At low tide, you can walk from the German mainland to the sea to some islands like the East Frisian island Baltrum run. Where ships usually sail and the raging sea foams, you can see groups of hikers walking through the mudflats (that’s what they call the coastal areas that are dry at low tide). But where does the water flow to at low tide and why are there low and high tides actually? Can one observe the tides only at the North Sea?
The vastness of the Wadden Sea: At low tide, the water retreats – leaving a wet landscape of mudflats that is the habitat of countless animals and microorganisms.
Anyone who has spent a vacation on the North Sea knows the phenomenon: You want to go swimming on the beach, but where there was sea just a few hours ago, there is now nothing but land for miles around. The sea has retreated – from the coast to the high sea. It comes and goes in a time sequence that is always the same. Together, high and low tide last about twelve hours and 25 minutes. High tide is the time when the water rises and low tide is the time when the water level falls.
But why are there high and low tides and who or what is responsible for it?? If you want to understand the tides, as the interplay between high and low tide is also called, you have to imagine our planet Earth in space. Because the tides are determined by the earth, moon and sun. Two physical forces play an important role in this process: the force of attraction and the centrifugal force. Who knows these forces, understands also better, why there is ebb and flood. But let’s start with the influence of the moon and its attraction on the water of the earth.
The moon: Lord of the tides
The moon has a decisive influence on the seas of our planet.
Have you ever wondered why the earth attracts us and keeps us firmly on the ground?? It is due to gravity – and this in turn is caused by the gravitational pull of the earth. This force is also called gravitational force. Without them we would float freely in the area. But the earth does not only attract us and keep us on the ground. We also attract the earth – or each other. However, we don’t notice this because we have much less mass and therefore weight than the earth and our gravitational pull is therefore much lower. Two laws of nature are important for us in this context: The more mass a body has, the higher its force of attraction. Generally every body attracts every other – just with different forces.
By gravitation the earth attracts the moon and the moon in turn attracts the earth. Since water is liquid, thus mobile, the moon with its attraction can "pull" the water on earth a piece to itself. At the earth side, at which the moon stands straight, the water level of the seas rises thus. A so-called "flood mountain" is formed on the side of the earth facing the moon. Since the earth turns around itself, it moves through this flood mountain. That’s why the tides alternate. The water "wanders thus not simply with the moon further, but the earth turns thereby also under the flood mountain through. But why does it take about twelve hours until the next high tide?? The earth rotates once around itself within 24 hours, i.e. in one day, and thus once under the "high tide" Throughout. But there is a second high tide – this one is directly opposite the first one on the side of the earth facing away from the moon. But how does this second tide rise? To this we must add the second physical force: the centrifugal force.
A "celestial merry-go-round
Anyone who owns a globe knows, that you can rotate it around its axis – axis means "pivot point" in Old High German. The axis is therefore the area, around which one or more rotating bodies move. The rotation is also called rotation. Forces are generated that "fling" everything around the axis of rotation outward. At the same time, a force acts toward the center of the circular motion, i.e., toward the axis of rotation, and keeps the rotational motion stable. The inward directed force is called "central force" or also "centripetal force (latin: petere means "to strive for"). The outward force is called "centrifugal force" or also "centrifugal force (Latin: fugere means "to flee"). An example for the centrifugal force is the chain merry-go-round. The faster it spins, the greater the centrifugal force, and the higher it "floats" also the drivers in the air.
We have now learned two things that help us: Namely, that the Earth and the Moon revolve around a common center of gravity, and that centrifugal forces are created during any rotational motion. Another example will illustrate how centrifugal forces affect the water on Earth: Let’s imagine the two celestial bodies like two people with long hair, facing each other and holding hands tightly. Now they start to rotate around each other – so fast that the hairs start to "float" in the air. Just as centrifugal force pushes the hair outward away from the common axis of rotation, the water on Earth is also thrown outward. The centrifugal force, which arises with the rotation of earth and moon, is thus responsible for the second, opposite flood mountain.
The one flood mountain lies on the side facing the moon, the other one on the side facing the moon
-averted side of the earth. This moves around the common center of gravity of earth and moon. Thus, on the side facing away from the moon, the centrifugal force is stronger than the gravitational force of the moon there.
The axis of rotation of earth and moon is still within the globe, but not in the center of the earth. This zone is also called the barycenter, which is derived from the Greek and means "center of gravity" is called. Therefore it is not quite correct to say that the moon simply revolves around the earth, in fact they both revolve around their common center of mass (see animation above). The moon moves in a big circle (radius about 380.000 kilometers) around the earth, the earth moves in a smaller circle (radius about 4.600 kilometers) around the moon orbiting it.
There is still one problem: It takes twelve hours and 25 minutes from one tide to the next – and not just twelve hours. This is due to the fact that the moon moves further around the earth within one day. If you divide the globe into 360 degrees, the moon moves around the earth by about 13 degrees every day. It becomes descriptive, if one notes the time, at which the moon rises. Let us assume that it rises exactly at twelve o’clock. If we would measure again one day later, it would rise about 51 minutes later, at 12:51 p.m. So let’s add these 51 minutes to the 24 hours the earth needs to rotate once around itself. Let us divide the sum now by two, because during this time the earth rotates exactly once under the two flood mountains. We come then to twelve hours and 25.5 minutes. That corresponds to the time, which lies between two floods.
At new and full moon the tide is higher
If sun, earth and moon stand on a line, it comes to the so-called spring tide.
The sun also has an influence on the ebb and flow of the tide. But although the sun has much more mass than the moon, its influence on the tides is not even half as great. This is due to the great distance of the sun to the earth. This is because the greater the distance between two bodies, the smaller their mutual gravitational pull – the gravitational pull of the sun on the earth is therefore about three times smaller.
Depending on the position of the moon in relation to the sun and the earth, the sun has a strengthening or weakening effect on the tides. If the sun, the earth and the moon are on the same line (see picture on the right), the tidal forces add up of sun and moon. The "tidal range is then particularly large. Tidal range is the difference in height between the water level at low and high tide. As a lot of water flows to the tide peaks, the tide is higher than usual. This special type of tide is called a spring tide. The ebb tide is also more pronounced than usual during spring tide.
If the moon is at an angle of 90 degrees to the sun, the ebb and flood tides weaken.
In some bays there is sometimes even a visible tidal wave during a spring tide. A spring tide can become dangerous when a storm forms and increases the water level even more. This happened, for example, in Hamburg in 1962. At that time, the dikes could not cope with the masses of water and finally collapsed. Several parts of the city were under water and 315 people lost their lives.
Fortunately, such extreme floods occur only rarely. A normal spring tide occurs every 14 days, namely at new moon and at full moon. Always then all three celestial bodies are on one line. When the crescent moon is waxing and waning, the earth, moon and sun are approximately at right angles to each other (see picture on the left). The tidal forces of the moon and the sun then work against each other and the tidal peaks are smaller than during a normal high tide. This special tidal form is called neap tide.
The tide is still low. But again and again people get into danger because they are surprised by the fast rising tide.
Again and again people drown, because they are surprised by the fast rising tide. Mostly it is tourists who unknowingly expose themselves to the dangers of the tide. At the same time, at low tide, they go unsuspectingly to collect shells or walk through the mudflats. Often they run long distances towards the water to swim and do not know much about low and high tide. But suddenly the tide rises so fast that bathers can’t get ashore fast enough.
In the wet and muddy tidal flats one does not move too fast. The extreme water currents often make it impossible to head against them under one’s own power. Another problem is a suddenly appearing sea fog. Imagine you’re on a mudflat hike – and within minutes, an increasingly dense fog rolls in. You can no longer see where the mainland is and where the open sea is. People in such a situation usually lose their orientation and wander around until the tide comes in – a life-threatening situation.
As recently as 2004, 18 Chinese tourists drowned on the English coast. They had walked through the mudflats to collect shells and were surprised by the tide. Despite the efforts of the army and police, not even half of the group could be rescued. This illustrates how extremely dangerous ignorance about the tides can be. Therefore, one should follow some rules when walking through the tidal flats.
Important rules in the tidal flats:
In the tidal flats you can quickly lose your orientation. The approaching tide is often underestimated, because in the mudflats one does not move fast and the current is often strong.
– One should only hike under local guidance.
– It is important to be aware of the times when the water level rises again.
– Since sea fog appears very suddenly, you should definitely hike with a compass to keep your bearings.
– Never walk through the mudflats during thunderstorms – the flat, damp landscape means the risk of a lightning strike is particularly high!
– Since the sun reflects very strongly on the wet ground, the risk of getting a severe sunburn is very high – therefore, it is better to rub sunscreen on before a mudflat hike.
Click on the "Next" arrow, to get to the second part of the article. In it, you’ll learn some interesting facts about the tides. Did you know, for example, that the tides are responsible for the days gradually becoming longer and longer or that the tidal forces also affect other celestial bodies??
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