Unit 11
The Tides
When sun, moon, and earth are in one line, then because of the added tidal energy, there are particularly high daily high tides in the oceans and seas.
We call this event spring tide. As such a lining up of the heavenly bodies happens at full moon as well as at new moon, about every fort- night there is a spring tide (s. picture).We can illustrate that phenomenon as follows:

 


1. We put the moon in full-moon position. It is then in a line with
the earth and the sun.


2. With the washable felt pen we draw a line from pole to pole via
Greenwich near London along the prime meridian and lengthen it on
the opposite side to a full circle (180th meridian).We do the same
at the 90th east and west meridians and thus have divided the earth
into 4 equal parts.


3. We turn the prime meridian towards the sun.


4. We put a somewhat wider tape or string , which is 2 inches longer
than the globe, around the pole and have it tightened by 2 pupils,
i.e., drawn towards the sun respectively off the sun. At the prime
meridian and at the 180th meridian we now have spring tide.
A third pupil now turns the earth underneath the tape (or string). After a quarter turn, i.e., after 6 hours, the height of the tide is at America and Asia.
After another quarter turn, i.e., after twice 5 hours =12 hours, we have spring tide again along the starting line.
So in 24 hours the earth turns about twice underneath the high-water "mountains" respectively the low-water "valleys". Therefore on the oceans there are twice daily the highest tidal levels(daily high tide) and the low-water levels (daily low tides).


Tip: The causes in detail are very much more complex, i.e., it is not only a question of forces of gravitation respectively attraction but also of centrifugal forces. In this process earth and moon form a system of gravitation whose common centre of gravity, because of the mass of the earth, lies below the surface of the earth. As the moon moves on a bit every day, two tides do not last exactly 24 hours but about one hour longer.
A week after the spring tide earth, sun, and moon are at right angles to each other. We have half moon. The forces generating the tides get weaker. Now all over the world is neap tide. At neap tide the high-water levels are lower and the low-water levels a little higher, i.e., the difference between high and low water, the tidal range, is smaller.

 

 

Tip: The moon-conditioned forces are stronger than the sun-conditioned ones.
The differences between spring tide and neap tide can be some decimetres on the German North Sea coast. The concurrence with an accidentally strong storm -event leads to particularly high water levels on the coast. Such spring tides are especially dreaded