Rules of Thumb & Things to Remember
- Diurnal tide – once a day
- Semi-diurnal – twice a day (two high and two low sea levels per day)
- Tidal cycle: 24h & 50 min
- Time between highs and lows: 6:12 hours
7.1. What causes tides?
The main forces generating tides is the gravitational force of the moon. This force pulls the water in the oceans towards the moon. The force is stronger on the water near to the moon and it is weaker on the water further away to the moon. Thus, the shape of the ocean water has some ovality with two high areas (bulges). The first bulge is at the side near to the moon due to the stronger pull from the moon. The second bulge is at the furthest away side because the moon does not pull the water strong enough.
Because the Earth is rotating around itself, different areas experience the high and low pull of the Moon. For example, if one location is now exactly at the point nearest to the moon and experiences a high tide, after approx. 6 hour, the earth would have made a 90 degrees rotation and the area will be experiencing a low tide. After another 6 hours the location will be at side furthest away from the sun and it will have a high tide once more.
(d) The gravity of force the Sun
(b) The centrifugal forces from the rotation of the Moon-Earth system. Moon and earth are rotating together around the barycenter (i.e. the center of gravity of the Earth-Moon system). Since the earth is much larger than the moon the barycenter is very close to the center of the earth and many times we think that the moon is rotating around the earth. Nevertheless, the fact is that both the noon and the earth (as a system) are rotating together around the barycenter. This rotation introduces a centrifugal force that tents to move the water of the ocean. Of course, gravity is much stronger and does not let the water leave from the earth. Nevertheless the centrifugal forces moves the water as shown in the graph, from the places with small centrifugal force to the places with larger centrifugal force..
(c) The centrifugal force from the rotation of the Sun and the Earth as a system
From the figures it is obvious that two bulges are formed because the tractive and centrifugal forces act on opposite directions. To make it simpler, if the moon were positioned in the plane of the equator and there were no continents this balance would create two bulges, one under the Moon and one on the opposite side of the earth and there would always be low tide in the poles and high tide close to the equator.
7.2. Diurnal tide – Explanation
A point P at the rotating surface of the earth will experience a varying water depth with two maximum (two high tides). The first when it is in the moon’s side and one when it is in the opposite side. Also tow low tides as it passes through the belt of the low water. This is a semi-diurnal tide M2. If P is located at a high latitude such that the low water belt (center line of the belt) is not crossed by point P as it rotates, then the water depth will encounter a single maximum (high tide) and a single minimum (low tide) during the day. This is a diurnal tide M1.
The same types of tides occur due to the sun S1 S2
7.3. Spring tide – Neap tide
The sides generated by the Sun and Moon coincide of counteract depending on the position of the moon relative to the sun. When the Sun and the Moon are aligned (twice a month, either in the same or opposite sides) the amplitude of the total tide is at a maximum. That is a spring tide. When the position of the moon is perpendicular to the Sun, the tidal amplitude is minimum (neap tide)