Tides are the periodic rise and fall of sea level caused mainly by the gravitational pull of the Moon and, to a lesser extent, the Sun on the Earth's oceans. Isaac Newton's equilibrium theory explains the basic mechanism: the Moon's gravity creates a bulge of water on the near side of the Earth and a second bulge on the far side, so most coasts see two high tides and two low tides roughly every 24 hours 50 minutes (the lunar day) (NOAA; Pugh & Woodworth, 2014).

When the Sun and Moon align (new or full moon), their effects add up and tidal range is largest—these are spring tides. When they are at right angles (first or last quarter), the range is smallest—neap tides. Predictions use harmonic analysis: dozens of tidal constituents (e.g. M2, S2, K1) with known periods are combined to forecast water levels. Storm surge and wind can raise or lower the actual level compared with the astronomical prediction.

Tide tables are produced by national hydrographic services (e.g. Rijkswaterstaat, NOAA) using long-term gauge data and harmonic constants. Levels are given relative to a datum such as NAP (Netherlands) or MSL (mean sea level). The data on this page comes from the source you have configured and is updated periodically.

Did you know? The Moon's gravitational pull on the oceans is about 2.2 times stronger than the Sun's, yet the Sun still drives the spring–neap cycle. The world's largest tidal ranges occur in the Bay of Fundy (Canada), where the range can exceed 16 metres.