Our Moon is unique. Of all the planets, only the Earth has a moon just the right size and distance to barely cover the Sun. If the Moon were smaller, or farther away, all eclipses would be annular. In such a world, the solar corona and prominences would remain forever invisible. If the Moon were larger, or closer, the Sun would still be entirely blocked, but so, too, would the corona, and once more the phenomena that have fueled the curiosity of astronomers, astrologers, and philosophers throughout history would go unseen. In either case, the existence of helium and the process that fuels the stars would have eventually been discovered through some other means, but it is hard to imagine astronomers asking questions about phenomena we would never have seen.
No other planet has such a large moon in comparison to its own size. While both Jupiter and Saturn possess moons just as big, both of those planets are also hundreds of times more massive than the Earth. Venus, our planetary twin in composition and size, has no moon at all, while Mars has nothing but the two tiny rocks of Phobos and Deimos. Only Pluto and its largest moon, Charon, are comparable double-planets to our own (never mind that Pluto is no longer a planet), but out at a distance of fifty Astronomical Units from the Sun, Charon utterly dwarfs the Sun in apparent size. It all seems so improbable.
The story of how we came to have our Moon is interwoven with the history of eclipses. While our myths and legends are filled with lunar deities that delight us with their tales of its origins, influences, and reasons for going through phases, our scientific understanding of how it came to be begins with the story of where it is now. It’s a question of orbits.
Edmond Halley, whose story has been so linked to that of eclipses, was a good friend of Newton’S, whose gravitational studies he helped pay to have published. Halley used these new laws of Newton’s to calculate the orbits of everything from comets to the Moon, but to do so he needed a record of where they appeared in the sky and when.aa For the Moon, the best data comes from the times and locations of solar eclipses, since at that moment the Moon’s location is exactly known in relation to the Sun and the observer. String enough of these together, and the Moon’s orbit should become obvious. This is why Halley was so eager to have the public see and record the duration of the 1715 total eclipse over London.
But Halley also pored through historical records looking for other eclipses he could use to refine his calculations even further. A result of such a precise lunar orbit would be the accurate dating of eclipses in millennia-old Greek texts, pinpointing the exact dates for the great battles and events that had shaped civilization. Francis Baily, who would attempt to date the eclipse predicted by Thales in the Histories of Herodotus (and become famous for his own eclipse observations), concluded that the precision with which the unmistakable spectacle of a total solar eclipse could be calculated meant that “all attempts at imposition or deceit are easily detected by our knowledge of astronomy: and the unintentional errors of the historian are soon rectified and adjusted.”
Strangely, the farther back in time Halley and his contemporaries looked, the worse their predictions matched the historical records. Either the records were wrong about where the eclipses were seen, or the times were wrong for when they occurred. Neither ever quite fit. Halley makes reference to this problem in a 1695 paper on the wonders of the ancient city of Palmyra in modern war-torn Syria. On the very last page of his paper, he asks if his readers would be so good as to please determine the exact positions of Palmyra, Baghdad, Cairo, and other cities where ancient eclipses had been seen, so that he could clear up some issues he was having with his calculations.
A hypothesis is only as good as the observations that constrain it. It’s at the heart of the scientific method that the late nineteenth-century American astronomer Simon Newcomb was a principal force in promoting. He was a public advocate for the power that science, through a rigorous method of observation, hypothesis, and experimentation, could bring to bear on the mysteries of both space and time. He wrote, “The real test of progress is found in our constantly increased ability to foresee either the course of nature or the effects of any accidental or artificial combination of causes. So long as prediction is not possible, the desires of the investigator remain unsatisfied. When certainty of prediction is once attained, and the laws on which the prediction is founded are stated in their simplest form, the work of science is complete.”
But the future can’t be predicted if the past can’t be explained; it’s the heart of what a successful scientific theory must do. Newcomb felt that unless Newton’s laws could accurately date ancient eclipses, it would be impossible to accurately predict future ones and thus understand how the Moon behaved. Unfortunately, the quality of eclipse records hundreds and thousands of years in the past was not always of the quality necessary to pin down exactly where the Moon had eclipsed the Sun and by how much and at what time. Trying to tease the necessary astronomical information from dusty parchment scrolls written by scribes reporting on events possibly second- or third-hand was tremendously tricky work. According to Newcomb, “There are tens of thousands of men who could be successful in all the ordinary walks of life, hundreds who could wield empires, thousands who could gain wealth, for one who could take up this astronomical problem with any hope of success. The men who have done it are therefore in intellect the select few of the human race, an aristocracy ranking above all others in the scale of being.”
Newcomb was not a modest man, though he was largely self-taught from the backwoods of Nova Scotia. But he was unflinchingly dedicated to the power of physics, provided one was absolutely precise in one’s observations. During the mania over the putative planet Vulcan, he was a meticulous observer, and when he failed to detect any such planet, he was critical of others’ claims of discovery. Even Halley’s “citizen-science” timings of the 1715 eclipse fell short in Newcomb’s strictly mathematical eye. “Halley, notwithstanding his scientific merits in some directions,” he wrote, “seems to have been extremely unskilled in every branch in the art of practical astronomy.” So Newcomb sifted the historical records for solar and lunar eclipses recorded with precision, going so far as to travel to the Paris Observatory in the midst of the Franco-Prussian War to recover the original observation logs for eclipses that had occurred during the previous two hundred years. With bombs bursting in the distance, he confirmed what earlier astronomers as far back as Halley had suspected: the Moon is accelerating. His new calculations derived from the best observations yielded the most precise value yet for just how much. And just as important, he realized that part of the problem was that the Earth itself was slowing down, so much so that with every century, our days grow two milliseconds longer. This is really what Halley had discovered when the time of day he calculated for ancient eclipses didn’t quite match with when or where they had been seen.
Lunar tides are why this is happening. Since gravity grows weaker with distance, the Moon attracts the Earth by different amounts from one side of our planet to the other. Rock doesn’t deform much—at least not under the Moon’s feeble mass—but water is fluid. The Moon therefore raises two liquid bulges on the Earth, one on the near side—as the oceans are pulled up from the planet—a second on the far side, as the planet is pulled out from underneath the sea. As the Earth spins on its axis, shorelines encountering these bulges experience two high and low tides every day. For any person standing on the seashore, the evidence for astronomy is present in the perpetual flow of the tides.
Since the Earth turns faster than the Moon orbits, friction with the seafloor drags the tidal bulges along with it. The greater the friction between seafloor and sea, the more energy the Earth loses and the slower it turns. The continental shelves, like the shallow Bering Strait, where North America and Asia once connected, are places of particular friction; they are the continent-sized speed bumps that the tidal bulges keep hitting. The effect, however, is not solely on the Earth. As the seafloor drags the bulge along with it, its gravity leads the Moon, pulling it forward. This constant acceleration sends the Moon spiraling away. Since the two effects are tied together, the rate at which the Earth slows down mirrors the rate at which the Moon grows more distant. Reflectors placed on the Moon by the Apollo astronauts confirm that today we are losing the Moon at a rate of 1.5 inches per year, about the same rate as fingernails grow. While that may not seem like much, it’s the first piece of evidence to suggest how the Moon once formed.
George H. Darwin, son of Charles Darwin, was the first to mathematically work out the evolution of these tides. In 1878, based on the Moon’s rate of recession, he found that it was only 60 million years ago that the Earth and the Moon must have been in contact—with the Moon orbiting a rapidly spinning Earth, and each taking only six hours to complete a full turn. To Darwin, the implication was clear: the Earth and Moon were once a single, rapidly spinning body that split into two when a chunk of the crust was flung into space. As evidence, he wrote of the tidal forces from the newly formed Moon, which he said would have raised the great ridges of liquid rock on the Earth that we now see as continents. The Pacific Ocean was the basin left over when the Moon split away.
The sciences of radiometric dating and continental drift wouldn’t be proposed for another three decades, so Darwin had no way of knowing the Earth was much older and the continents much younger than his physics suggested. In addition, later physicists could find no plausible mechanism that could get the Earth to spin so fast that it would break into two. Still, even as late as the 1970s, the fission hypothesis found some supporters, including the American chemist Harold Urey.
Urey, a Nobel Laureate at the University of Chicago, was one of the first to propose that the Apollo moon rocks could be used to understand how the Moon and solar system formed. By understanding the mechanism by which planetary bodies formed, he hoped to be able to say how rare they might be in the cosmos. By the end of the Apollo missions, three principal findings had come out of the rocks the astronauts brought back. One was that lunar rocks have low amounts of volatiles, elements that can be easily vaporized or boiled away. Second, lunar rocks are depleted in metals. On average, the Moon has much less metal than the Earth and a metallic core that, if it exists, is smaller in relation to its size than the Earth’S. Both of these conclusions speak to a world somewhat different from our own. The third finding was that the ratio of certain elemental isotopes—oxygen, in particular—was almost identical on the Earth and the Moon.bb Since oxygen isotope ratios in Earth rocks are wildly different from those in meteorites from the asteroid belt or from Mars, this similarity tells us that the Earth and the Moon must share a common event in their origin.
By the end of the Apollo missions, no single hypothesis had yet been proposed to explain all of these findings. Some scientists thought the Earth and the Moon formed together, while others thought the Earth must have captured the Moon from elsewhere; some, like Urey, still favored a Moon that formed out of the Earth. Into this confusion of hypotheses, the astronomers Alastair Cameron and William Ward proposed a revolutionary idea in 1976: What if the Moon really had split off from the Earth, but for a reason that had nothing whatsoever to do with the Earth’s rotation? Cameron was a Canadian astrophysicist who had written extensively on the origin and evolution of planets and stars. He and Ward proposed that, yes, the Moon had spun away from the Earth, but only after the Earth was hit by another planet. The Moon is the result of the violence of worlds.
As much as their idea may seem the stuff of science fiction, it does follow naturally from how astronomers think planets should form. Stars and planets coalesce out of clouds of spinning gas and dust. Over time, dust in these cosmic clouds is drawn together by gravity. Over millions of years of matter colliding and sticking, small things grow to become big things. Within such a hypothesis, the last stage of such collisions would surely have seen a few large impacts from nearly planet-sized objects, called planetesimals. The impact that formed the Moon was simply one of the last our planet experienced.
Imagine the Earth four and a half billion years ago as a hot, rapidly spinning world of glowing rock. Inside, the majority of high-density metals had already settled into a molten core, leaving lower-density rock floating on top to form a mantle and a crust. On such a hot world, volcanoes would have spewed lava across a barren surface, their plumes releasing the first gases that would form an atmosphere. Someone standing on such a hellish world, looking high overhead, would have seen a star-filled expanse in which no moon yet existed. Then one night, one of those millions of stars was noticeably brighter than it had been the night before (although the time between sunsets was no more than five hours). Soon, had anyone been there to see it, the star would have become a disk the same size as Mars. Planetary scientists have nicknamed this world Theia after the mother of Selene, the goddess of the Moon in Greek mythology.
As the two worlds approached each other, the tidal forces between them would have grown so much that as our planet bulged outward, earthquakes would have rocked the landscape. On the fatal final day, when the two planets struck in a glancing blow, one entire hemisphere of Theia would have instantly vaporized, exploding outward and carrying massive amounts of molten debris into orbit around the now rapidly coalescing remnants of the two previous worlds. The new planet that was created on that day is the Earth on which we all now live, and for a brief period it possessed a ring out of which our Moon eventually formed.
This is a hypothesis that explains all that has been seen. From a glancing blow, computer models reveal that the Moon would have formed out of the relatively low-density elements in the mantles of the two planets. The violence of the collision would have vaporized all the volatiles in the newly created Moon, including most of its water. Since the two new worlds formed from the same hot molten mix of planets, the Earth and the Moon now exhibit nearly identical isotope ratios.
Had that been the last major collision our Earth ever encountered, our Moon would now orbit in the same plane as the Earth around the Sun, and we’d experience solar eclipses every month. But recent computer models reveal that over the next few tens of millions of years, any remaining solar system debris nearly as large as the Moon that passed too close to our double-world would have contributed just enough of a gravitational tug to send the Moon’s orbit tilting at the angle at which we now find it. The evidence for these close flybys and occasional impacts is found in our precious metals. Precious metals like platinum and gold react strongly with iron. On a molten Earth, high-density iron would sink to the core and take these elements with them. But the collision of even half a dozen lunar-sized objects following the formation of our Moon would have brought just enough additional gold and other precious metals to litter our surface in the amounts we now see. In the words of the planetary scientist Robin Canup, “Had such a population of objects not existed, the Moon might be orbiting in Earth’s orbital plane, with total solar eclipses occurring as a spectacular monthly event. But our jewelry would be much less impressive—made from tin and copper, rather than from platinum and gold.” Eclipses are literally worth a world of wealth.
From the moment of the Moon’s creation, as the bodies cooled and tidal friction grew, the rapidly spinning Earth slowed and the Moon began its long spiral away. We will never lose the Moon. But a day will come when the Earth has slowed so much that it turns at the same rate as the Moon orbits, and a day on Earth will be as long as a month. The Earth will be tidally locked to the Moon as the Moon is now locked to the Earth, and a single hemisphere on each world will forever stare at the other. When that happens, both the Moon and the Earth as seen from the other will never move in the sky, never rising or setting.
By that time, the Moon will be so far away that it no longer fully covers the Sun—the last total solar eclipse will have occurred. At the rate the Moon is currently receding, we only have another 563 million years of total solar eclipses left to us. That last eclipse will be a brief one, the Moon’s disk only barely the size of the Sun that it’s covering. Perhaps totality will be no more than a second in length, and every one after that, for the rest of the Earth’s life, will be no more than an annular eclipse. The spectacle will be gone. The total solar eclipses we now see—that were once an omen of terror, that then became a scientific tool, and that have now turned into a tourist attraction—will have been just a transitory phase in the life of our planet.
Ironically, for a phenomenon that we can talk about with such great certainty, where coronaphiles can make plans to see one decades in advance, it’s impossible to say the exact date and location of that last ever eclipse. The reason is that over its lifespan the rate at which the Moon recedes has varied with time. At its current, precisely known rate, only 1.5 billion years ago the Moon should have been so close that the Earth’s mantle would have melted and the Moon ripped apart. Needless to say, there is no record of this happening. The only solution is that the Moon must have been receding at a slower rate in the past. Evidence from ancient seafloor sediments shows that the length of the day has similarly fluctuated over billions of years.
All of this is due to the slow motions of continents and seafloor altering the rate at which friction slows down the Earth and speeds up the Moon. As it has changed in the past, so, too, will it change in the future, and the exact rate of recession becomes impossible to predict. But thanks to the long chain of human beings who have watched the Moon and the Sun and divined the patterns in their eclipses—including Halley and Newcomb, who used the timing of eclipses to discern the subtle motions of the Earth and the Moon—the science is clear that such a day will eventually arrive. Enjoy an eclipse while you can.
Newcomb once wrote, in relation to the predictive power of science, that “whenever the subject becomes so well understood that the chain of natural causes can be clearly followed, miracles and final causes cease, so far as the scientific explanation of things is concerned.” It is ironic, then, that in an echo of the “God of the Gaps,” by which we once attributed eclipses to demons and gods, there are those who still attribute eclipses to a deity. They see proof of a world designed by a Creator in the size and distance of our Moon, and in the fact that eclipses will have occurred on this planet only during a time when an intelligent species was here to behold it.
Some proponents of this idea go so far as to claim that proof of its truth will be found by looking for intelligent life only where there are moons large enough to cause eclipses; wherever one finds eclipses, according to this theory, one will certainly find intelligent life.cc I suppose this bodes well for NASA’s upcoming mission to Jupiter’s moon Europa, as all four of its Galilean satellites are at the right size and distances to eclipse one another every six years.
Interestingly, given the importance some creationists place on its existence, our Moon may actually play a crucial role in the answer to the question of how rare life may be. Were it not for our Moon, then 4 billion years ago there would have been few tide pools to harbor the “primordial soup” of complex organic molecules that chemists like Urey and Carl Sagan studied as the building blocks of life. Once formed, these organics needed millions, then billions, of years to make the transition from soup to sentience. That is a journey that requires a hospitable planet, constant in its climate over great spans of time.
Tidal forces are the reason our climate has remained relatively constant over billions of years. Mars has no similar-sized moon, and recent evidence shows that its obliquity, the angle its pole tilts relative to its orbit, changes chaotically with time. This is important because a planet’s obliquity determines the severity of its seasons. Right now, Mars’s obliquity is nearly identical to Earth’S, but models show that over millions of years Mars’s tilt has wandered from as little as 0 degrees to as high as 80 degrees. These changes have made Mars’s summer seasons so warm that water could flow across its polar surface, while creating winters so fierce that glaciers buried the surface beneath miles of ice. This changing obliquity is due to the gravitational tug of the rest of the solar system, primarily the Sun and Jupiter. We are affected by them as well, but our Moon’s tidal embrace keeps our planet in check. It’s hard for the tidal bulge around our equator to wobble too much if the Moon is always there to pull it back into place. The result is that the obliquity of our planet, first likely set in the collision that formed the Moon, hasn’t changed by more than a degree over the past half million years. Even so, these tiny shifts have helped give rise to a cycle of gradual ice ages, the most recent of which ended only 10,000 years ago. As great as these ice ages may seem, imagine what would have happened had our poles tilted even more.dd Intelligent life on Earth might only be possible in the presence of a moon like our own.
This is a sobering thought, given what such a chance occurrence our Moon may have been. What does it mean that we may only be here because of an accident? People criticize science for making us small, for taking us from the center of creation and demoting us to tiny inhabitants in space and time of a vastly larger impersonal universe.
Yet human history is inextricably linked with our need to accurately understand the world in which we live. From the time of our earliest ancestors, as they hunted for food and then developed agriculture, we would not have survived to the present without having unlocked the mysteries of the seasons and the clockwork patterns of the sky. This is a process that need not be opposed to religion, as astronomers going back to Kepler (a Protestant) and Galileo (a Catholic) attest. The current director of the Vatican Observatory, Brother Guy Consolmagno, a Catholic Brother and PhD planetary scientist, once told me that the proof of God is in the fact that the universe is knowable.
So what do we know? We know that we occupy a special place, on a special planet, with a special moon, during a special moment in history. We are privileged to inhabit a habitable world that has been capable of sustaining complex life over billions of years, thanks to a moon created by a chance event. But that chance event is a natural part of how solar systems form—and we now know of over a thousand others out there in space, with more being discovered every year.
All this we know, in part, due to eclipses. Through these momentary alignments, the inhabitants of this planet—in ancient Chinese palaces, Babylonian observatories, Mediterranean cities, and Mayan temples—all deduced that our world is one of repeating patterns that are knowable. From these beginnings we have followed the path of totality to every corner of the Earth to learn more about the Moon, the Sun, and a galaxy of planets. The secrets of the universe have been revealed by shadows stretching over the light-years between stars. Because of these shadows, we now have a galactic context in which to understand how common planets may be, while realizing, as we look around at our own solar system, how utterly inhospitable most planets are. While we may not be alone in the universe, we are still precious, as is the world that sustains us. All this we see when the Sun disappears behind the Moon and for a brief moment each one of us is aligned with the heavens.
a This is how he found that observations of what appeared to be multiple comets appearing over a period of centuries was in fact a single comet returning every seventy-six years. Now known as Halley’s Comet, it is the most famous comet in the world.
b Different elements have different numbers of protons in their nucleus. Different isotopes of a single element have the same number of protons, but different numbers of neutrons.
c Google “recession of the Moon” and seven of the first ten results to come up are from creationist websites.
d Changes in our obliquity, orbital eccentricity, and precession of the equinoxes are all part of what is known as the Milankovitch cycle of climactic change. They lead to only slow changes in climate over thousands of years, not the changes the Earth is experiencing now over the past two hundred years.