What leads to a flood zone designation

The forces of the tides

When the water slowly recedes on the beach, the moon has its invisible hand in play - every child knows that. But how the height and duration of the tides come about in detail can only be explained by the interaction of the forces of attraction between earth and moon and between earth and sun. The inclination of the earth's axis, the inclination of the earth's orbit to the orbit of the moon and the geography also influence the tides.

The basic phenomenon, however, takes place between the earth and the moon. The gravitational force of the moon pulls the matter of the planet. Because the strength of this force diminishes with distance, the tides arise.

On the side of the earth that faces the moon, the gravitational force it exerts on the earth is slightly greater than in the center of the earth. The earth's crust hardly gives in to this difference in forces, but the water of the oceans follows the dragging of the lunar gravity - the water moves towards the moon and forms a flood mountain.

On the opposite side of the earth, however, it is exactly the opposite. There the attraction of the moon is slightly less than in the center of the earth. That is why the water there loses the ground, so to speak, and the sea level rises - the water moves away from the moon and forms a second flood mountain.

The two flood mountains are not exactly the same size, however. Because the force that causes the piling up of a flood mountain is seven percent stronger on the side facing the moon than on the back. This difference is due to the fact that the gradient of the gravitational force - the degree of its spatial change - depends nonlinearly on the distance: the strength of the gradient is inversely proportional to the cube of the distance between two masses (while the force itself is inversely proportional to the square).

In the areas between the two flood mountains there is ebb, i.e. low water, because from there the water is "pulled away" into the flood areas.

Doesn't the centrifugal force of the earth also play a role with regard to the earth-moon system for ebb and flow? It is still often used to explain the second flood mountain on the side facing away from the moon. However, taking the centrifugal force into account only makes the explanation more cumbersome, without the centrifugal force being important for the creation of the tides (see info box).

Influence of the rotation of the earth

For this, the earth's own motion, i.e. the earth's rotation around itself, is extremely important for the tides as we perceive them - without the earth's rotation, the tide and ebb would not move across the earth's surface, but would be fixed in one place. But because the moon rotates around the earth in 27.3 days, i.e. 27.3 times slower than the earth around itself, the earth's surface moves away constantly in an easterly direction under the flood mountains and ebb valleys. The flood mountain is thus moving in a westerly direction around the earth.

The combination of the self-rotation with the combined rotation of the earth and moon means that it does not take exactly 24 hours until the same point on the globe faces the moon again and thus has a flood mountain, but rather a little longer: 24 hours and 50 minutes. A tide - that is the time interval between high tide and high tide or between ebb and ebb - is half as long: it lasts 12 hours and 25 minutes each.

No spring tide without the sun

Nipptide and Springtide

In order to understand the phenomenon of ebb and flow, in addition to the moon, the influence of our other close cosmic companion must be considered. Because the central star influences the tides through its gravitational force in a similar way to the moon - albeit less strongly. The gravitational pull that the sun exerts on the earth is about 200 times that of the moon due to the large mass of the sun, but because the sun is about 400 times as far from the earth as the moon, the tidal force is the sun (the gradient of their gravitational force) smaller than that of the moon. Because the gradient of gravity falls faster with the distance than the force. That is why the tidal force of the sun is only 46 percent compared to our satellite.

Depending on how the sun, moon and earth are positioned relative to one another, the sun either strengthens or weakens the tides. The two constellations in which all three celestial bodies are on the same line, i.e. the new moon and full moon, are particularly important. Then the tidal effects of the sun and moon intensify, and there are spring tides with a particularly high tide mountain and a particularly low ebb (see the article in "Physics behind things" on spring tide). In the case of a half moon, on the other hand, both influences partially offset each other. There are nippptides with only weakly pronounced high and low tides.

The tides and the coast

Anyone who has already traveled a few coasts of the world knows of course that the tides are not equally strong everywhere. In the Mediterranean, the tidal range is barely noticeable in some places, while the difference between high and low tide in the North Sea is several meters. The cause of these differences can be found in geography.

The distribution of the continents alone creates obstacles and thus affects the movements of the water. The depth and shape of the oceans also affect the tides. From the point of view of physics, one has to do with a system of forced oscillations in interconnected water basins around the globe. Depending on the geographical conditions, this can even lead to resonance phenomena, which rock the tidal range. Small marginal seas on the continental shelf (continental shelf) such as the North Sea are coupled to these vibrations. As a result, the strength of the tides in such areas of the earth is often largely generated by these vibrations.

How the position of the moon affects the tides

In the midst of large oceans like the Pacific, the tidal range does not even reach a meter. Strong tides are only observed near mainland coasts. As the water depth decreases, the speed of the tidal movement decreases - but at the same time the tidal range increases. The shape of the coast also has a strong influence on the local formation of the tides. This can lead to fluctuations in the tide differences from a few centimeters to well over ten meters. The Bay of Fundy in eastern Canada holds the record, where the difference between high and low tide is up to fourteen meters.

How the earth's tilt distorts ebb and flow

The explanation of the tides gets a bit trickier when you consider that the earth's axis is inclined and the lunar orbit is tilted relative to the earth's orbit. This causes asymmetries at ebb and flow. Because the flood mountains are always on the connecting line earth-moon, but this line does not necessarily lead through the equator. The moon can move up to 28.7 degrees from the celestial equator on its trajectory across the sky. That is why the flood mountains periodically change their position relative to the equator.