The Moon's Gravitational Fingerprint
Twice each day, the ocean rises and falls in a rhythm dictated by the Moon. The mechanism is differential gravity: the Moon pulls harder on the near side of Earth than the far side, stretching the ocean into two bulges. As our planet rotates beneath these bulges, any coastal location experiences two high tides and two low tides roughly every 24 hours and 50 minutes — the lunar tidal day.
Spring Tides and Neap Tides
The Sun also generates tides, about 46% as strong as the Moon's. When Sun and Moon align during new and full moons, their forces add constructively, producing spring tides with the largest tidal range. During quarter moons, the forces act at right angles, partially canceling to create neap tides with the smallest range. This two-week spring-neap cycle dominates tidal patterns worldwide.
Perigee and Apogee
The Moon's orbit is elliptical, varying from 356,500 km at perigee to 406,700 km at apogee. Since tidal force varies as the inverse cube of distance, perigean tides are about 20% stronger than apogean tides. When perigee coincides with a spring tide (perigean spring tide or "king tide"), coastal flooding risk increases significantly — a growing concern as sea levels rise.
Tidal Resonance and Extremes
The Bay of Fundy in Canada experiences the world's largest tides — over 16 meters between high and low water. This extreme range occurs because the bay's natural resonant period nearly matches the 12.42-hour tidal forcing period, amplifying the tide like pushing a swing at just the right moment. Ocean basin geometry, not just celestial mechanics, determines local tidal behavior.