The moon is drifting away from us.
Each year, our moon moves distinctly, inexorably farther from Earth—just a tiny bit, about an inch and a half, a nearly imperceptible change. There is no stopping this slow ebbing, no way to turn back the clock. The forces of gravity are invisible and unshakable, and no matter what we do or how we feel about them, they will keep nudging the moon along. Over many millions of years, we’ll continue to grow apart.
Given this rather melodramatic description, you might wonder: Don’t you have better things to think about than the moon? Well no, not really, because I’m a space reporter and it’s my job to contemplate celestial bodies and write about them. And also because a representation of this phenomenon recently played out in China during festivities for the Mid-Autumn Festival, which marks the full moon closest to the fall equinox. A giant balloon designed to resemble the moon, craters and all, broke free and rolled into the street. Video footage of the unscripted moment shows two people running after the massive moon as it tumbles away. Bye!
The moon used to be closer. When it first formed, about 4.5 billion years ago, molded out of rocky debris that had been floating around Earth, the moon orbited 10 times nearer to the planet than it does today. The debris, scientists believe, had come from a collision between Earth and a mysterious Mars-sized object. Fresh out of the cosmic oven, the moon was hot and molten, glowing red in the night sky. Back then, scientists say, the moon was moving away at a rate of about eight inches per year.
Our planet and its moon were always going to grow apart like this. The gravity of moons, small as they are in comparison, can still tug at their planets, causing the larger worlds to bulge outward a little bit. On an ocean-covered planet like ours, the effect shows up in the shifting tides. The moon pulls at our oceans, but those oceans pull back, making the moon speed up in its orbit. And “if you speed up while orbiting Earth, you are escaping Earth more successfully, so you orbit from a farther distance,” James O’Donoghue, a planetary scientist at JAXA, Japan’s space agency, explained to me. Scientists refer to this phenomenon as “lunar retreat”—a delightful term, as I’d prefer to imagine the moon enjoying itself at a relaxing getaway, bending its rocky body into various yoga poses, rather than slowly ghosting Earth.
Scientists have measured this retreat by beaming lasers at mirrors that the Apollo astronauts left on the moon, using that data, along with other sources, to estimate past movements. The rate of lunar retreat has shifted over the years; spikes have coincided with significant events, such as a bombardment of meteors on the moon and fluctuating ice ages on Earth. The constant retreat has influenced Earth beyond the ebb and flow of its tides. The forces that draw the moon away from us are also slowing down the planet’s rotation, stretching out the length of our days. In the beginning, when the moon was cozying up to us and Earth spun faster, a day lasted just four hours. At the current rate of lunar retreat, it would take a century to tack on an extra two milliseconds or so to the length of the day.
The moon is expected to continue drifting this way for the very scientific measure of forever. And, despite the premise of an upcoming action movie called Moonfall, it’s not going to smack into us either. Someday, about 600 million years from now, the moon will orbit far enough away that humankind will lose one of its oldest cosmic sights: total solar eclipses. The moon won’t be able to block the sun’s light and cast its own shadow onto Earth. But the moon will remain bound to Earth, looking out onto a very different, much hotter version of the planet, as oceans start to evaporate. Of course, a few billion years after that, the sun will derail the moon entirely, and Earth too, when it runs out of fuel, expands, and engulfs the inner solar system in a spectacular act of star death.
This weekend, I looked through a telescope for the first time, into a much calmer solar system. (I know, right? Some space reporter I am!) A neighbor had set one up on my building’s roof, and I tried to pay attention as he explained the different lenses and their amplification capacity, but I was too excited, thinking only, Let me see, let me see. I had seen the moon just as a bright two-dimensional orb in the sky, with dark spots that play tricks on our brains, making us see familiar patterns where none exist. People have interpreted these glyphs in many ways: a human face, the silhouette of a rabbit. What has the moon seen in us? “The moon had been observing the earth close-up longer than anyone,” the Japanese writer Haruki Murakami wrote in his novel 1Q84. “It must have witnessed all of the phenomena occurring—and all of the acts carried out—on this earth.” The moon is still watching. What must it be thinking now, after such a horrid year and a half?
My neighbor swiveled his telescope across the cloudless sky. There was Jupiter and its twisty bands, faint but unmistakable, and three tiny points of light just off to the side—its largest moons. There was Saturn, a perfect ball, its rings sticking out at each side. And then there was the moon: covered in craters and cracks and shadows, so richly textured that the skin of my fingertips prickled at the sight, as if I were rolling the moon around in my hand like a marble, feeling its jagged edges. I decided not to spoil the moment for everyone else on the roof that night by telling them that the moon was, slowly but surely, distancing itself from us. The experience of distance—from our families, from a time of relative normalcy—had already tormented many of us enough. Better to focus on the little image in the lens, on seeing the moon properly for the first time. It may be wishing Earth a very long goodbye, but it was nice to say hello.