What are tides?

Tides are the short term periodic rise and fall of the world’s oceans. They are due to the combined effects of gravitational attraction and the revolution of the Earth-moon system about its common centre of mass.

At this point (which lies within the solid Earth) the gravitational attraction between Earth and moon exactly balances the forces required to maintain the moon’s orbit. Elsewhere the two forces are not in balance and give rise to the so-called tide generating force. This explains why there are two tides per day at most places around the globe – which we call semi-diurnal tides. Some parts of the world’s oceans (e.g., Vietnam) do not respond strongly to these forces and have only one tide per day (a diurnal regime).

The gravitational pull of the Sun acts in a similar way but – because it is so much further away – the solar tide generating force is only 0.46 that of the moon. Every fortnight when the moon is full or new the solar and lunar tides combine give higher than usual tidal ranges. These are called spring tides. At the moon’s quarter phases the lunar and solar forces cancel out to some extent giving smaller tides called neap tides.

The tide at any place can be represented as the sum of a large number of constituents, each of which is associated with a distinct physical (usually astronomical) cause. As long as the periodicity of the constituent is known, its amplitude (size) and timing with respect to other constituents can be found by a mathematical process called harmonic analysis. The first practical method was put forward by Sir William Thomson (Lord Kelvin) in 1867. Once the amplitudes and phases of all constituents have been calculated, then a tide table can be produced. Modern computers carry out tidal prediction in a fraction of a second and typically construct the tide from 120 constituents.

The following topics relate to the tidal and sea level research carried out at the National Oceanography Centre in Liverpool.

Tides: questions and answers
Sea level: questions and answers

Answers to a selection of questions asked by the public.

The Doodson-Légé Tide Predicting Machine

Tidal predictions were first carried out in 1924 at Bidston Observatory (University of Liverpool Tidal Institute), initially by hand and then by early tide predicting machines – Read more →

Tide clocks and watches – lunitidal interval

Tide clocks and watches are practical ways of indicating the state of the tide, in addition to consulting local tide tables – Read more →

Tidal river bores

In most tidal rivers the change from ebb to flood is gradual: the ebb current downstream slows, there is a period of slack water, then slowly the flood tide starts flowing upstream. In a few rivers, the behaviour is remarkably different and the onset of the flood tide is marked by a distinct, sometimes very vigorous wave – a bore – Read more →

The River Dee

The River Dee, between Wirral and North Wales, is unusual in that comparatively little water occupies so large a basin. A recent theory is that the estuary was not formed by water, but by ice being pushed southwards by the pressure of an icecap over the Irish Sea – Read more →

The River Mersey

The River Mersey begins at the confluence of the River Tame and River Goyt, and flows 70 miles to the Irish Sea; its estuary has the second highest tidal range in the UK – Read more →

Permanent Service for Mean Sea Level FAQs (PSMSL website)

Find out about 'mean sea level', 'altitude above sea level', global sea level rise, and how measurements by GPS (Global Positioning System) and tide gauges help us to understand sea level change – Read more →