The distorting forces caused by variations in the strength and direction of gravity are called tidal forces, whether they are due to the Moon, Earth, Sun, or any other astronomical mass. As the Earth turns one revolution under the bulges, each point experiences two high tides. The result is two bulges on opposite sides of the Earth, one facing toward the Moon and the other facing away. One way to think about this is that on the near side, the Moon pulls the water away from the Earth, but on the far side, it pulls the Earth away from the water. For the same reason that the tall man’s head is pulled away from his feet, the water on both sides of the Earth-near and far-bulges away from it. You might expect the Moon to create a single oceanic bulge on the closer side, but that’s wrong. The Moon’s pull on the oceans is strongest on the side of the Earth facing the Moon and weakest on the far side. But in this case, it’s the Moon’s gravity, not the Earth’s. The cause of the twice-daily rising and falling of the seas is exactly the same as the cause of the 2,000-Mile Man’s discomfort: the non-uniformity of gravity. “Let’s return to the question of the ocean tides. The force on his head and feet will not be the same because gravity, as it pulls toward the center of the Earth, will push his head toward his feet, leading to the strange sensation of being compressed. Instead of pulling in a single direction, gravity pulls directly toward the center of the planet, like this: This creates a new problem for the giant if he falls horizontally. Both the strength and the direction of gravity vary. The 2,000-Mile Man would have no trouble in this environment, whether he fell vertically or horizontally-not until he hit the ground anyway. But more than that, the strength of the gravitational pull is entirely uniform. The vertical lines, together with the arrows, indicate the direction of the gravitational force-not surprisingly, straight down. To understand why this is so, let’s temporarily imagine that the Earth is flat. He feels as if his head is being pressed toward his feet. Yet when the giant tries it, he finds a new discomfort the stretching sensation is replaced by an equal feeling of compression. “Perhaps he can avoid being stretched by falling in a horizontal position, legs and head at the same altitude. The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics Perhaps he can avoid being stretched by falling in a horizontal position, legs and head at the” The net effect is an uncomfortable feeling that his head and feet are being pulled in opposite directions. The poor man is so tall that the pull on his feet is much stronger than the pull on his head. For the 2,000-Mile Man, this presents difficulties even while he is in free fall. But as he draws closer, the pull of gravity increases. Far from the Earth, it is almost entirely absent. The cause of his discomfort is that gravity is not uniform throughout space. ![]() The problem is not the giant’s overall acceleration toward the Earth. ![]() But as he gets closer to the Earth, strange sensations arise in his long body-sensations not of falling but of being stretched. Far out in outer space, gravity is weak-so weak that he feels nothing. ![]() Imagine the 2,000-Mile Man-a giant who measures 2,000 miles from the tip of his head to the bottoms of his feet-as he falls, feet-first, from outer space toward the Earth. “Tides and the 2,000-Mile Man What causes the seas to rise and fall as if they were breathing two big breaths every day? It’s the Moon, of course, but how does it do it, and why twice a day? I will explain, but first let me tell you about the fall of the 2,000-Mile Man.
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