More notes from my reading.
A couple of passages from Chris Impey, University Distinguished Professor and Deputy Head of the Department of Astronomy at the University of Arizona, taken from his book How It Began: A Time-Traveler’s Guide to the Universe (New York and London: W. W. Norton and Company, 2012):
It’s a dark family tale. Gaia is born on cue and emerges shrieking into the night. Her twin sister Luna is born slightly later, after having been starved of nutrients inside the womb. Luna is a runt, small and fragile. She’s active for a while but then she fades away. Gaia can’t bear losing her sister so she keeps her close, even after her body becomes lifeless and her skin pockmarked with time. Determined to avoid the fate of her sibling, Gaia lives life hard. A woman now — sad and sweet — she’s always making herself over in a frantic and unbecoming effort to hang on to her youth. Yet perversely, the proximity of her dead twin keeps her stable and stops her from becoming completely unhinged. (14)
You may have guessed that this is a depiction of the relationship between the Earth (Gaia) and its Moon (Luna).
Impey proceeds to recount the widely-accepted scientific story of the origin of the Moon as the product of materials ejected from the still-young and molten Earth following a glancing collision with a passing Mars-sized rock.
A chance encounter 4.5 billion years ago played a significant role in making the Earth hospitable for advanced life. Tides create a unique transition zone where creatures can experiment with the adaptations needed to move from the sea to the land. The Moon stabilizes Earth’s tilt axis. Without a similarly large Moon the axial tilt of Mars wobbles from zero to 60 degrees. The result is that the Earth has less extreme climatic variations. When it was closer to the Earth, the Moon helped generate the tectonic crust that life needed to get established.
It’s dangerous to push these arguments too far, but the early impact dusted the Earth’s mantle with metals, without which humans might not have progressed beyond flint arrows and stone wheels. We’ve already seen that lunar cycles spurred thinking about regularities in nature. The Moon was pivotal in the ancient Greeks’ awareness of the Earth as a sphere, in Galileo’s concept of the “plurality of worlds,” and in the first test of Einstein’s theory of gravity.
The story of the Moon’s creation spurs the question, What if? With a slightly different trajectory, the Mars-like object would have sailed harmlessly by. Subjected to more violent extremes of climate, the planet might never have become hospitable for the hairless ape that can only survive a slender temperature range. And if the collision had been more direct, the Earth would probably have been obliterated rather than augmented. It’s sobering to think that our existence hinges on serendipity in the chaos of the early Solar System. (16-17)
And here’s another scientifically useful function of our Moon:
The near-perfect blotting out of the Sun by the Moon only takes place because the 400-times-larger Sun just happens to also be 400 times farther away. Creatures in distant solar systems are probably denied this spectacle. (12)
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