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Question to Marilyn: "Are there tides in fresh water or just the oceans?"

Marilyn's answer: "There are tides everywhere on Earth. (...) If you stood still enough, there would even be tides in your tummy."

Physicist's comments: See below.

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"During the full moon, the moon has a higher gravitational pull, creating a higher tide. The Miami Beach psychiatrist Arnold Lieber says that pull affects oceans and people in a similar way, since the human body is mostly water."

Physicist's comments: See below.

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In his interesting book, Dr. Lieber presented a compelling evidence of a relationship between the lunar phase and violence, based on homicide records kept by coroners of Dade County (Miami, Florida) and Cuyahoga County (Cleveleand, Ohio), and spanning 15 and 13 years, respectively. In addition, Dr Lieber dicovered a similar correlation between lunar phase and psychiatric ER visits and fatal traffic accidents.

Unfortunately, Dr. Lieber went on to speculate about a possible mechanism for such a correlation and wrote:

"I believe the gravitational force of the Moon, acting in concert with the other major forces of the Universe, exerts an influence on the water in human body - in you and in me - as it does on the oceans of the planet. Life has, I believe, biological high and low tides governed by the Moon."

"When the Moon’s gravitational pull upsets our fluid balance, the result makes us tense and liable to emotional outbursts."

Physicist's comments: See below.

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"Here in Southern Illinois as in much of the world, we experience two periods of high and low tides every day. These tides are caused by the gravitational force of the moon.(...) At perigee, the moon's gravitational tug on Earth is at its maximum and so the waters of the world rise and fall to their maximum extent."

Physicist's comments: See below.

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Here I only give the main idea in a nutshell:

• Tides arise because of differences in the lunar gravitational pull at different points in the Earth. Remember, the gravitational force of the Moon decreases with the distance, so a rock on the side of Earth closest to the Moon feels slightly stronger gravitational pull than an identical rock at the Earth's center does, while yet another identical rock on the side of the Earth that faces away from the Moon feels slightly lesser gravitational pull than does the one at the Earth's center. These tiny differences in gravitational pull of the Moon are called tidal forces.

• The Earth is not just sitting in space - the Earth is in free fall toward the Moon, i.e. it accelerates toward the Moon, because the Moon is pulling on it. (At the same time, the Earth pulls on the Moon, so the Moon accelerates toward the Earth. Fortunately for us, the Earth and the Moon do not crash head-on because each has also a sideway speed, so they just end up orbiting the common center of mass (the barycenter), located (on average) some 1710 km below the Earth's surface. The Moon orbits the barycenter with average orbital speed of 3675 km/h, while the Earth's center orbits the barycenter with average orbital speed of only 44.9 km/h.)

• Under the force of lunar gravity, the rock on the side of Earth closest to the Moon should accelerate faster and get ahead of the center of the Earth, while the rock on the side of Earth facing away from the Moon should accelerate less and be left behind the center of the Earth. Fortunately, the Earth's own gravity easily overcomes these tendencies of both rocks, so the net result is that the Moon's gravity tries to deform the Earth into a football shape. Since the Earth is quite rigid, it can't deform much in response - in fact the resulting tidal stretch of the Earth is only about 2 ft.

• Unlike the Earth's crust, the water in the oceans is not rigid, and it can flow. For simplicity, imagine that there is only one big ocean on the Earth, that covers the whole planet.  The water would flow unobstructed by continents, and the ocean would take a football shape, with one tip of the water football pointing towards the Moon, and the other tip pointing away from the Moon. As the Earth rotates, a point on the ocean floor periodically gets under either tip of the water football, only to emerge from it a while later. This is the origin of high and low ocean tides. (The model for real tides is much more complex, as one has to take into account the shapes of the continents, the effects of winds and atmospheric pressure etc.)

• The Sun also generates tidal forces, as the Sun pulls a tad harder on a side of the Earth closest to it, and a tad weaker on the side of Earth away from it, as compared to the Sun's pull on the center of the Earth. Note that the Sun's pull at the center of the Earth is about 178 times stronger than the Moon's pull. However, because the Sun is so far away from the Earth, the differences in Sun's powerful pull on various parts of the Earth are actually smaller than differences in much weaker gravitational pull of the Moon. As a result, the Moon is responsible for some 70% of the magnitude of tides, while the Sun contributes another 30% (a contribution from Jupiter and other planets is simply 0%).

• Note that just as the Moon exerts tidal forces on the Earth, the Earth exerts tidal forces on the Moon, deforming the Moon into a football shape with one tip pointing toward the Earth.  The Moon was initially molten some 4.6 billion years ago, but retained this football shape when it solidified, and the Earth's tidal force keeps the long axis of the lunar football pointing toward the Earth. To keep the long axis always pointing towards the Earth, Moon spins once on its axis as it makes one revolution around the Earth. That's why we the same side of the Moon always faces the Earth.

• By now you should see that there are no tides in your tummy, your body, a swimming pool or a small lake, simply because the water at different locations within each of these reservoirs is still at the same distance from Moon, so no difference in gravitational pull exists.

Warning: I would like to add a caveat here that the explanation of tides given in the article "Our Restless Tides" posted by the Center for Operational Oceanographic Products and Services (CO-OPS) of the National Ocean Service (NOS) of the National Oceanic and Atmospheric Administration (NOAA) is unfortunately another example of convoluted, if not outright bad physics. Could you see why?

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