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Macmillan Childrens Publishing Group

What If the Earth Had Two Moons?

And Nine Other Thought-Provoking Speculations on the Solar System

Neil F. Comins

St. Martin's Griffin



What If the Earth Had Two Moons?
They came in the dead of night. One moment the bedroom was filled with the sounds of forest animals behind the village, the next with the deep shouts of men, rattling of heavily armored horses, and the ominous creaking of a carriage. A whip snapped; dogs began yelping; the leader of the group issued orders. They stopped right in front of his cottage. Lying in bed he heard his neighbors shuttering their windows and barring their doors. He knew that such actions would not protect them. The intruders would get in anywhere they wanted. As he finished this thought, his front door flew off its hinges, landing on the floor with a brittle crash.
He was up now, but as he moved to get out of bed, his mistress grabbed his arm. She was trembling, her eyes wide with the same terror he felt, but which he hid so she would not fear the worst. "Why?" she asked, hoarsely, softly.
"I don't know. Maybe," he smiled wanly, "I forgot to pay our taxes."
"They wouldn't be here for that."
"It was ajoke," he said, lightly. She gave him the "not now" look, but he missed it as the stairs were filled with the clattering of hobnailed boots. Doors opened and slammed shut. The children began crying.Then their bedroom door opened and an officer, followed by four soldiers, strode in.
"Galileo Galilei?" the officer demanded.
Galileo nodded. Without taking his eyes off Galileo, the man issued an order. "Take him."
"You will leave her and the children?" Galileo inquired, meekly.
"We only have orders to arrest you," the officer said, adding, "in the name of the Holy Inquisition."
If there is any justice in the world it occurred then, as a cluster of meteorites burst through the roof. A pair of them plunged into two of the soldiers, who fell dead.
Galileo laughed. The officer cursed and said, "Damned nuisance." Then he turned to the remaining soldiers and ordered them to take Galileo. Marina screamed as they dragged him out of bed. Galileo watched her, and their arms reached out to each other and then separated in slow motion.
The prison carriage, merely a cage on wheels drawn by a sickly horse, clattered and clanged as it carried the nightshirt-clad Galileo through the town and up to the castle. His body glowed red in the light of the moon Lluna shining and spurting molten rock overhead. He saw the eyes of hundreds of people watching him through slits in their shuttered windows. For perhaps the first time in his unruly life he wondered what they were thinking.
Over the next two weeks Marina, disguised as a scullery maid, brought Galileo his food, passing it through a small opening in the locked door, talking to him in whispers. She told him about the children and how the inquisitors had taken all his papers. He asked if "they" had asked her about anything in particular that he was working on. She shook her head.
For six weeks, Galileo sat in his prison room on a bed of straw and rough burlap, wearing the same nightshirt, which literally rotted on his body. During the first week, his arrogance kept him aloof, as he waited for inquisitors to question him. They never came. During the second week, his reserve turned to anger. The people taking turns monitoring him through a series of hidden mirrors saw him circlingaround the room, rubbing against the wall opposite his bed, then against the wall with the door, then against his bed, and finally against the outer wall, with the commode and window. As he circled, he rubbed the walls with his hand until it was raw. After the second week, Marina stopped coming. She could not stand the smells.
During the third week, as his anger dissipated, he found solace in continuing his studies. The day before his imprisonment, he had received a letter full of technical details of the observations made by Martinelli on the island of San Salvador in the New World. He had spent that day memorizing the results in it. Now he compared those observations, meticulously recorded by his friend, with the ones he had made at the same time and the ones that they both had made twenty years before. Sifting through all this data chiseled in stone in his eidetic memory, and scribbling calculations in the dust on the floor, he completed the work that had obsessed him for years. And he was right! The angles between the telescopes observing the same place on the moon Lluna had changed over the decades. Lluna was moving away from his world, Dimaan. Despite the squalor, the arrogance returned.
During the fourth week he started pounding on the door, demanding to be released. No one came to tell him to stop, and the door never opened. During the fifth week, confusion surrounding his arrest, imprisonment, and this interminable isolation broke through his defenses. The original question returned. Why had they arrested him? It must be something to do with his observations of the heavens, he reasoned, but what? His renowned ability to focus and concentrate evaporated.
During the sixth week, he started thinking about errors he had made: errors of commission and errors of omission. Maybe, just maybe, he should have married Marina. Perhaps he shouldn't have fired Sestilia. Vincenzo really deserved the raise he had requested all those years ago. And what about his girls ... .
The day after the tears appeared, an orderly opened the door, gagged, and vomited. Then he ordered Galileo out. Nearly naked, Galileo stumbled into the hallway. The gaggle of guards all backed away. With spears, they prodded him down the corridor and into a room through the middle of which ran a stream of water. He was tossed apiece of soap, a towel, and a robe, and ordered to bathe, which he did with as much zest as he could muster.
The courtroom was a study in contrasts. On the side where the three judges sat, the walls were covered with dark wood panels and an immense tapestry. Glasses and pitchers filled with crystal-clear water sat in front of each of them, along with baskets of fruits and nuts. On the other side, Galileo stood on a bare wooden platform in an alcove surrounded by gray, rough-hewn walls.
"How do you plead and do you agree to recant what you have said?" the central judge, tall, with a goatee beard, demanded.
"Are you out of your tiny mind?" Galileo demanded. "That is, if you have one at all. I have done nothing wrong. Nothing," he hissed at them.
The three judges and the guards against the side walls all gaped.
The judge on Galileo's right regained his composure first. Scribbling something on the sheet in front of him, he half turned his head toward the astronomer. "Do you really believe that? Do you think you would be here if we didn't have proof positive of your transgression?"
Galileo glared at him. The silence filling the room became so thick that several guards shook their heads to clear their minds.
"Do you deny ..."
"I have done nothing wrong and made no mistakes," Galileo interrupted, through clenched teeth.
The third judge sat back, smiled briefly, and began speaking, putting his index finger to his lips as Galileo opened his mouth.
"We think that you misunderstand us." He motioned for the guards to leave. When they were gone, he rounded on Galileo. "We know that our planet, Dimaan, and the heavens have been here forever. They are immutable. Unchanging in Essential Essence."
"Earthquakes, volcanoes, sunspots," Galileo interjected, unsure where this was going.
"Mere challenges to humans," the judge said, smiling. "Our Creator does not want us to think we live in paradise here. You, however, claim that you can prove that there are irreversible changes in the universe. These, in turn, lead to the conclusion that our planet, and by extension, the universe, has not been here, fundamentally unchanged, forever."
"I ... I don't follow," Galileo murmured, the bravado draining.
"Then I will explain ... if you have a brain to understand," the judge sneered. "Dimaan has two moons, Lluna and Kuu. Lluna is ninesixteenths the distance of Kuu, a relationship of perfect squares that the Church finds consistent with its teachings. You," he stabbed a finger at Galileo, "have secretly proposed that Lluna is moving away from Dimaan, toward Kuu. You have a co-conspirator, Luigi Martinelli, in San Salvador. He is there to provide you with data that will allegedly prove your hypothesis. And he has sent you that information."
The judge waved papers at Galileo, who squinted, unable to make out what was on them. The judge rose regally and, walking around the desk, handed them to Galileo. They were a copy of the letter he had received before his arrest.
"I don't unders--," he began, but suddenly he did. "This data, with my own, proves that Lluna is moving away. Hence it was closer yesterday and closer still each previous day. Once upon a time it must have been captured or it was part of Dimaan, flung off," he did a quick calculation in his head, "nearly a billion years ago."
"So if what you propose is true, Dimaan and Lluna were not the same in Essential Essence a billion years ago as they are today."
"Doesn't it give you a headache to think that everything has been as it is forever? What happened before forever?"
"Your caustic blasphemy will gain you nothing," the central judge said, his voice icy.
"If I can prove Lluna's recession, then science can rethink the evolution of our world. Otherwise, we are stuck with a universe that has lasted forever, a Lord who has also existed forever, and life that has only recently been put here. But why now? Why were we not created a trillion years ago? Or a billion years from now? It makes no sense to me."
Silence hung in the air like molten lead. "Our Lord works in mysterious ways. She has given us Her teachings and we are here to make sure that no one ... I mean no one ... goes astray. You, sir, are going astray and you are beginning to take others with you. This is absolutely unacceptable. You will ... you must recant these heretical beliefs here and now and vow to never mention them again, except as errors."
"And if I refuse?"
"The past six weeks of confinement will only be the beginning. We will tortu ... teach ... you using every tool at our disposal. They are many tools, each more--instructive--than the previous one. Halfway through your instruction, you will plead for death--but it will be denied. Eventually we will rip your arms ..."
"And what if ... what if I am right and you are wrong? What if I can prove what I claim is true?"
The three judges looked at each other and the one on Galileo's right nodded slowly. "I think it would be a mistake to start with your 'education.' We will begin by educating your mistress and children."
Earth is unique in the solar system1 for many reasons. Some distinctive properties are entirely obvious (complex surface life comes to mind), whereas others are more subtle, such as Earth having one Moon. In comparison, Mars has two moons, Jupiter has at least sixty-three, and the other planets have numbers between these two. (Venus and Mercury have none.) Furthermore, our Moon2 has the mass of the Earth, whereas all the other moons in the solar system have masses less than the masses of their planets.
The Moon's existence, combined with its large mass compared to the Earth and the fact that it is the only natural body orbiting our planet, has led to many changes from what the Earth would have been like without it. Because of the Moon's existence life formed relatively rapidly; the day is twenty-four hours long rather than being roughly eight hours long; tides are three times higher than they would be otherwise; many species of animals that are active at night could not exist without moonlight to aid their hunting, navigating, and mating activities; our planet's rotation axis does not randomly change direction as it would otherwise do; and we have an essentially constant cycle of seasons, which we wouldn't otherwise have, among many other things. Each of these results of our having a Moon, in turn, has affected myriad other aspects of the Earth and life on it. Any modification to the Earth or its astronomical environment leads to fascinating changes inour world, as well as providing new perspectives and insights into our planet as it is now. In this first of ten alternate worlds we explore what Earth would be like if we had two massive moons today instead of just one.
The Earthlike planet in this chapter, called Dimaan, begins its life identical to the early Earth in size, composition, and distance from the Sun. Based on geological and fossil evidence, the Earth was initially spinning much faster than it is today. Although that rate is not yet known, I give Dimaan a plausible eight-hour day when it first formed. Neither Earth nor Dimaan had a moon at first. Ours came into existence within about 200 million years of the Earth's forming.
Moons can form in four ways: from impacts, in which the planet is struck and thereby ejects debris that becomes one or more moons; simultaneously with a planet, in which the moons and planet condense together (Appendix); by fission, wherein the moons are literally thrown off a rapidly rotating planet (Appendix); and by capture of the moons after the planet has formed.
Most astronomers believe that our Moon formed as the result of a collision between Earth and a Mars-sized body. The intruder hit Earth at an angle that ejected debris into orbit in the same general direction in which our planet was spinning. This rubble formed a short-lived ring that was much smaller but, interestingly, much more massive than all of Saturn's rings combined. As this material orbited, it began colliding with itself and bunching together under the influence of its own gravitational attraction until it coalesced into the Moon. This is how I posit Dimaan's first moon, Kuu, formed.
Although it is entirely possible for an impact of a small planet onto a larger one to splash enough debris into orbit to form two moons similar to ours, such moons would drift together and collide billions of years before advanced life evolved on Dimaan (Appendix). Because I want that second moon around for people to enjoy, I posit that Dimaan captures its second moon long after the first one formed.
The process of forming a star and its host of orbiting objects is a very, very messy affair involving countless collisions. The star system begins as aslowly swirling eddy of gas and dust in a giant interstellar cloud that begins to contract under the influence of its own gravitational attraction. In the case of our solar system or that of Dimaan, the central region of this eddy condenses to become the Sun. The material in its outer reaches becomes a disk of gas and dust, parts of which condense to form the planets, moons, and smaller orbiting pieces of debris such as asteroids and comets.
Most moons are potato-shaped bodies typically a few miles across that were originally not bound to planets. As these small bodies drifted past them, the planets captured them with relative ease. In our solar system, the tiny moons include Phobos and Deimos orbiting Mars, and at least 150 moons orbiting the giant planets Jupiter, Saturn, Uranus, and Neptune. For each piece of space debris that was captured, millions of similar objects struck planets or sped past too rapidly to go into orbit.
Even though our solar system and the one destined to become Dimaan's home began identically, it is entirely plausible that Dimaan acquired a second massive moon. This happened because the orbits of the debris from which all the bodies in the solar system formed were chaotic, a concept that transcends the intuitive definition of the word.
The mathematical and physical concept of chaos reveals that exceedingly tiny changes (such as a butterfly flapping its wings in Africa) can have monumental consequences (such as a hurricane in Louisiana) that would not have occurred without the tiny initiating event. In the formation of the solar system, tiny gravitational tugs from small pieces of debris led to huge unpredictable changes in the orbits of all the objects that formed in it, compared to what the orbits would have been had the small pieces not been there. For example, imagine two mountain-sized chunks of rock that gently collided in the young solar system, thereby creating a slightly larger object. This bigger body then collected other matter, eventually growing into the Earth. Now suppose that the initial collision was ever-so-slightly faster so that the impact pulverized the two bodies, rather than forming a larger, more massive one. In that case, the Earth would not have formed as it did. Another collision of different debris, perhaps at a different distance from the Sun, would have led to the formation of an Earthlike planet, but with a different orbit and different physical properties than the Earth has today.
Chaos also justifies the assertion that if another system very similar to the solar system had formed with Dimaan in the same orbit as Earth, therecould easily be a few "extra" Moon-sized bodies in it that never existed in our solar system. One of these is destined to be captured and become Lluna, Dimaan's closer moon in this chapter.
Let's now set the stage for Lluna's capture by considering how Kuu evolved into its present orbit. Once we have gotten it out there, we can capture Lluna. Because Kuu and our Moon formed identically, I simplify the discussion by referring here to how our Moon got out to its present orbit.
If the impact that splashed the debris that became our Moon into orbit had been powerful enough to put the Moon directly into its present orbit, the Earth would have been destroyed in the process. Keeping the Earth intact required that the debris splash into orbit much closer to the planet. We don't yet know how near to the Earth the Moon was originally, but it could plausibly have been ten times closer.
Tides are the key to understanding how the Moon got out to where it is today, along with why the day is twenty-four hours long. Indeed, the tides play roles in so many of the scenarios in this book, I believe it is essential that we briefly explore them here.
For the first twenty years that I taught astronomy, I explained the tides incorrectly. The explanation in the existing astronomy texts back in those days, even the advanced ones, from which I had learned about the tides, was wrong. It went like this. The force of gravity decreases with distance (which is true). The part of the Earth closest to the Moon feels the greatest gravitational attraction to it, and the central region of the Earth feels less attraction. Because the ocean water closest to the Moon is pulled hardest, that water creates a high tide. The oceans on the far side of the Earth feel the least gravitational attraction, so they are "left behind," meaning that they create a high tide on the side of the Earth farthest away from the Moon.
My culpability in propagating this misconception runs deeper. During that period I began writing an astronomy text that included this explanation. Our understanding of the cosmos changes rapidly, so astronomy texts are revised every few years. Each time we revise one, it goes out to many astronomers for review. Eventually one reviewer (out of over a hundred) complained about the above explanation of tides. Needless to say, I thought hewas wrong, but in the spirit of "due diligence" I decided to check. I went to the font of much wisdom about the oceans, the National Oceanographic and Atmospheric Administration. They have a Web site entitled Our Restless Tides that convinced me that there is more to tides than I and my colleagues had been writing and teaching.
After reading this, I invite you to let me know whether you had been given the correct scoop on tides by visiting this book's Web site.
Although I describe tides for the Earth--Moon system, this presentation applies to any objects on which tides occur. To simplify matters, let's begin by ignoring the Earth's spin (technically, rotation) on its axis and the tidal effect of the Sun. We put them in later.
There are two parts to the tidal picture. First is gravity, the only universal force of attraction. Although the effect of gravity from any object extends infinitely far, the strength of the gravitational attraction decreases with distance: the farther you are from something, say the Moon, the less gravitational force you feel from it. Consider, then, the Moon's gravitational attraction on the Earth's oceans. The oceans closest to it at any moment feel the strongest gravitational attraction from it. Those halfway around to the other side of the Earth feel less force from it, whereas the oceans on the far side of the Earth feel the least gravitational attraction from it. This change in attraction of the oceans by the Moon with distance is not enough to explain the tides.
The second piece of the tidal puzzle relates to the orbit of the Moon. Contrary to popular belief, the Moon does not orbit around the Earth. Rather, the Earth and Moon together orbit around a common point, their center of mass, like two dancers who are holding each other and waltzing around. When gliding straight across the dance floor, the dancers whirl around each other and their center of mass moves in a straight line.
The center of mass of the Earth--Moon system is called the barycenter. It is located 1,064 miles under the Earth's surface on a straight line between the centers of the two bodies. The barycenter follows a smooth elliptical orbit around the sun, whereas the Earth and Moon waltz around it (Figure 1.1a).
The motion of the Earth around the barycenter creates a force everywhere on the Earth that is directed straight away from the Moon. This is similar to the outward force you feel on a merry-go-round. The tides result from a combination of the gravitational force from the Moon pulling the oceans towardit and the force away from the Moon created by the motion of the Earth around the barycenter. Keep in mind that the force of gravitation decreases with distance and the outward force is constant over the Earth. When you subtract these two effects everywhere (Figure 1.1b), you discover that the closest point on the Earth to the Moon feels a net (i.e., gravitational minus outward) force toward the Moon, whereas halfway around the Earth to the other side the net force is zero, and on the opposite side of the Earth from the Moon, the net force is directed away from the Moon.
Aha! you say. The net force toward the Moon lifts the waters closest to it, and the net force away, on the opposite side of the Earth, pushes those waters away from the Moon. Not quite. If those net forces were the only cause of tides, then tides could only be a matter of inches high, because that is the maximum height that the Moon's gravitational force can lift ocean water. What really causes the tides is that the water nearly halfway round to the other side from the Moon is pulled by it, sliding along the Earth's surface, piling up close to the Moon, and causing the oceans closest to it to bulge outward (a high tide). At the same time, the tides just beyond halfway to the opposite side are thrust away from the Moon, sliding along the Earth's surface, and piling up opposite the Moon in another, simultaneous, high tide as shown in Figure 1.1c.
The above discussion implies that the high tide closest to the Moon should be in a straight line between the centers of the Moon and Earth. It isn't. We need to put the Earth's rotation back in the mix. As the Earth turns, it pulls the high tides around with it. As the one closest to the Moon is pulled away from being directly under it, the Moon pulls it backward, trying to keep that tide lined up between the centers of the two worlds. Because the Earth spins nearly twenty-eight times faster than the Moon appears to orbit the Earth, our planet's motion combined with the friction between the ocean and the surface of the Earth keep that high tide about ten degrees ahead of the Moon (Figure 1.1d).
Finally, as concerns the tides, it is worth noting that although the Sun is about twenty-seven million times more massive than our Moon, the Sun is 390 times farther from the Earth. The relatively short distance between the Earth and Moon, compared to the distance between the Earth and Sun, causes the tidal effect of the Moon to be greater than the tidal effect of the Sun. Today, the Moon creates two-thirds of our tides and the Sun creates most of the rest3.
If the high tide closest to the Moon were directly between the centers of the Moon and Earth, the gravitational attraction of that water on the Moon would pull it in the same direction as the rest of our planet does, keepingthe Moon in its present orbit. Because the closer high tide is not on that line, the water in it creates a gravitational attraction on the Moon that pulls it forward (Figure 1.1d), giving the Moon extra energy, which causes it to spiral outward. With this as background, we can now return to the evolution of the moon Kuu around Dimaan.
Forming ten times closer than the Moon is to the Earth today, Kuu created tides on Dimaan that were 1,000 times higher than the tides we nowexperience from the Moon. These towering tides, pulling Kuu forward in its orbit, provided the tug necessary to cause that moon to spiral outward. After about 4.2 billion years, Kuu was out nearly as far as our Moon is today.
Life began on Dimaan when it had just the one moon, Kuu. Aquatic plant life in Dimaan's oceans evolved first. It was just when plants were about to transition onto the land, about 4.2 billion years after Dimaan and Kuu formed, that Lluna began drifting toward it.
Lluna, a spherical body similar in size and mass to Kuu, formed far from Dimaan. Not orbiting a planet, we call such bodies asteroids or "small solar system bodies." In its early years, Lluna acquired a companion body the mass, like our dwarf planet Pluto and its main moon, Charon (pronounced like the name Karen). Indeed, many small objects in our solar system have orbiting companions. Lluna's companion will play a crucial role in the moon's capture by Dimaan.
Capture of massive moons isn't easy. Gravity pulls objects together, but a near miss doesn't imply capture. If the incoming body or bodies are moving too fast, they will slide by, changing direction as they go, and then recede back into interplanetary space. To be captured, an object from afar must slow down or, put technically, it must lose the energy it has as a result of its motion, called kinetic energy.
Kinetic and Potential Energy
Two kinds of energy must be considered in understanding how one body can capture another: kinetic energy and potential energy. Imagine jumping upward, rising perhaps a foot. The energy you have when you are moving is called kinetic energy. Once you leave the ground, of course, you begin slowing down, which means that your kinetic energy is decreasing. The Earth's gravitational attraction (or force) is what slows you. In situations like this, however, energy is conserved, meaning that your kinetic energy has to be converted into another kind of energy in your body. Your kinetic energy is transformed into potential energy, which gives you the "potential" of falling back down. Assuming that nothing prevents that from happening, your potential energy is converted back to kinetic energy as you descend.
This kinetic energy/potential energy interplay occurs for all objects moving through the solar system. Consider the object destined to becomeLluna. Far from Dimaan, Lluna and its companion are traveling along, feeling the planet's gravitational pull. Therefore, Lluna has both kinetic and potential energies. As it nears Dimaan, the force between them increases, the kinetic energy of Lluna increases (it speeds up), and its potential energy decreases. After the closest approach, Lluna and its companion recede and slow down the same amount they had originally sped up. In other words, they leave the vicinity and don't necessarily ever return. If that were all there were to it, Dimaan could never capture Lluna because Lluna must lose overall energy (i.e. kinetic plus potential) to be captured. If we can figure a way for Lluna to lose energy, it will be unable to move as far outward as it was originally. In other words, it would be captured. This is the same as if you jumped upward a second time, but with less vigor (think, energy). In that case, you would not rise as high.
The Capture
Four effects contribute to the capture of Lluna: most important is the fact that Lluna's companion feels a slightly different gravitational attraction from Dimaan and Kuu than does Lluna itself. This occurs because as Lluna and the companion approach Dimaan, these two intruders are at slightly different distances from the planet and its original moon. Therefore they feel different amounts of gravitational force from them. This difference can be enough to pull the companion free of Lluna and fling it away, taking with it a substantial amount of energy, which has the effect of slowing Lluna down, making it possible for the final three effects to complete its capture.
Upon approaching the Dimaan-Kuu system, Lluna's gravitational attraction pulls on the moon Kuu, causing its orbit to become more elongated (more elliptical). Moving Kuu causes Lluna to lose energy. At the same time, Lluna creates tides on the planet Dimaan that pull back on Lluna, slowing it down further. Finally, the gravitational pull of Dimaan on Lluna coupled with the planet's orbit around the Sun cause Lluna to lose even more energy. In this final process, energy is taken from Lluna and given to Dimaan. The combination of all these effects can remove enough energy from Lluna for it to become locked in orbit around Dimaan.
I set Lluna's initial orbit around Dimaan to be half Kuu's distance from the planet, with both moons orbiting in the same direction and in about thesame plane that our Moon orbits Earth. As we show shortly, this leads to eclipses related to both Kuu and Lluna. Virtually all objects in the solar system have elliptical orbits (egg-shaped), however, most of these are very close to circular. Lluna and Kuu will initially have more elliptical orbits than any other moons because the capture of Lluna was so messy.
It will take roughly two weeks from the time that Lluna is first close enough to generate noticeable tides on Dimaan until this moon is securely in orbit. During that transient period, all hell breaks loose on the planet. Lluna's gravitational pull creates tides on Dimaan eight times higher than those from Kuu. While Lluna is settling into orbit, it will also create monster tidal waves on Dimaan that will make any tsunamis that we have on Earth seem like tiny ripples in comparison. The water will slosh like the waves created in a large pan filled with water as you carry it from the sink to the stove.
These tidal waves and the tidal bulges generated by Lluna will create Dimaanquakes and increased volcanic activity that will persist for years. The dust released by the volcanic emissions will darken the skies and cool the atmosphere dramatically. The volcanoes active during this time will also release vast volumes of water vapor, carbon dioxide, sulfur dioxide, carbon monoxide, stinky hydrogen sulfide, and hydrochloric acid, among other gases. All of this activity will cause a mass extinction in the ocean life of Dimaan.

Lluna's capture and the damage to Dimaan and life on it in the process don't mean that the planet will thereafter be lifeless. Life on Earth has experienced over half a dozen similarly catastrophic mass extinctions, episodes caused by geological and astronomical events during which large fractions of all life on our planet were eradicated. Perhaps the most dramatic of these events, the Permian--Triassic extinction, occurred 251 million years ago. It wiped out over ninety-five percent of all species of life. Nevertheless, the remaining life-forms grew, diversified, and became the progenitors of the life on Earth today. What Lluna's presence does mean is that the sequence of evolutionary events on Dimaan would be profoundly different from whatoccurred here on Earth or that would occur on Dimaan had Lluna not appeared on the scene. Let's explore some of the differences that would result.
At half the distance, Lluna will have twice the diameter as does Kuu in Dimaan's sky (or does our Moon in our sky). Twice the diameter means that the area Lluna covers in Dimaan's sky will be four times greater than that of Kuu. Because moonlight is sunlight scattered from the surface of a moon, Lluna will be four times as bright on Dimaan as is Kuu.4 Combining the light from both moons, nighttime on Dimaan when both moons are full will be five times brighter than the nighttime surface of the Earth under a full Moon. It would be quite easy to read a book under those conditions.
Lluna and Kuu orbit Dimaan at different speeds, therefore it is more likely that at least one of the moons is up at night than it is for us with our single Moon. When a moon is high in the sky at night it is at least half full (technically the moon is in either a gibbous or full phase). Therefore, Dimaan will have more nights brightly lit with moonlight than does the Earth.
In what follows, let's assume that the sensory equipment available to life on Dimaan is the same as on Earth. That means people there will evolve seven senses: touch, taste, smell, sound, sight, heat, and gravity. The last two of these are often left off lists of senses taught to children, but we have them nevertheless. Sensitivity to heat is straightforward: put your hand near a fire and you know that it is hotter than its surroundings. Sensitivity to gravity is our ability to know our posture and to sense when we are falling.
Because it will be easier for predators to see their prey at night on Dimaan, camouflage will be more highly refined than it is on Earth. This, in turn, will require more acute hunting skills using sight, sound, smell, and heat detection for animals that are active at night. The cycle of protection and detection driven by the brighter nights on Dimaan could well lead to creatures that are more aware of their surroundings than early land animalswere on Earth. This, in turn, is likely to increase various aspects of intelligence in these creatures compared to what was necessary for survival here. Perhaps the first sentient creatures on Dimaan will evolve from nocturnal hunters rather than from arboreal creatures, as occurred on Earth.
Eclipses happen when the shadow of one world falls on another. Solar eclipses on Earth occur when the shadow of the Moon crosses the Earth when the Moon is in the new phase (between the Earth and Sun). Conversely, lunar eclipses occur when the full Moon moves into Earth's shadow. However, you have probably noticed that solar eclipses do not occur at every new Moon, nor do lunar eclipses occur at every full Moon.
Eclipses don't happen every month because the Moon's orbit is tilted five degrees compared to the plane of the ecliptic (Figure 1.2). The ecliptic is the plane defined by Earth's orbit around the Sun. When the Moon is new, it is usually slightly above or below the ecliptic, meaning that the Moon's shadow is slightly above or below the Earth. When our Moon is crossing the ecliptic in the new phase, however, its shadow speeds across a swath of the Earth, prevents sunlight from striking this region, and thereby creates a solar eclipse. The same arguments about the Earth's shadow apply to lunar eclipses when the Moon is full.
Lluna does not cause solar eclipses every time it passes between the Sun and Dimaan because I put Lluna in an orbit that is also tilted 5° from the ecliptic. Lluna is only 1° across as seen from Dimaan. Therefore, when Lluna passes between Dimaan and the Sun, its edge closest to the Sun will appear as much as 3¾° from it. At such times, Lluna is too far in angle from the Sun to cover it as seen from the planet.
When solar eclipses on Dimaan caused by Lluna do occur, they will be different from those caused by Kuu (or, equivalently, by our Moon on Earth) in two ways. Total solar eclipses occur when a moon's shadow touches its planet. If you are inside that shadow, the moon would appear dark (new phase), with the Sun completely blocked out behind it. On Earth that complete shadow region behind the Moon, called its umbra, is never more than about 160 miles wide on the Earth's surface. If you are just outside the umbra, in the penumbra, you will see a crescent Sun on the edge of the Moon throughout the eclipse. The umbra sweeps across the Earth's surface so rapidly that an eclipse never lasts for more than about seven and a half minutes here.
The first difference for Lluna's eclipses is that its larger apparent size in Dimaan's sky will cause its umbra to cover twice the area on the planet's surface as does our Moon's umbra. The second difference is less obvious: the total solar eclipse caused by Lluna lasts a shorter time than such eclipses do on Earth today. This difference in duration occurs because Lluna orbits closer to Dimaan. Kepler's third law of orbital motion, worked out by Johannes Kepler (1571--1630), reveals that the closer a moon is to its planet, the faster it orbits compared to a moon farther away. Hence Lluna moves faster across the sky than does Kuu. Its higher speed more than makes up for the larger size that Lluna has in Dimaan's sky as the shadow passes across Dimaan.
Phases of the Moons
The two moons will have cycles of phases that last different lengths of time as seen from Dimaan. The phases of a body are caused by seeing different amounts of its sunlit side from hour to hour or day to day. From Earth, for example, we see varying amounts of the lit side of the Moon as it orbitsaround us. Starting when the Moon is between the Earth and Sun, this cycle of phases changes as follows: new (when we see the least of the lit side), waxing crescent, first quarter (when we see half the lit side), waxing gibbous,5 full (when it is on the opposite side of the Earth from the Sun and we see all the lit side), waning gibbous, third quarter, waning crescent, and back to new. This cycle takes about 29½ days to complete.
Lluna and Kuu will both go through the same order of phases. When Lluna is first captured, Kuu's cycle will have essentially the same length as our Moon's phases, namely about 29½ days to go from, for example, new moon to new moon. Lluna, orbiting more rapidly, will have a more rapid cycle of phases that takes about 10 days to complete. That cycle is so rapid that you could go out and see Lluna in one phase and return later that day or night and see it in a distinctly different phase!
We see the phases of our Moon vary uniformly because the Moon's orbit around us is nearly circular. Because they have significantly more elliptical orbits, the phases of Kuu and Lluna will change much less smoothly. When they are each closest to Dimaan, their phases will be changing most rapidly, and conversely the phases will flow most slowly when they are farthest away.
Tides and Shorelines
With a few notable exceptions, such as Swansea, Wales, and Mont Saint Michel, France, where the ocean bottom slopes downward especially slowly, beaches on Earth tend to vary in width by a few hundred feet or less between high and low tide. Although tidal erosion is significant, it usually takes place over periods of decades or longer, giving people who live or work on the shore time to develop defenses against the changing landscape. The presence of Lluna will make both the range of tides and the speed at which they erode the shoreline of Dimaan much greater.
As noted earlier, Lluna creates tides 8 times higher than Kuu. Combiningtheir tidal effects with that of the Sun leads to tides on Dimaan that are as much as 61/3 times greater than the range of tides on Earth today. This occurs when Dimaan, Lluna, Kuu, and the Sun are in a straight line. Therefore, the typical intertidal region (the area that is exposed at low tide and hidden at high tide) on Dimaan will be much more extensive than it is on Earth. Because more tidal water is flowing on Dimaan each day, the amount of erosion of the shoreline there will be much greater than it is here. The shore would therefore wear away and expose coastal buildings to damage more rapidly than occurs on Earth.
Tidal erosion would also be more profound in the rivers on Dimaan than on Earth. Where the rivers run into the oceans on Dimaan, the tides would carve deeper channels than they do here. These would lead to tidal waves called tidal bores that even run up some rivers here on Earth, such as those off the Bay of Fundy between Maine and Nova Scotia, and the Bristol Channel between England and Wales.
Cities built near the mouths of rivers, such as New York, San Francisco, New Orleans, or even London (as far upriver on the Thames as it is), would experience unacceptable erosion problems due to the tides and tidal bores generated on Dimaan. Shorelines would erode so quickly that without heroic effort, such as thick concrete walls lining the rivers, cities could not be built on most ocean shores or on especially active rivers.
It is worth noting that building cities on waterways has the advantages of providing easy sewage removal and of transporting people and goods to and from many locations via ships. It is therefore likely that the challenges caused by high tide cycles would lead to differences in the way coastal civilizations developed on Dimaan from the way they have on Earth.
The month on Earth has its origins in our Moon's cycle of phases. That cycle takes roughly 29½ days. The fact that this is not a whole number of days, and especially not a whole number divisible by seven (for the days in the week) has made the month less useful and more symbolic than the day, week, or year.
Dimaan's calendar could conceivably be even more convoluted, becauseLluna and Kuu have different cycles of phases and the two cycles will not be whole number multiples of each other. Dimaan's people could plausibly have calendars with days, weeks, parmos,6 and months.
Lluna's Rotation (and Its Change Due to Tides)
As with all similar-sized bodies in our solar system, Lluna will be rotating when it is captured by Dimaan. The rate at which it rotates will be unrelated to any property of Dimaan: Lluna's rotation will have been established long before, when it and its companion were orbiting around each other. In our solar system, the dwarf planet Pluto rotates once every six days which, not coincidentally, is the same length of time it takes Charon to orbit around it.
Charon, our Moon, and Dimaan's moon Kuu all have an interesting orbital property, namely that they rotate at exactly the same rate as they orbit their planets. This is called synchronous rotation. Objects in synchronous rotation always have the same side facing the body they orbit. That is why we always see the same side of the Moon.
Synchronous rotation of spherical moons is not an accident. When moons form or are captured, they are unlikely to rotate at the same rate they orbit. However, the planet creates tides of the land on their spinning moons called land tides. These are analogous to the tides that moons create in oceans. As land tides cause the ground to rise and fall, friction inside the moon between adjacent pieces of moving rock creates heat (as you would feel if you rubbed your hands together vigorously). This heat, in turn, melts that rock, creating molten rock--magma--that leaks out through volcanoes and cracks in the moon's surface. This liquid rock flows on the surface, enhancing the tidal effect.
This idea of land tides is not unique to Lluna, nor does the rock that moves have to be molten to be tidal. Surprisingly for those of us who are not geologists, Earth's continents have tidal motion today (commonly called Earth tides, but that name will confuse matters later on). They were proposed in the 1880s by Sir George Howard Darwin and measured as early as 1909. The highest ones here, over eight inches high, occur in the equatorial regions of our planetas the land is pulled upward by the gravitational attraction of the Moon and Sun. They can be measured using technology, but you can't feel or see them.
The regions of high land tides act as handles that the gravitational attraction of the planet can pull upon to change a moon's rotation rate. At the same time, friction between liquid rock flowing on the moon's surface--lava--and the solid rock below it changes the rotation rate in the same way. Eventually these two effects combine to change the moon's rotation rate from whatever it was to a rate equal to the time it takes the moon to orbit the planet. In other words, the moon's rotation becomes synchronous. At that time, the high tides on opposite sides of that moon become fixed (i.e., they will no longer travel along the surface) and so the rubbing that created heat before it was locked into synchronous rotation will cease. The surface then cools and solidifies.
Most of the moons in our solar system are in synchronous rotation around their planets. Lluna will be brought into synchronous rotation. However, another source of heating will keep Lluna's interior molten.
Volcanoes on Lluna
By far the most spectacular thing about Lluna's presence at the time people exist on Dimaan will be the moon's active volcanoes. Their existence is analogous to the volcanoes that occur on Jupiter's moon Io today. Recall that because of the way it was captured, Lluna's orbit around Dimaan is not especially circular. Eventually, its orbit becomes more so, but Kuu prevents it from ever being perfectly circular: when Lluna is between the planet and the outer moon (Figure 1.3a), Dimaan pulls it in one direction, and Kuu pulls it in the opposite direction. As a result, Lluna is pulled into an orbit slightly farther away from Dimaan than when Lluna is on the opposite side of the planet (Figure 1.3b). In the latter position, both the planet and the other moon are pulling it toward Dimaan and so Lluna is then closer to the planet than it would be if Kuu were not there.
The result of the noncircular orbit is that when Lluna is closer to Dimaan, the land tides on the moon are higher than when Lluna is farther away.7 Seenfrom afar, it would look as if Lluna were breathing as it orbits Dimaan. This change in height of the land creates the same friction that kept it molten before it was in synchronous rotation. As a result, the inside of Lluna will be molten throughout its existence in orbit around Dimaan and that magma will continually be leaking out through volcanoes and cracks in its surface.
Lluna is going to present a spectacular sight from Dimaan. Peppered with huge volcanoes, Lluna will be a world alive with red-hot lava being ejected in several places at once. Some of these events will be seen along the edge of the moon, like mammoth mushroom-shaped fountains leaping miles into the air and then crashing silently back down. These are analogous to stratovolcanoes on Earth. Other features on Lluna will include volcanoes that ooze lava, and rivers of lava that flow and, upon cooling, freeze into place. Although that would be very romantic today, I can imagine that prescientific civilizations on Dimaan would create a wide variety of mythical scenarios based on it. Hell, yes.
Unlike our Moon, there will be few, if any, impact craters on Lluna by the time people view it from Dimaan. Lluna's surface will be re-covered by fresh lava so often that craters will be quickly erased.
Some of the lava ejected from Lluna will be racing upward so rapidly that it will escape into space and never return. Some of that debris will drift away from the Dimaan system, some of it will form a tenuous ring around the planet, and some of it will hurl toward the planet.
Pieces of debris, pebble-sized and bigger, that are ejected Dimaanward from Lluna will continually be entering the planet's atmosphere. The smaller pieces will vaporize, creating meteors, just as occurs in our skies. Whereas we experience frequent meteors only a few times a year, during meteor showers, every clear night on Dimaan will be filled with meteors (aka shooting stars) streaking down every few seconds. That would be very romantic from our perspective. Would Dimaanians become so inured to these events that they would lose interest?
Larger pieces of debris falling through an atmosphere do not completely turn into dust. Their solid remains strike whatever is in their paths. Sufficiently large ones create craters upon impact, but the pieces of debris from Lluna are likely to be too small to do this. Nevertheless, life on Dimaan is going to have to contend with more frequent meteorite impacts, like bullets from the sky, than we experience on Earth.
The Collision Between Lluna and Kuu
Lluna and Kuu are destined to collide. Our Moon spirals away from the Earth. Kuu spirals away from Dimaan. After Lluna forms, the tides it creates on Dimaan will act back on it, forcing it, too, to spiral outward. Calculations reveal that after Lluna is captured, it recedes from Dimaan faster than does Kuu. Just as the recession of our Moon slows the Earth's rotation and Kuu's recession slows Dimaan's rotation, the recession of Lluna will make Dimaan's day even longer. As Lluna closes in on Kuu, the day on Dimaan will exceed twenty-eight hours.
The collision will take place tens of thousands of miles farther from Dimaan than our Moon is from the Earth. As seen from Dimaan, Lluna will approach Kuu from behind. In their final days, the gravitational forcesthat each moon exerts on the other will cause the two to become more and more egg-shaped as land tides miles high form on them. This will cause Lluna to crack open, allowing its molten interior to pour out and cover its surface in glowing lava. Kuu, heated by friction as it distorts, will also have an outpouring of molten rock. Then the worlds will kiss.
Inexorably Kuu and Lluna will come together. The collision, however, will not be even remotely as horrendous as the impact on Dimaan that created Kuu or the one on Earth that created our Moon. The intruders in these latter impacts were moving much more rapidly, compared to the planets they struck, than Kuu and Lluna will be moving when they collide. People on the side of Dimaan facing their moons can prepare a jug of their favorite libation, set up a comfy chair, and watch the first phases of the event over a period of hours, as they might watch a science fiction movie at home. Only this event will be real and its consequences deadly.
After the kiss, rings of crushed rock will fly off the moons from the regions where they come into contact. At the same time, the sides of the moons opposite the impact site will erupt with lava shooting out as their liquid interiors collide and bounce away from each other. By the time a quarter of their mass has come into contact, both moons will begin to break apart. Seen from Dimaan, they will appear to explode in slow motion. For many hours the sky will be filled with bright red light from the impact region and wherever else molten rock is emerging.
Debris from the impact will fly in all directions, most notably perpendicular to the direction that the two bodies were moving when they struck. Put in blunt terms, a lot of stuff is going to fly toward Dimaan. The devil of this impact is in the details (such as the relative speed between the moons, their internal temperatures, and their chemical compositions), but some of their debris, including chunks big enough to create craters miles across, will drift toward the planet for several days and then fall into Dimaan's atmosphere.
Fortunately, this event is going to occur long after advanced civilizations have been established on Dimaan. They are likely by then to have the technology and techniques (Hollywood, take note) to prevent impacts of collision debris that would otherwise lead to a mass extinction on their world. Whether they would be able to save the civilizations they had established on the two moons before the collision is another question altogether.

The two moons will eventually become one. Following the impact, debris that wasn't blown completely out of orbit would form a significant ring around Dimaan. Within a few years, the densest part of the ring would clump together due to its own gravitational attraction and due to relatively slow collisions between its pieces. A single body would form, growing as it collected more and more of what used to be Lluna and Kuu. Eventually this new moon would absorb the remaining ring debris and a new era in the life of Dimaan would begin.

WHAT IF THE EARTH HAD TWO MOONS? Copyright © 2010 by Neil F. Comins. All rights reserved. For information, address St. Martin's Press, 175 Fifth Avenue, New York, N.Y. 10010.