Why do tides happen twice a day, because the moon is one?
Here's the deal. The Earth and the Moon revolve around one point - their common center of mass. The moon is on one side, the earth is on the other. At the center point of the Earth, the centrifugal force of rotation and the force of gravity of the Moon counterbalance each other. But the Earth is not a point (alas ...) and the points e of the surface facing the Moon rotate in a circle of a smaller radius, which means that a smaller centrifugal force acts on them. And the distance from them to the Moon is less, which means that the force of attraction of the Moon is greater. The result is an additional force directed towards the moon. On the other hand (pun intended!), The opposite points of the earth's surface rotate in a circle with a larger radius, and a large centrifugal force acts on them, and they are farther from the Moon and the Moon attracts them weaker. Therefore, an additional force is obtained for them, directed from the moon. In total, a force constantly acts on the Earth, tearing e in the direction of the Earth-Moon. It acts on all points, and on water, and on air, and on the earth. Only the solid does not move so easily, there are no elastic forces inside. And what you take from the air and water - you get two tidal waves. One is on the side facing the moon, the other is on the opposite. The same tearing force acts on the Moon, as well as on all satellite planets. Under unfavorable conditions, this force can turn out to be so significant that it can surpass the forces of elasticity and the forces of gravitational attraction, and the satellite will burst. This is at a small radius of rotation of the satellite around the base planet, the so-called Roche limit. The moon is very far from this limit, but Phobos near Mars will be torn apart. The fact that these are far from threats is very well confirmed by the rings of Saturn.
The moon is one, but the tidal wave is not a displacement of the entire water mass of the world ocean towards the moon, but stretching in the direction of the earth - moon. So the tide is formed on both sides Earth - the one that faces the moon, and the opposite. Therefore, the tidal period is half a day, not a day.
Solar tides are on their own, and there are also two of these tides. They can coincide in phase with the lunar ones, or, on the contrary, they can slightly extinguish them (the amplitude of the solar tides is noticeably less than that of the lunar ones).
Because throughout the day the position of the earth relative to the sun and the moon changes.