The origin of tides and currents
The sun and the moon exert a gravitational pull on water. This causes water to move, both vertically and horizontally (so-called tidal phenomena). These movements create a high (high water level) and a low (low water level). From the moment of low water level to the moment of high water level, the water rises (high tide), and from the moment of high water level to the moment of low water level, the water recedes (low tide). The gravitational pull that the sun and moon exert on the Earth is not uniform everywhere, but depends on the position of a location on Earth relative to the sun and moon at any given moment. Differences in gravitational pull arise from the rotation of the Earth on its axis and the rotation of the Earth around the sun; as a result, the water on Earth experiences a regularly recurring movement: the tidal movement.
The tidal movement can best be imagined as a huge wave rolling across the earth. When the crest of this wave reaches us, we have high tide; when the trough reaches us, we have low tide.
Duration
The duration of a tidal cycle due to the influence of the moon is approximately 24 hours and 50 minutes, the duration of a tidal cycle due to the influence of the sun is 24 hours and is much less strong than that of the moon. On the Dutch coast it is high and low wind twice a day, the duration of a tidal movement is therefore approximately 12 hours and 25 minutes.
Spring tide:
When the tidal waves caused by the moon and the sun interact, i.e., when they have high and low tides at approximately the same time, it is called a spring tide. The water level is then greatest; in other words, high tide is the highest and low tide the lowest compared to previous water levels. The water current is also strongest at this time due to the greatest water level drop. Spring tide occurs approximately every two weeks (the time between a new moon (PM) and a full moon (AM)).
Neap tide:
When the tidal forces counteract each other, a neap tide occurs. This is the situation during the first quarter (EC) and last quarter (LS) lunar phases. EC and LS occur approximately seven days after PM and PM, respectively. This means that a neap tide occurs approximately seven days after a spring tide, and a spring tide occurs again seven days later.
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When the sun and moon are aligned, as it were, with respect to the Earth, they combine their forces and attract more water. This is called spring tide. The water level is then higher at high tide and lower at low tide. The moon and sun can also counteract each other. This happens when the two celestial bodies are at right angles to each other. In this case, the water is pulled from two different directions, resulting in a much lower than average water rise. This phenomenon is called neap tide.
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Spring tides occur twice per lunar month (29.53 days). You might expect them to occur during the full moon and the new moon. However, they occur two days later. The solar tide, with a period of 12 hours, constantly overtakes the lunar tide, with a period of 12 hours and 25 minutes. This causes the lunar tide to experience a waveform: it is periodically strengthened and weakened. It also affects the timing of the tide. Between spring tide and neap tide, the solar tide has an advancing effect on the tide, while between neap tide and spring tide, it has a delaying effect. |
Below is a video from Buienradar explaining spring tides and neap tides.
It starts at 49 seconds.