Many believe that the reason for the ebb and flow is the direct attraction of the moon, that it moves the masses of water in the ocean by its gravity. Such a phenomenon does take place, but its effect is negligible because the attraction of the Moon is several times weaker than the attraction of the Earth and is completely compensated by it. Why, then, does the ebb and flow occur? Let's take a closer look at this.
The cause of ebb and flow is two things:
- Uneven distribution of the Moon's gravity on the Earth's surface.
- Centrifugal force arising from the rotation of the Earth around its axis.
The influence of centrifugal force is small and causes only a constant rise in the water level at the equator in comparison with the poles. But the heterogeneity of the lunar attraction is decisive.
The law of universal gravitation says: "the attraction of bodies is directly proportional to the product of their masses and inversely proportional to the square of the distances between them." For people far from science, I will explain: the smaller the distance between the bodies, the stronger their attraction.
As an example, we can say that when the Moon is over the Atlantic Ocean, its waters will be affected by a greater force of gravity than the waters of the Indian Ocean, which are at that moment on the other side of the globe. The attraction of the Earth also plays a role, since it is many times larger than the lunar one, it does not allow the waters of the oceans to fly up to the Moon. As a result, the water only rises a few meters higher, but does not fly away into space.
But we still haven't answered the main question. Why does the Moon affect the oceans, but not affect smaller objects? The thing is that on small objects you don't feel any difference in the force of attraction acting on different parts of the object. Because of this, they are not affected by either the Moon or the Sun. At the same time, the object does not have to be very small, tides do not occur in shallow bodies of water, so in the largest fresh lake in the world, Baikal, there are also no ebb and flow, although its dimensions are 636 by 80 kilometers, and the depth in some places exceeds 1500 meters.
And by the way, in addition to the Moon, the Sun also significantly affects the water masses. It is because of this that the largest ebb and flow are observed on the day of the new moon, when the Moon and the Sun are on the same side of the Earth.