Tidal Phenomena

Tides — the periodic rise and fall of sea levels — are caused by the gravitational pull of the Moon and Sun on Earth's oceans. The Moon's gravity creates a tidal bulge on the side of Earth closest to it, and another on the opposite side (from inertia); as Earth rotates, most coastal locations experience two high tides and two low tides per day. The Sun contributes about 46% of the Moon's tidal force; when Sun and Moon align (new/full moon), 'spring tides' are highest; when perpendicular, 'neap tides' are smallest. The Bay of Fundy (Nova Scotia, Canada) has the world's highest tidal range — up to 17 meters between low and high tide, twice daily. This is due to resonance between the bay's natural oscillation period (12.4 hours) and the tidal period. Tidal power (harnessing the kinetic energy of tidal currents) is a proven renewable energy technology — the La Rance tidal barrage (France, 1966) has operated for 60 years. Tidal forces gradually slow Earth's rotation (days were 22 hours long 620 million years ago) and cause the Moon to recede 3.8 cm per year.