How a book written in 1910 could teach you calculus better than several books of today

[Towlie]

25. The bit about two tides per day not making sense is especially interesting.

The existence of tides is often taken as a proof of gravity, but this is logically flawed. Because if the moon's "gravity" were responsible for a bulge underneath it, then how can anyone explain a high tide on the opposite side of the earth at the same time? Anyone can observe that there are two — not one — high tides every day. It is far more likely that tides were given us by an Intelligent Creator long ago and they have been with us ever since. In any case, the fact that there are two high tides falsifies gravity.

The best explanation I've encountered for why there are two high tides per day is part of a 1966 short story by Larry Niven, titled Neutron Star, which can be read here. The explanation is on page 3.

“Do you know what happens when a moon gets too close to its primary?”

“It falls apart.”

“Why?”

“I do not know.”

“Tides.”

“What is a tide?”

Oho, said I to myself, said I. “I’m going to try to tell you. The Earth’s moon is almost two thousand miles in diameter and does not rotate with respect to Earth. I want you to pick two rocks on the moon, one at the point nearest the Earth, one at the point farthest away.”

“Very well.”

“Now, isn’t it obvious that if those rocks were left to themselves, they’d fall away from each other? They’re in two different orbits, mind you, concentric orbits, one almost two thousand miles outside the other. Yet those rocks are forced to move at the same orbital speed.”

“The one outside is moving faster.”

“Good point. So there is a force trying to pull the moon apart. Gravity holds it together. Bring the moon close enough to Earth, and those two rocks would simply float away.”

The Moon has the same effect on the Earth, trying to pull it apart, but in this case the ocean's response is most visible because it's liquid and can flow. Whether the Moon is overhead or below our feet, it tries to pull the earth apart. Therefore there are two tide cycles per day.