It is helpful, but not essential, to understand the reason why the tides vary in height and time on different days. Twice daily, the tide comes in, and each day it will be approximately 50 minutes later than the previous day. Astronomically, this is because as the Earth moves around on its axis every 24 hours, the Moon during the day and night moves through one-thirtieth of its orbit around the Earth. This means that it will be approximately one-thirtieth in a different position 24 hours later.
This, of course, influences the tides because of the importance of its gravitational pull on the world's oceans and seas. The water pull is best visualised as a horizontal movement, not as a vertical up and down movement as viewed from the shore. There is a considerable variation in the height of the tide from week to week.
Because of its nearness to the Earth, the gravitational pull of the Moon is the stronger influence than that of the Sun. As the Moon orbits around the Earth it exerts its gravitational pull as illustrated (diagram to follow):
(1) Highest Spring Tides - Moon and Sun in conjunction, with combined gravitational pull producing the highest tides. High tides on the opposite side are due to the centrifugal force of the Earth's rotation.
The highest tides in England occur about two days after the New Moon.
(2) Neap Tides - Moon's orbit moves towards 90° to that of the Sun, when they are at quadrature, and their relative pulls, about 7 to 3 in favour of the Moon, result in Neap tides, with smaller variations.
(3) Spring Tides - Moon and Sun in opposition, with combined gravitational pull.
(4) Neap Tides - Moon's orbit moves toward the quadrature.
The highest spring tides of the year occur after the equinoxes in March and September. However, the weather may not be so good, and rain and gales are frequent. The Tide Tables cannot be relied on for precise highs and lows of the tides. A storm surge out in the Atlantic can put at a metre on a high tide in exceptional circumstances, and the wind can also prevent the tide from receding as much as it should.
It is interesting to note that high spring tides occur at approximately the same time of the day every year in each location on the coast. This means that high springs occur at dawn and dusk in around Mumbles and Swansea.
The configuration of the shore has a large bearing on tide heights and range, tending to increase in bays and funnel-shape inlets like the Bristol Channel and St. Malo, near the Channel Islands, in France.
UK Hydrographic Office Tide Predictions
National Ocean Service
Tides (UK) (a few locations only) INSTANT ACCESS
Tide Predictions: Neptune (fee required to unlock the download)
Tide Prediction Program*** WXTide 32
Equinox Times & Dates (Eric W. Weisstein's Astronomical pages)