Tides

Tides are the regular rise and fall of sea level caused primarily by the gravitational forces exerted by the Moon and the Sun on the Earth's oceans, together with the rotation of the Earth. The Moon's gravity pulls on the near side of the Earth more strongly than on the far side, producing a bulge in the ocean toward the Moon and a corresponding bulge on the opposite side. As the planet rotates, different areas pass through these bulges, creating rising and falling water levels that occur about every 12 hours on most mid-latitude coastlines. The Sun also exerts a gravitational force, though much weaker than the Moon's; when the Sun, Moon, and Earth align during full or new Moon phases, their tides combine to produce higher high tides and lower low tides, known as spring tides. Conversely, when the Sun and Moon are at right angles relative to Earth, they partially cancel each other, producing lower high tides and higher low tides, known as neap tides.

Tiding patterns vary by coast: semidiurnal tides have two roughly equal high tides each day, diurnal tides have one high and one low, and mixed tides show two highs with unequal elevations. Amplitude depends on latitude, coast geometry, seafloor depth, resonance in ocean basins, and meteorological factors such as winds and atmospheric pressure. Tidal ranges are described as microtidal, mesotidal, or macrotidal, with Bay of Fundy among the highest in the world during spring tides.

Tides influence navigation, coastal erosion, and ecology; many coastal habitats such as estuaries and mudflats rely on tidal inundation. Tidal power aims to generate electricity from predictable tidal cycles using barrages, lagoons, or in-stream turbines.