# How do ebb and flow arise?

At low tide you can walk from the German mainland to some islands such as the East Frisian island of Baltrum. Where else ships sail and the thundering sea foams, you can see groups of hikers walking through the mudflats (this is how the coastal areas that are dry at low tide are called). But where did the water flow to at low tide and why are there ebb and flow? Can you only watch the tides on the North Sea.

Anyone who has ever vacationed on the North Sea knows the phenomenon: You want to go swimming on the beach, but where the sea was just a few hours ago, you can only see land for miles. The sea has withdrawn – from the coast to the high seas. It comes and goes in the same sequence of times. Together, the ebb and flow of the tide last about twelve hours and 25 minutes. High tide is the period in which the water rises and ebb is the period in which the water level falls.

But why are there ebb and flow and who or what is responsible for it? If you want to understand the tides or tides, as the interplay between ebb and flow, 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 here: the force of attraction and the centrifugal force. Anyone who knows these forces will also better understand why there are ebb and flow. But let’s start with the influence of the moon and its gravitational pull on the water of the earth.

The moon: lord of the tides

The moon has a decisive influence on the seas of our planet.

Wikipedia

Have you ever asked yourself why the earth attracts us and holds us firmly on the ground? It’s 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 be flying around freely in the area. But the earth not only attracts us and keeps us on the ground. We too attract the earth – or each other. We do not notice this, however, because we have much less mass and thus weight than the earth and our gravitational pull is therefore much less. Two laws of nature are important to us in this context: The more mass a body has, the higher its attraction. In general, every body attracts every other – with different forces.

Through gravity, the earth attracts the moon and the moon in turn attracts the earth. Since water is liquid, i.e. mobile, the moon can “pull” the water on earth a little towards itself with its force of attraction. On the side of the earth on which the moon is currently standing, the water level of the seas rises. A so-called “flood mountain” is created on the side of the earth facing the moon. Since the earth rotates on itself, it wanders through this flood mountain. That is why ebb and flow alternate. The water does not simply “move” with the moon, but the earth also rotates under the flood mountain. But why does it take about twelve hours until the next flood? The earth rotates within 24 hours, i.e. in one day, once around itself and thus once under the ”

A “heavenly carousel”

The red cross marks the barycenter, ie the common pivot point in the “Earth-Moon” system. It takes the moon about 27 days to orbit this point.

Anyone who owns a globe knows that it can be rotated around its axis – axis means “pivot point” in Old High German. The axis is the area around which one or more rotating bodies move. The rotation is also called rotation. This creates forces that “fling” everything around the axis of rotation outwards. At the same time, a force acts towards the center of the circular movement, i.e. towards the axis of rotation, and keeps the rotary movement stable. The inwardly directed force is called “central force” or “centripetal force” (Latin: petere means “strive for”). The force acting outwards is called “centrifugal force” or “centrifugal force” (Latin: fugere means “to flee”). An example of centrifugal force is the chain carousel.

We have now learned two things that will help us: namely that the earth and moon rotate around a common center of gravity and that centrifugal forces arise with every rotation. Another example will illustrate how the centrifugal force affects the water on earth: Let us imagine the two celestial bodies as two people with long hair who stand opposite each other and hold hands tightly. Now they start to turn around each other – so fast that the hair starts to “float” in the air. Just as the centrifugal force pushes the hair away from the common axis of rotation, the water on the earth is also thrown outwards. The centrifugal force that arises when the earth and moon rotates is responsible for the second, opposite flood mountain.

One flood mountain is on the side facing the
moon , the other on the side facing away from the earth. This moves around the common center of gravity of the earth and moon. As a result, on the side facing away from the moon, the centrifugal force is stronger than the attraction of the moon there.

Wikipedia

The axis of rotation of the 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 something like “center of gravity”. Therefore it is not entirely correct to say that the moon just rotates around the earth, in fact both rotate around their common center of mass (see animation above). The moon moves in a large circle (radius about 380,000 kilometers) around the earth, the earth 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 flood to the next – and not just twelve hours. This is because the moon continues to move around the earth within a day. If you divide the globe into 360 degrees, the moon moves around the earth by about 13 degrees every day. It becomes clear if you note the time at which the moon rises. Let’s say it opened at exactly twelve o’clock. If we were to measure again a day later, it would open about 51 minutes later, at 12:51 p.m. So let’s add these 51 minutes to the 24 hours it takes for the earth to turn around itself. Let us now divide the sum by two, because during this time the earth rotates exactly once under the two flood mountains. We then come to twelve hours and 25.5 minutes.

The tide is higher at the new and full moon

DISPLAY

If the sun, earth and moon are on one line, what is known as a spring tide occurs.

Wikipedia

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 from the earth. Because the greater the distance between two bodies, the smaller their mutual attraction – the attraction of the sun to the earth is therefore about three times as small.

Depending on the position of the moon in relation to the sun and earth, the sun has an intensifying or weakening effect on the tides. If the sun, earth and moon are on one line (see picture on the right), the “tidal forces” of sun and moon add up. The “tidal range” is then particularly large. The tidal range is the difference in height between the water level at ebb and flow. Since a lot of water flows to the flood mountains, the tide is higher than usual. This special form of tide is called spring tide. The ebb tide is also more pronounced than usual during the spring tide.

If the moon is at an angle of 90 degrees to the sun, the ebb and flow weaken.

Wikipedia

In some bays there is sometimes even a visible tidal wave during a spring tide. A spring tide can be dangerous if a storm forms and the water level increases. This happened, for example, in Hamburg in 1962. At that time, the dikes were unable to cope with the water masses and finally collapsed. Several parts of the city were under water and 315 people were killed.

Fortunately, such extreme floods rarely occur. There is a normal spring tide every 14 days, namely with new and full moon. Then all three heavenly bodies are always on one line. With the increasing and decreasing crescent moon, the earth, moon and sun are roughly at right angles to each other (see picture on the left). The tidal forces of the moon and sun then act against each other and the tide mountains are smaller than with any other high tide. This particular shape of the tide is called the nipp tide.

Dangerous flood

The tide is still out. But time and again people are in danger because they are surprised by the rapidly rising tide.

Time and again people drown because they were surprised by the rapidly rising tide. Most of the time it is tourists who ignorantly expose themselves to the dangers of the flood. At low tide they go unsuspecting to collect mussels or go for a walk through the mudflats. Often they walk long stretches towards the water to bathe and don’t know much about ebb and flow. But suddenly the tide rises so quickly that the bathers cannot come ashore quickly enough.

In the damp, muddy mudflats, you don’t get on too quickly. The extreme water currents often make it impossible to steer against them on your own. Another problem is a sudden appearance of sea fog. Imagine you are on a mudflat hike – and within a few minutes an increasingly thick fog comes in. You will then no longer see where the mainland is and where the open sea is. People in such a situation usually lose their bearings and wander around until the tide comes – a life-threatening situation.

As recently as 2004, 18 Chinese tourists drowned on the English coast. They had run across the mudflats to collect mussels and had been surprised by the tide. Despite the efforts of the army and police, not even half of the group could be saved. This illustrates how extremely dangerous ignorance of the tides can be. Therefore, there are a few rules to follow when hiking through the mudflats.

Important rules in the mudflats:

You can quickly lose your bearings in the mudflats. The approaching tide is often underestimated, because in the tidal flats you cannot make fast progress and the current is often strong.