The Earth (diam. = 12.756 Km) has a big satellite, the Moon (diam. = 3476 Km), rotating around it at an average distance between the respective gravity centres of 384,400 Km, with a period of 29.5 days (709 hours), much slower than the rotation of the Earth around its own axis (24 hours). In our solar system, only Pluto (not considered a planet anymore since 2006) has a bigger satellite (Charon) respect to its dimensions (2274 Km of diameter for Pluto against 1206 Km for Charon) and rotating closer, at 19,640 Km.

This means that the Moon exerts a remarkable gravitational attraction on the Earth and just this is responsible of the tides that make cyclically vary every day the sea level everywhere in the world. This attraction (tidal force) is slightly stronger on the closest side to the Moon and weaker on the opposite side.

The Earth is not rigid so that this attraction tends to stretch it along the direction between the gravity centre G of the Earth-Moon system, placed still inside the Earth, but closer to the Moon, so that the same Earth rotates around G with its own gravity centre E. The rigid continental masses are deformed for only few mm, but the swelling affects much more the oceanic masses with an increase of their level of 0.5-1.0 m in the open ocean, about 2 m along the linear coasts and up to 16-18 m in the narrow estuaries and in closed oceanic bays with high cliffs, where sea water can move only upward.

This swelling occurs in each point P of the Earth surface twice a day, the first, when P is closer to the Moon because here the lunar attraction is maximum and the centrifugal force is minimum respect to the G point (much closer to P) and the second when P is at the opposite side of the planet respect to P because, now, it is just the lunar attraction to be minimum here, while the centrifugal force, respect to G, is maximum and makes swell as well the oceans. So, in those opposite areas, the tide is at the maximum (high tide) while, in the meantime, there's a minimum level (low tide) on the two other sides of the Earth. This movement of the oceans is complicated and interrupted by the continents but, in any case, the bulges created by the lunar attraction on the oceans tend to follow the Moon while the Earth rotates during the 24 hours.

Given the presence of the continents and the only relative fluidity of the oceans, there's a certain inertness by the oceans masses in following this movement so that they are shifted ahead respect to the point along the direct Earth-Moon distance. This means that the attractive force by the Moon is equally shifted respect to this straight direction and this creates a little torque on the Earth. The result of this torque is a transfer of rotational energy from our planet to the Moon so that the Earth's rotation around its own axis is slowed down of about 1.5 milliseconds/century and the Moon is shifted on a higher orbit (3.8 cm far from our planet every year. Making few calculations, this means that, 100 millions years ago, during the dinosaurs era, the Moon orbited around our planet 380 millions cm, or 3800 Km closer than today, in the average (about 1%).

Also the friction caused by the oceanic tides on our planet are considered a cause of the slowing down of its rotation that is progressively increasing the length of our days. Tides are more intense in the oceans than in the closed seas like the Mediterranean Sea, the Persian Gulf or the Baltic Sea because the water masses are much bigger in the oceans, with more space available to move. This is the reason for which tides in the Mediterranean Sea don't reach 1 m of height.

References:

http://www.nineplanets.org/lun a.html

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