# Water in motion

The fact that the tides are influenced by the moon is almost generally known. But, apart from the gravitational forces between moon and earth, various other factors determine the movement of water. So height and duration of tides can only be understood if we take into account the rotation of the moon, the turning of the earth and the gravitational forces between earth and moon, in addition to geography and the weather.

## Forces at work

Earth and the moon are gravitating towards each other. However, they do not plunge into each other, because they revolve around a common centre of gravity. We perceive this movement as a rotation of the moon around the Earth, since the common centre of gravity is situated within the mantle of the Earth. On the side of the earth inclined towards the moon, gravitational power is slightly greater than needed for circular movement around the centre. This has an impact on ocean water. It moves in the direction of the moon and forms a tidal bulge. On the opposite side of the earth, however, things occur in exactly the opposite way. There, the gravitational forces are slightly smaller. Thus, the waters cannot be contained on the circular path and form a second tidal bulge. In the sections between the bulges, ebb tide will be forming.

## The influence of the earth’s rotation

Without the rotation of the Earth around itself, ebb and flow would be locally restricted to one place. Since the Earth rotates around its own axis every 24 hours, every location on Earth will move into a tidal bulge twice a day. Therefore, ebb and flow will be occurng, two times every day.

## Spring and neap tides

To understand the amplitude of the tides, influence of the sun has to be taken into account. Depending on the position of Sun, Moon and Earth, the sun‘s influence weakens or strengthens tidal movement. This is especially the case at new or full moon, when all three planets are aligned. So-called spring tides with especially strong ebb and flow will occur. At the so-called neap tides, ebb and flow are less distinctive because at half moon, the relative impact of sun and moon will partly cancel out each other.

## The influence of coastal regions and the weather

The amplitude of ebb and flow also depends on the formation of the coast and position of the continents. The weather also has an impact on sea currents and can change water levels together with the tides. Especially in low-water areas like the German North Sea coast, winds play a decisive role: Onshore winds will lower the water level, while offshore winds cause an íncrease. Mainly at the onset of rivers like the Weser, water levels are also impacted by surface waters. There is, for instance, often flooding in spring, when strong rainfall is topped up by melt water.