Intro
Tides are not a well understood concept. Many misconceptions arise while trying to derive its origins. However, tides are quite well behaved in terms of natural phenomena. Weather, for example, is extremely chaotic compared to tides, which exhibit periodicity and have links to many astronomical objects around the Earth such as the moon and the sun. Understanding and being able to predict tides is crucial for navigations, especially in areas where the water level difference can reach 14 meters, such as in Brittany, France. To predict tides, the complex sinusoidal wave needs to be broken down into its components, which all have a periods related to astronomical factors, such as the time taken for the Earth to rotate around its axis, the Moon to rotate around the Earth, and the elliptical orbit of the Moon. The more components are included, the more accurate the prediction of the tides will be. However, even just a few of the most important components, namely the ones relating to the Earth, the Moon, and the Sun, are sufficient to get a good model for the tide.
Tides
Fundamentally, tides arise from gravity. This was first realized by Newton. Further work was done by Laplace to describe how tides behave between continents. However, the common explanation of tides is wrong from a physics perspective. The traditional view has however been proposed by many reputable sources on the matter such as the NOAA. Understanding why it is wrong however offers a lot of insight into the actual phenomenon behind tides.
The traditional explanation does give an intuitive reason for the two tidal bulges. Firstly, let’s assume that Earth is covered completely with water and that it only interacts with the Moon. The gravitational force experienced by an object is given by Newton’s law of Gravitation:
Therefore, one unit weight of water on the side of the Earth near the Moon experiences more force than the center of the Earth which itself experiences more force than the side of the Earth away from the moon. This definition of a tidal force on Earth is therefore the difference between the gravitational pull at a point and the gravitational pull at the center of the Earth. Therefore:
If we imagine three particles, one on each side of the Earth and one in the center, and we imagine their trajectories as they go towards the moon, two bulges clearly emerge, in accordance with observations. The Earth now simply rotates with no friction underneath these two bulges in order to have two high tides and two low tides per day (approximately because in a day, the moon has time to move).
In this way the combination of gravity and inertia create two bulges of water. One forms where the Earth and moon are closest, and the other forms where they are furthest apart. Over the rest of the globe gravity and inertia are in relative balance. Because water is fluid, the two bulges stay aligned with the moon as the Earth rotates (Ross, D.A., 1995).
However, this explanation is flawed. If this tidal force due to the differential in gravitational force was sufficient to pull the water up, the effect would be visible in lakes and in closed basins. However, tides are no visible in lakes and closed bodies of water, so it is clear that this tidal force cannot pull water up a noticeable amount, and simply decreases the acceleration towards the center of the Earth.
So how do the two bulges form? To understand that, the tidal forces that occur at the poles need to be taken into account. The three particles are once again attracted towards the moon. Due to Earth falling towards the Moon, a non-inertial frame, the force is directed mostly inwards towards the center of the Earth.
Source: https://www.youtube.com/watch?v=yg6t2rZBWGY (The Physics of Tides by Higgsino physics)
The tidal field can be visualized around the Earth. The pushing force summed over large stretches of water over large portions of ocean generates a wave which can be observed as tides. This explains why tides are not noticed in lakes or small water basins, since there is not enough water to be pushed around by a large difference in tidal forces.
Overall, thinking that the moon pulls the water up from the surface of the Earth is misleading. Tides should be thought as the circulation of water due to the sum of pushing forces and the poles and pulling forces at the Equator. The water is displaced from the poles, flowing towards the Equator to form two bulges.
Another factor which effects the tides is the sun. The sun's effect is approximately half of the moon's effect in regards to the tides on the Earth. However, the Sun and the Moon can work together or against each other when generating the tides. If the Moon and the Sun are in line with each other, either on the same side or opposite sides of the Earth, then their attractions combine generating a spring tide, which leads to a large tidal range. If the Sun and the Moon are at 90 degrees to each other, then their attractions oppose each other, generating a neap tide, which leads to a small tidal range.
Conclusion
The common misconception of the moon pulling water up from the Earth was analyzed. A preferable explanation was given which explains why tides are not visible in lakes. Finally, the principle of spring tides as the combinations of the tidal forces from the Moon and the Sun was given.
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