Extraterrestrial Civilizations - Isaac Asimov [59]
Red and infrared are less intensively energetic than the remainder of the visible light spectrum, and there would be many chemical reactions that yellow, green, or blue light could initiate that red and infrared could not. However, life is not based on photochemical reactions, except for photosynthesis and that is initiated by red light. No doubt we would not have to stretch matters intolerably to imagine life on such a world—so far.
Let us, however, take up a new issue:
The gravitational field of any object decreases in intensity with the square of the distance. If distance is doubled, the intensity falls to ¼ of what it was; if the distance is tripled, it falls to 1/9 and so on.
This affects the manner in which the Moon and the Earth attract each other.
The average distance between the center of the Moon and the center of the Earth is 384,390 kilometers (238,860 miles). This varies somewhat as the Moon moves about its orbit, but that doesn’t affect the line of argument.
Not all parts of the Earth are, however, at the same distance from the Moon. When the center of the Earth is at its average distance from the center of the Moon, the surface of the Earth that directly faces the Moon is 6,356 kilometers (3,950 miles) closer. The surface of the Earth that faces directly away from the Moon is 6,356 kilometers (3,950 miles) farther.
This means that while the surface of the Earth directly facing the Moon is at a distance of 378,034 kilometers (234,910 miles) from the Moon’s center, the surface of the Earth facing directly away from the Moon is at a distance of 390,746 kilometers (242,810 miles) from the Moon’s center.
If the distance of the Earth’s near side from the Moon’s center is set at one, the distance of the Earth’s far side is 1.0336. This difference, only 3.36 percent of the total distance from the Moon, does not seem like much. However, the gravitational pull of the Moon falls off over that small distance by an amount equal to 1/1.03362 and is only 0.936 at the far side as compared with 1.000 at the near side.
The result of this difference in the Moon’s pull at the near and far sides of the Earth is that the Earth is stretched in the direction of the Moon. The near surface is pulled toward the Moon more forcibly than the center is, and the center is pulled toward the Moon more forcibly than the far surface is. Both the near and far surface bulge, the former toward the Moon, the latter away from the Moon.
It is a matter of a small bulge only, half a meter or so. Still, as the Earth rotates, each part of its solid matter bulges up when it turns toward the side facing the Moon, reaching its greatest height when it passes under the Moon, then settling back. The solid matter bulges as it turns toward the side away from the Moon, reaching another peak when it is directly opposite the position of the Moon, then receding.
The water of the ocean bulges up also, to a greater extent than the solid land does. This means that as the Earth turns, the land surface passes through the higher bulge of water and, as it does so, the water creeps up the shore and then back down. It does so as it passes through both bulges of water, one on the side facing the Moon and one on the side away from it. This means the water rises and falls along the shore twice a day; or, we can say, there are two “tides” a day.
Because this difference in gravitational pull causes the tides, it is referred to as a tidal effect.
Naturally, the Earth also exerts a tidal effect on the Moon. Since the Moon is smaller than the Earth, the Moon’s diameter being 3,476 kilometers (2,160 miles) as compared with Earth’s diameter of 12,713 kilometers (7,900 miles), the drop in gravitational pull across the Moon is smaller than the drop