The tide, as Geoffrey Chaucer observed, waits for no man. Even out at sea, when all seems calm on the surface, the tide is there, constantly changing, flooding in and then ebbing away. Remorseless and powerful, its effect is most dramatic close to the shore. For sailors, knowing the state of the tide can make the difference between passing harmlessly over an obstruction, or being wrecked on it. Between reaching the sanctuary of a port in a storm, or spending hours waiting for high water in the offing. But what causes tides, and why are their effects so varied?
If our planet was alone in space it would have no tides. We only have them thanks to our powerful neighbours, the sun and the moon. Although the moon is much smaller, it exerts a force two and half times as great as the sun because it is so much closer to earth. Its gravitational field draws the surface of the sea towards it as a bulge of water on the side of the earth facing it, and a corresponding one on the far side of the earth. These are the high tides. On the sides of the earth between these bulges, the sea level is correspondingly less, producing low water. As the earth spins beneath the sea, these bulges and troughs move across the world, producing a continuous rise and fall of sea level.
High tide and low water occur roughly 6 hours and 20 minutes apart, which is why high tide in a given location advances from day to day. The size of a tide also varies over the lunar cycle, thanks to the effect of the sun’s gravity. When both sun and moon are aligned, the largest spring tides occur, always just after each full and new moon. When they are in opposition, the smallest tide, called a “neap” happens. Between the two extremes, the tide grows and shrinks, day by day.
Tides are also affected by land masses that block the water bulge of a rising tide as it travels across the globe. Seas that are enclosed, such as the Caspian Sea, have almost no detectable tide. Even substantial bodies of water that open into the ocean, like the Baltic, Mediterranean and even the Caribbean have tides measured in inches that most visitors to their coasts will barely notice.
As well as blocking tides altogether, land can also channel its flow, which can produce some strange effects, depending on the shape of the coastline. The port of Southampton in the UK, for example, has always been a prime location for ocean-going shipping, thanks to it having twice the normal number of high tides. This is because the rising sea has to pass around the large mass of the Isle of Wight, taking longer to arrive via the Eastern Solent than from the west. The Bay of Fundy in Nova Scotia is a huge enclosed inlet into which over a billion tons of Atlantic seawater flows in and out each tide. As a result, it has the highest tidal range in the world, with up to 50 feet difference in sea level between high and low water. What was seabed becomes dry and what were islands become columns of rock, before the Atlantic returns to reclaim its own.
The constriction of land can produce powerful tidal races that ships can struggle to sail against. The Bristol Channel in the UK, which has the world’s second highest tidal range, is shaped like an enormous funnel between Wales and the northern coast of Devon and Somerset. It narrows from almost 40 miles wide at its Atlantic end to the width of the River Severn. Here the tide can flow at over 8 knots, dangerously fast for many boats. The inrush of water is so powerful that a spring tide will produce the Severn Bore, a wave large enough to be surfed, that travels up the river for over twenty miles, deep into the heart of the English countryside.
In the age of sail, the gentler tidal race in the English Channel was used by shipping for their benefit. London, the busiest port in the world until well into the 20th century, lies beyond the Channel’s eastern end. The prevailing wind, however, is a westerly - dead foul for ships trying to head out into the Atlantic. Sea captains developed a method of sailing that made use of the tide to help them. They would anchor when the eastward-flowing flood tide was running, and then beat into the westerly wind when the ebbing tide began to run in their favour, taking their ships with it. Through a series of tidal cycles, they would claw their way westward. This was called “Tiding Over,” a phrase that entered the English language to mean borrowing money for a short period until an expected change in circumstances.
Tides can be useful or dangerous to mariners in almost equal measure, but some biologists claim that they may have played a wider role in human history. Since life began in the sea and later moved to the land, they argue, then the strip of ground produced by the tides that transitions between the two, will have played a vital part in this process. It was here, they say, that life was able to move from out of the sea to inhabit the land. If correct, then we all have a great deal to thank the tide for.