Ebb and flow of a force of nature
The perpetual rise and fall of the tide is a fascinating and awe-inspiring phenomenon governed by the forces of nature, and the moon and Sun
IT IS A clear, fresh day, and the beach lies under a huge sky that seems to stretch endlessly to the sea. Glistening pebbles of purple and grey lie scattered among the driftwood and strings of kelp piled on the wet shore, trailing into the dips and ridges sculpted by the water. There are whorls of sand left by the lugworms and mermaid’s purses and razor shells long since abandoned by their occupants.
It is low tide here at St Annes in Lancashire, and the sea is only a thin string on the horizon; far past the ruin of the old jetty, which emerges out of the sand. But in a short while, the restless water will return; rising and falling on the sands in nature’s endless rhythm.
Infinite attraction
Swelling and shifting tides are the constant pulse of the coast; a perpetual beat accompanying the life of the sea and the people on its shores. High and low water dictate when ships sail, when samphire and shrimp can safely be gathered, and the best places for fishermen to trawl for their catch. A true sailor’s heart was said to beat faster on a flood tide, and it was firmly held that the dying took their final breaths with the ebbing waters. “He’s a going out with the tide”, says Peggotty as Barkis nears death in Dickens’ David Copperfield.
But what causes the tides? Once believed to be the result of the Earth’s respiration or water flowing through subterranean tunnels, it was not until the 17th century, and the work of the French mathematician and scientist, Descartes, and especially Isaac Newton, that the answer was revealed to lie not on Earth, but in the heavens.
“Everything on Earth has a gravitational attraction to both the moon and Sun, and it is this that creates the tides,” explains Professor Kevin Horsburgh, of the National Oceanography Centre. “The moon is the more important of the two, as it is much closer to us. But the effects of gravity
“One day I wrote her name upon the strand, But came the waves and washed it away: Again I wrote it with a second hand, But came the tide, and made my pains his prey”
Edmund Spenser, ‘Amoretti LXXV’
are complicated by the fact that the Earth and moon are revolving around each other. This motion produces centrifugal forces, which push things outwards from the centre point, like being on a fairground ride or driving around a sharp corner.”
As the moon moves around the Earth, the oceans are pulled by gravity towards it, with the bulging water creating a high tide on that part of the planet. “Here, gravity wins out over centrifugal forces, but on the opposite side of the Earth, the moon’s gravitational force is weaker, and the reverse is true,” says Kevin. On this side, centrifugal forces dominate, but a high tide is still created as they push the water outwards. Low tides occur in between these bulges of water.
The tides, therefore, follow a lunar day: the water rising and falling back approximately every six hours as the moon travels around the Earth. Most places in the UK have two high and two low tides a day, occurring approximately 45 minutes later on successive days as that specific point on the spinning Earth catches up with the orbiting moon.
Effect of the Sun
If the moon was the only gravitational pull on the seas, the tides would be almost uniform. But the Sun also has an effect, boosting the gravitational pulling power of the moon when the two come into alignment.
“Every fortnight, when the moon is full or new, the Sun and moon are in a straight line, and their gravitational strength is combined,” explains Kevin. “Then you get a bigger tide, known as a spring tide. It’s nothing to do with the season: the name comes from the Old English for ‘leaping forward’. One set of spring tides can be bigger than another because the moon reaches ‘perigee’; its very closest position to Earth, when it’s popularly called a ‘supermoon’.”
On the alternate two weeks, between spring tides, the Sun and moon are pulling on different sides of the Earth, and their combined force is diminished. This gives rise to smaller, neap tides; the name possibly derived from the Old English ‘nēp’, meaning ‘to become lower’.
“Around the spring and autumn equinoxes on 21 March and 21 September, the Sun and moon can be aligned, and the Sun is overhead at the Equator,” says Kevin. “This twice-yearly occurrence is when the oceans are most responsive to the tidal forces, and you can get an exceptionally big global tide.”
The difference between high and low water is known as the tidal range. Britain has some of the highest tidal ranges in the world; the Bristol Channel having a mean spring tidal range of 40ft (12.3m) and Liverpool Bay 27ft (8.3m). At
Lowestoft, on the eastern coast, on the other hand, the range is only 6ft (1.9m). In areas with shallow, flat beaches, such as St Annes, the sea may recede a long way out, whereas waves may continually crash against cliffs towering from deep water, and the tidal range only becomes noticeable over a period of several hours.
Taking shape
Although the gravitational pull of the moon causes all water molecules to oscillate, even those in a cup of water or a garden pond, the combined effect of this movement is so slight as to be negligible. Only in large bodies of water, such as the oceans, does the energy of the moving molecules build into a wave that travels across the water as a tide: not to be confused with surface waves created by the wind. Even
“The winds and the waves are always on the side of the ablest navigators”
Edward Gibbon