Sea level rises and falls in a predictable rhythm. Tide is the periodic rise and fall of sea level caused mainly by the gravitational pull of the moon and the sun on the Earth’s oceans.

Tidal range and timing vary by location. Tide tables and software predict high and low water and the height of tide at any time. Navigators use tidal height to calculate under-keel clearance and tidal stream to estimate set and drift. Some ports can only be entered at high water.

Spring tides produce larger ranges; neap tides produce smaller ones. Local effects of coastline and bathymetry can amplify or distort the astronomical tide. Accurate tidal information is essential for safe pilotage and berthing.

• Periodic rise and fall of sea level
• Driven mainly by lunar and solar gravity
• Predicted in tide tables for each location
• Critical for clearance, berthing, and tidal streams