Book Review: The Secrets of Earth’s ‘Silvery Sister’

By Angela Chen, a science journalist whose work has appeared in The New York Times, The Wall Street Journal, The Atlantic, The Guardian, and National Geographic, among other publications. She is the author of “Ace: What Asexuality Reveals About Desire, Society, and the Meaning of Sex.” Originally published at Undark. 

In the beginning, according to the Book of Genesis, God created the heavens and the Earth. First there was light itself, separated from the darkness; then, on the fourth day, God specialized and created the “greater light” of the sun and the “lesser light” of the moon, in that order. Ever since, the moon has seemingly been relegated to this inferior position. Even the Apollo program that landed humans on the moon was named for a Greek god of the sun.

Yet no matter how much inhabitants of this planet may focus on the star we orbit, the moon is not incidental and its fate is not separate from ours. It is, as science journalist Rebecca Boyle writes in “Our Moon: How Earth’s Celestial Companion Transformed the Planet, Guided Evolution, and Made Us Who We Are,” “more sibling than subordinate.”

The moon has changed the balance of light and dark in our days, the musculature of our bodies and capacities of our minds, and acted via the tides as a significant factor in battle. In a book that spans science, culture, and history, Boyle beautifully and convincingly argues that this quietly luminescent satellite, “a crater-pocked wasteland that smells of doused firecrackers,” is our “silvery sister” and “Earth’s biographer, its first chronicler, and its most thorough accountant.”

At first glance, this wasteland has little in common with the lush and roiling contours of our watery world. There, crater shadows confuse the eyes, all is dust, daylight lasts for weeks, and humans’ ability to know up from down becomes warped by gravity one-sixth of that of Earth. Still, to call the moon our sister is more than poetic license. The astronauts sent to the skies brought home rocks revealing that the moon and Earth are partly made of the same stuff.

Ever since, scientists have been refining theories for the moon’s origin. According to one, a Mars-sized object called Theia collided into proto-Earth on an apocalyptic day about 4.5 billion years ago. Both structures shattered, then spun so quickly that they transformed into a doughnut-shaped rock-lava cloud called a synestia. From the synestia emerged both the moon and the Earth in its more-or-less current form.

Had it gone another way, much about our blue marble would be altered. Without the moon’s gravity to act as heavenly defender, Jupiter “would push Earth around like a playground bully,” Boyle writes, and Earth’s axis, currently at a fairly stable 23-degree tilt, could tip to 85 degrees, or zero.

Absent the moon, our bodies and minds might be unrecognizable. Without tides, our swimming ancestors may never have gone upright. (Some scientists speculate that during the Devonian period the moon, then 5 percent closer to Earth, caused stronger tides that beached fish and pushed them to walk on land and breathe air.) Menstrual cycles may not have developed to last about 28 days, so strongly correlated, that some people’s periods start to synchronize with the moon before they get pregnant — even though the exact mechanism remains unknown.

And, for some creatures, the biological understanding of time would lack a key feature. Aristotle, writing in 350 BC, noted that the lunar cycle influenced the size of sea urchin eggs. Two thousand years later, scientists have learned that it is the moon that coordinates the mating schedule of corals, which are just one of the creatures that exhibit this circalunar clock.

The strength of “Our Moon” comes from its gorgeous writing and vast scope in both time and space. The book’s final section on more recent intellectual history trods familiar ground — perhaps unavoidable because its subjects (Kepler, Copernicus) are such giants of Western science — but in other chapters Boyle takes readers along more far-flung trajectories.

An image of Earth and the moon, taken from Mars with a powerful camera called HiRISE. On average, the moon orbits the Earth from about 239,000 miles away. Visual: NASA/JPL-Caltech/University of Arizona

We follow her, historically, to Babylon, where the Stela of Nabonidus offers a glimpse into moon worship, then to the Johnson Space Center’s Lunar Sample Lab to look upon those rocks brought back from that orb. We tag along as Boyle treks to Scotland’s Warren Field, where Mesolithic pits form a calendar that tracks both the phases of the moon and the winter solstice, thus providing clues about when humans discovered the very concept of time. We trace the saga of an archaeologist working to take down smugglers and obtain the Nebra sky disk, a treasured depiction of the firmament that could rewrite the history of the Bronze Ages.

In today’s age of private spaceflight and prospective lunar mining, our moon remains a symbol of shifting and sometimes conflicting values. Debates over funding for space exploration projects in a rapidly heating world echo Apollo 11-era discussions with the same tenor: Right before the 1969 mission began, Black protesters led by Ralph Abernathy gathered at the Kennedy Space Center to call attention to the poor Americans who could be helped by money from the space program. The hope was not that NASA would scrub the launch but that it would also consider the problems on this planet and not lose itself in other worlds. Though the NASA administrator at the time, Thomas Paine, reportedly told Abernathy that his agency’s work was “child’s play” compared to the challenges of addressing civil rights issues, ongoing arguments about both the future of the moon and the future of Earth only highlight that little has been resolved.

Apollo 11 showed the world that we had nothing to fear from our celestial companion. Moon dust did not, as some feared, react badly with oxygen. The lunar surface was not crawling with alien pathogens ready to devastate Earth. The astronauts, after quarantining for two weeks back on Earth, were fine. Their precious rocks were quarantined too — then carefully exposed to plants and shrimp and fish and birds. Nothing happened to the creatures. Arguably, today the greater question is what the moon has to fear from us.

“We came in peace for all mankind,” reads the plaque that the Apollo 11 astronauts put on the moon, but the intentions of those who come to the moon now have not remained as idealistic.

At the end of last year, the president of the Navajo Nation asked NASA and the U.S. Department of Transportation to postpone a private mission putting human remains on the moon. To do so, the tribe argued, would profane a sacred object. So even as the religious and cultural significance of the moon endures for some, for others, the moon is now a place “to build, to extract, to maybe get rich or die trying,” Boyle writes.

The 1967 Outer Space Treaty technically prevents nations from owning any part of the moon, but a clause banning countries from interfering with each other’s bases creates a convenient loophole. Far from being a place for all, the moon remains a site of geopolitical struggle as China, America, and India all rush to return and control, notes Boyle. Everyone wants a piece. Scientists advocate for further exploration; NASA hopes to land humans on the moon again in 2026; private companies and even the European Space Agency dream of the lunar economy and a Moon Village. Who gets to decide?

The moon has always been with us, changed with us, changed us. Now, as humanity’s grasp continues to expand, it’s time to make decisions about the moon’s legacy and its future, ideally ones that protect it from overreach. “The moon will accompany us forever,” writes Boyle, “but it will be lonely forever, if we treat it right.”