Robert M. Hazen

“No direct evidence yet documents Earth’s tidal cycles more than a billion years ago, but we can be confident that 4.5 billion years ago things were a lot wilder. Not only did Earth have five-hour days, but the nearby Moon was much, much faster in its close orbit, as well. The Moon took only eighty-four hours—three and a half modern days—to go around Earth. With Earth spinning so fast and the Moon orbiting so fast, the familiar cycle of new Moon, waxing Moon, full Moon, and waning Moon played out in frenetic fast-forward: every few five-hour days saw a new lunar phase. Lots of consequences follow from this truth, some less benign than others. With such a big lunar obstruction in the sky and such rapid orbital motions, eclipses would have been frequent events. A total solar eclipse would have occurred every eighty-four hours at virtually every new Moon, when the Moon was positioned between Earth and the Sun. For some few minutes, sunlight would have been completely blocked, while the stars and planets suddenly popped out against a black sky, and the Moon’s fiery volcanoes and magma oceans stood out starkly red against the black lunar disk. Total lunar eclipses occurred regularly as well, almost every forty-two hours later, like clockwork. During every full Moon, when Earth lies right between the Sun and the Moon, Earth’s big shadow would have completely obscured the giant face of the bright shining Moon. Once again the stars and planets would have suddenly popped out against a black sky, as the Moon’s volcanoes put on their ruddy show. Monster tides were a far more violent consequence of the Moon’s initial proximity. Had both Earth and the Moon been perfectly rigid solid bodies, they would appear today much as they did 4.5 billion years ago: 15,000 miles apart with rapid rotational and orbital motions and frequent eclipses. But Earth and the Moon are not rigid. Their rocks can flex and bend; especially when molten, they swell and recede with the tides. The young Moon, at a distance of 15,000 miles, exerted tremendous tidal forces on Earth’s rocks, even as Earth exerted an equal and opposite gravitational force on the largely molten lunar landscape. It’s difficult to imagine the immense magma tides that resulted. Every few hours Earth’s largely molten rocky surface may have bulged a mile or more outward toward the Moon, generating tremendous internal friction, adding more heat and thus keeping the surface molten far longer than on an isolated planet. And Earth’s gravity returned the favor, bulging the Earth-facing side of the Moon outward, deforming our satellite out of perfect roundness.”
― Robert Hazen, The Story of Earth: The First 4.5 Billion Years, from Stardust to Living Planet