Small Talk #8 - The Moon
The new year is a great time for small talk here in the upper Midwest. On short days and cold nights, we huddle around cafes or taverns or fireplaces, shooting the breeze and spinning yarns. This time of year invariably has us thinking about cycles: the Sun creeps, the trees sleep, the birds wait, the calendar turns. But nothing is more cyclical than our only natural satellite, Luna, queen of the night, goddess of the tides. So let’s have a chat about the Moon.
The Moon
When we consider an interesting topic, we usually have to choose between science and fable. There is the analytical explanation of things, and then there is the magical. But in the case of the Moon, they are one and the same. No one knows exactly how the Moon came to be, but it almost certainly emerged from a cosmic embryo that also produced the Earth. The Moon and Earth are so similar in chemical and geologic composition that they must have a shared origin. The prevailing theory is that another celestial body—say, a tiny planet—crashed into the proto-Earth, cleaving from it a mass that, over time, became the Moon.
And so the two bodies evolved together, each shaping the other’s development. In the solar system, our moon is the largest satellite relative to its planet. So while Jupiter reigns supreme over its 69 little moons, the Earth is engaged in a singular dance with our Moon, and without each other, we would cease to be what we are. Consider this: the Moon always has the same side facing the Earth. Its rotational period (the time it takes to spin on its axis) is the same as its orbital period (how long it takes to orbit the Earth). Contrary to legend and prog-rock album titles, the side of the Moon we don’t see is not dark. It’s also illuminated by stars, so from our viewpoint, it is simply the far side of the Moon.
By human standards, the Moon got the bad deal in its separation from Earth. Although the Moon, like our planet, has an atmosphere and a magnetic field, neither are substantial enough to protect it from a cosmic beating. The craters you see on the Moon today are ancient, many of them formed about 4 billion years ago, during a period scientists call the “Late Heavy Bombardment,” when an arsenal of asteroids and meteors pounded the solar system. The Earth wasn’t spared this abuse, but as our atmosphere developed, it created conditions necessary for oceans, life, and weather—which washed away the consequences of that bombardment. The Earth can heal. The Moon, however, must bear its ancient scars.
Structurally, the Moon is like an elegant piece of truffle candy. It probably has a solid iron core, surrounded by a molten outer core, surrounded by a mantle of basalt, which is draped in a powdery crust. If you were to pop that candy Moon into your mouth, you might be surprised to find that it moves! That’s because the Moon is host to a lot of seismic tremors. Some are hundreds of miles deep, brought on by the stresses of Earth’s gravity. Others occur because of extreme lunar temperature changes. But the most interesting moonquakes take place near the surface, as strong as 5.5 on the Richter scale. That would be a powerful quake on Earth, but here they don’t last for long because water in the crust acts as a damper. On the Moon, where there is relatively little water, these surface quakes can continue for as many as ten minutes. The Moon sings like Tibetan bowl.
Most of us would describe the Moon as white. But that’s mostly because it’s reflecting light. Really, much of the Moon’s surface is dark colored, thanks to the preponderance of that basalt, a form of volcanic rock also common on Earth. Light travels 252,000 miles from Moon to Earth, but it only takes about 1.3 seconds. We all know that the phases of the Moon are caused by sunlight reflecting off its surface. Except during a lunar eclipse, half of the Moon is always illuminated by the sun. We see a partial illumination—its phases—depending on the angle we’re viewing from. But have you noticed that sometimes you can see part of the Moon ‘in shadow’ next to the illuminated crescent? That pale section of the Moon is caused by ‘earthshine,’ which is light reflecting off the Earth, onto the Moon, and back to us.
We are locked in a gravitational embrace, we two children of that cosmic collision. Technically speaking, and poetically as well, the Earth and the Moon orbit around a common point that lies between them. The point is within the Earth’s radius, but it is far from the planet’s center. So we are constantly whirling in a dance with our moon. Its gravity tugs at our oceans, causing the tides. Water piles up on the side nearest the moon, and drains from the opposite side. Meanwhile, the Earth’s gravity tugs back at the Moon, and over billions of years, it has deformed the Moon, creating a ‘mass concentration’ on one side, which is why that same side always faces us.
At one time, the Moon was much closer to Earth. Perhaps not so close as Calvino’s tale, where people built ladders high enough to step onto the lunar surface. But still, it dominated our sky. In those days the moon spun faster and all its sides could be seen on Earth by whoever might have been looking. Speaking of which, there is a theory that the Moon is partly responsible for life on Earth, as the earliest organisms probably evolved in mucky tidal flats, created by the Moon’s gravity. Perhaps those early life forms marveled at this great shining benefactor.
But just as the Moon was once closer, it is also slowly drifting away from us. I read that the moon is receding at the same speed as fingernails grow. That seems a bit anthropocentric, but then the Moon does seem too coincidental for a random universe. Think about this: On Earth, we experience complete solar eclipses because the Sun is 400 times the diameter of the Moon, just as the Moon is 400 times closer to Earth. That is, it’s aligned so the Moon covers the Sun perfectly. This doesn’t mean there’s some great plan (and if there were, we would avoid it in small talk). But it certainly is wonderful.
I think we feel wistful when we look at the Moon because we know it’s slowly drifting away. Goodbye, dear Moon, and thanks for all you’ve given us. Sorry we didn’t give you a proper name, but instead just called you Moon. That would be like calling a female child Daughter. Which my wife and I did not do, though we had three chances. When our oldest daughter reads this, she might remember a winter’s night in a brick courtyard in Chicago, when she was wrapped in a blanket so tightly—leaving only a hole for her enormous eyes, which reflected the mysteries of the firmament, and she said her first word. That word was Moon.
Now I really should let you go. Tonight the Moon is a waning crescent, its illumination at 19 per cent. I’ll probably step outside and have a look. Maybe I’ll see you there.
Have a good one,
Kipling Knox
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