What are tides?
Tides are recurring changes in sea level driven mainly by differential gravitational effects of the Moon and Sun.
Definition and processes
Core concept The Moon has the larger effect, with a contribution from the Sun. Earth’s rotation and ocean-basin shape determine local times and heights of high and low water. How it is studied Tide gauges record levels relative to a local datum. Two highs and two lows per day are common in some places, while others have different patterns.
Applications and caveats
Why it matters Tides matter for ports, coastal floods and ecosystems. Local predictions account for the basin; storm surge can add to normal tidal height. Common misunderstandings Tides do not cause every ocean wave. The Moon’s position alone cannot precisely predict the timing at every coast.