) stretch Earth into a prolate spheroid; a port rotates through both bulges and sees two highs a day. Align Moon and Sun for spring tide; put them at right angles for neap. The gauge at the bottom is a real harbour clock.

Two highs per rotation — spring when the Sun lines up, neap at quadrature.
The physics

Equilibrium tide: a test mass feels the Moon's gravity more on the near side and less on the far side than Earth's centre does, so both faces bulge. Height at longitude λ is h = Aₘ cos 2(λ − λₘ) + Aₛ cos 2(λ − λₛ), with Aₛ ≈ 0.46 Aₘ. Spring tide at syzygy (new/full, angle 0°/180°); neap at quadrature (90°). A sidereal day plus the Moon's orbital motion makes the true interval ~12 h 25 min. Declination (port latitude) trims the two highs unequal — the diurnal inequality sailors know.