More than 2,000 years ago we were able to connect the rise and fall of sea level during the tides with the movement of the Moon around the Earth. Even before we understood what gravity was or we considered the Moon as an object with a certain size and located at a certain distance. And yet, from the simplest theoretical models to practice there is a whole world (and a satellite and a star).
The easiest way to understand and show the creation of tides by the Moon is to imagine a planet Earth where water covers the entire surface to a uniform depth . That is, a planet Earth without continents and without an oceanic relief. In this way, the tides will be the consequence of the gravitational attraction of the Moon, which will deform the layer of water that surrounds the solid part of the planet. If this attraction were uniform, that is, it was equally intense in the part of the Earth near the Moon and in its distant part , then we would simply have that the entire layer of water would move in the direction of the Moon, creating a single bulge in that direction.
However, the gravitational attraction will depend on the distance and the far part will therefore feel less attraction than the close one, creating a second bulge on the other side of the Earth with respect to the Moon. These two bulges, which will rotate with the rest of the planet throughout the day, will cause the two high tides that are usually seen in most of the world’s coasts. But the story does not end here. It has only just begun, really. Because the Sun also contributes to this effect and because the Earth does not look like what we said before: 30% of the Earth’s surface is covered with continents and the remaining 70% does not have a uniform depth, but is plagued by elevations, depressions, mountains and, in general, obstacles , which influence the movement of water.
The Sun contributes to a lesser extent , not because its gravity is much less than that of the Moon, but because being much further away (150 million kilometers compared to the 384,000 of the Moon) the difference between the attraction it exerts on the two faces of the planet is much smaller . In addition, the Earth’s own inclination with respect to the Sun or the lunar orbit also has an influence, as well as the eccentricity of the lunar orbit . Our satellite does not orbit in a perfect circle, but rather an ellipse, which leads it to come closer and further away during different parts of its orbit. At the point of closest approach, called perigee, it can be only 355,000 kilometers from the planet, while at its closest distance, called apogee, it reaches up to 406,000 kilometers away. This difference will have effects on the tide. In addition, the relative position of the Sun and Moon will also create a cycle that will increase or decrease the amplitude of the oscillations. Basically when both stars are on the same line, their effect will be added and when they are separated by 90º, their effects will occur in different directions.
But all these effects and contributions end up being at the mercy of the fact that the Earth is far from being an object with an “ideal” shape . The arrangement of the continents and oceanic landforms can greatly contribute to the development of tides, even preventing them from occurring in some seas and areas of the world.
This occurs, for example, in the Mediterranean , where the difference between low and high tide barely exceeds 20 centimeters in some areas, insufficient to be perceived with the naked eye . We could expect this to be the case, since this sea only connects with the rest of the oceans through the Strait of Gibraltar, which is barely a few kilometers wide. However, a similar effect occurs in other seas, such as the Caribbean or the Sea of Japan , where the distribution of the islands that delimit them prevent tides from forming.
In these cases, the effect is not so much because the flow of water is limited and therefore the “wave” that supposes the movement of the bulge that follows the rotation of the Earth also has difficulty propagating. What happens here is that this wave tends to be reflected on the coasts and in the geographical accidents that populate the oceans , creating new waves that will interact with the primordial one to increase or decrease the final amplitude. In cases like the Caribbean and the Sea of Japan, the result of the many waves reflected at each point on the coast will have the effect of almost completely canceling out the main wave , making the tides disappear.
But this can not only happen in areas with narrow seas or where islands abound, but also in the open sea, in what are known as amphidronic points , which are nothing more than those points in the ocean where the tide resulting from all the aforementioned effects is zero. . In the North Atlantic, halfway between Europe and Canada, there is one of these points, as well as in the South Pacific, in the Indian Ocean and in a dozen more locations, such as the southwestern coast of Australia, which does not experience tides for this reason.
References:
IN Avsiuk, 1977, On the tidal force, Soviet Astronomy Letters, vol. 3
