Field guide training: Astronomy – Part 4

Everyone who has visited the shores of the North Sea before knows about the following phenomenon: Two times a day the water is drawn back from the coast and returns after around six hours. “Tides”, “ebb and flow” or “falling and rising tide” are some names to call it. But how can this phenomenon be explained? Is it influenced by the moon and how come that the development of the tides differs from one place to the other?

If speaking from rising and falling tide most people quickly associate it with the moon: “Isn’t it related to the moon?”. In fact, the moon is the driving force of the tides, but not only the moon. Moon and earth are influenced by their gravitational interaction. This why the earth “pulls” the moon and the other way round. This effect can be observed by watching the biggest, movable mass on earth: The water of the oceans. The matrix of the moon and the earth is rigid, but water can react to the gravitational force of the moon. This is why a mountain of water is formed on the side of the earth which is facing the moon and almost looks like a “bump” on the surface of the earth. Due to the centrifugal forces of the earth which are caused by its own rotation a second “bump” is shaped on the other side of the earth which is now facing the moon. In between, valleys of water are formed where the water is pulled back.

Because of the rotation of earth and moon this system is not static, but in steady movement. Every time a mountain of water is formed, water is pressed towards the coast and in places where a valley of water is situated the water is pulled back. As there are two mountains of water, which are moving around the earth, the phenomenon of the tides occurs twice a day within 24 hours. The cycle of the tides does not exactly take 12 hours, which is why there are daily changes of a couple of minutes in the low- and high-water time.

Additionally, to the usual tide there is an event called spring tide. It occurs when moon, sun and earth are in one line and the gravitational forces of the moon and the sun add up. Spring tide always takes place when it is full or new moon, which means every two weeks and is a very strong tide, which is higher or lower than the average high and low water. Contrary to the spring tide is the neap tide, a constellation, when sun, moon and earth form a right angle and the sun and the moon weaken each other’s gravitational influence on the earth which lessens the power of the tide. The above described effects can be increased through hurricanes and heavy storms. A spring tide in combination with a storm flood can have catastrophic effects.

From the southern hemisphere the following planets can be observed without binoculars: Venus, Jupiter, Mars, Mercury and Saturn. Venus is the planet closest to the earth which is why it can be easily spotted at night as the brightest of all stars. Jupiter is the biggest planet in our solar system. Like Venus, Jupiter mostly consists of gas which causes its surface to appear cloudy when observed through a telescope. Mercury is, after the dwarf planet Pluto, the second smallest planet in our solar system. It is very close to the sun which makes Mercury hard to spot from the earth.

But why are there seas where the tides are hardly perceptible? In Brittany, on the east coast of France, the difference between the tides can be up to 10 meters while in the Mediterranean Sea it is only a couple of centimeters. The reason for this is the size of the water body and the coastal structure. The Mediterranean Sea is simply too small and too flat to form a mountain of water. It is only connected to the Atlantic Ocean through a narrow sea gate which makes it impossible for the mountain of water that travels the Atlantic to influence the Mediterranean Sea.