In the previous section you read that the time of high water (HW) and low water (LW) is always a little bit shifting. That’s because the moon is circling the earth. But the moon is not the only one that creates the tides. The sun also pulls on the water. Not as hard as the
moon, but it’s still very noticeable. Only with the difference that the sun doesn’t rotate but stands still.
- Apparitions of the moon
The moon revolves around the earth and the sunlight falls on the moon. As a result, we see – from Earth – the moon illuminated a little differently every day. Those are the so-called phases of the moon.
With new moon (NM), the moon is dark from the earth. That’s because the moon is in front of the sun at that moment. The next night the moon shifted 12° and a narrow crescent is visible. That sickle gets a little bigger every night.
When the moon is half-lit after about a week, that first quarter of an hour is called (EK). The moon has then completed a quarter of its circle. Because the illuminated part is getting bigger and bigger, this is also called waxing moon. This will continue until the moon is fully illuminated as seen from Earth.
The dark half is now turned away from the earth. This phase of the Moon is called Full Moon (VM). The moon keeps spinning and we see the illuminated part getting smaller and smaller.
When we see only half of the light, the moon has arrived at its last quarter of an hour (LK) and will continue to reduce until it has become completely dark for us again.
This can be seen in the following diagram. The outer circle of moons shows the phase of the moon in relation to the sun and the earth. The inner circle of the moon shows how the moon can be seen from the earth. The black part is invisible. The sun is so far away that it doesn’t fit on the page. The direction in which he is standing is indicated.
- Spring tide and neap tide
At New Moon and Full Moon, sun and moon are in one line and move together at the water. That’s why the high tide is extra high; we call this spring tide. If it
high tide is at that moment we call this High Water Spring.
If it is low tide during spring tide we call this Low Water spring tide. The low tide is also extra low at spring tide. For all sorts of reasons, this extra high tide and extra low tide do not exactly coincide with New Moon, but about two days later.
Two days after the first and last quarter of an hour it is neap tide, because then the attraction forces of the sun and the moon work against each other. Both are at an angle of 90 degrees to each other.
There is a time span of about 7½ days between the four phases of the moon.
The drawing below shows spring tide and neap tide. The dotted area around the earth represents the water, but for the sake of clarity it is indicated in an exaggerated way.
Figure 28 clearly shows that when the sun and moon are in line, they both exert force on the water on earth. That’s why the water will be extra high at new moon and full moon. (spring tide). During the first quarter and last quarter of an hour, the sun and the moon work against each other and the water will be more evenly distributed over the earth. As a result, the high water is less high than in the first case (neap tide).