What Causes Tides?

The gravitational effects of the Moon and Sun, together with the motion of the Earth-Moon system, create the main astronomical tide-producing forces. A simple equilibrium-tide diagram with two bulges is useful for introducing the cycle, but it is not a timetable for a real harbour.

Tides travel through ocean basins as long waves. Basin shape, coastal geometry, depth, friction and resonance change their timing and size. That is why nearby ports can have different high-water times and why some places have double high waters, double low waters or prolonged stands.

Many UK ports are mainly semi-diurnal, with two high waters and two low waters in a lunar day of about 24 hours 50 minutes. A comparable phase is therefore often about 50 minutes later on average, but the interval varies. For passage planning, use the published prediction rather than advancing yesterday's time by a rule of thumb.