tide

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. The Moon's gravity pulls on the near side of the Earth more strongly than on the far side, creating a bulge. The Sun also exerts a gravitational pull, and although it is much farther away, its huge mass makes a significant contribution. As the planet rotates, different coastal areas move into and out of these bulges, producing the familiar cycle of high and low tides.

On most coastlines there are two high tides and two low tides each tidal day, roughly every 24 hours and 50 minutes. The exact pattern depends on geography, giving semi-diurnal, diurnal, or mixed tides. The difference between high and low tide is called the tidal range. When the Sun and Moon align (full or new Moon), their gravities reinforce each other to produce higher highs and lower lows, known as spring tides. When they are at right angles, their effects partially cancel, producing neap tides with smaller ranges.

Tides are influenced by coastline shape, ocean depth, seafloor topography, and local bathymetry. Narrow bays and estuaries can amplify tides and cause tidal bores in some rivers. Tidal information is published in tide tables and measured by tide gauges. Tides are important for navigation, coastal engineering, and flood risk assessment. They also drive tidal energy projects and influence coastal ecosystems through mixing and nutrient transport.