The first popular book to explain the dramatic theory behind the Moon's genesis
This lively science history relates one of the great recent breakthroughs in planetary astronomy-a successful theory of the birth of the Moon. Science journalist Dana Mackenzie traces the evolution of this theory, one little known outside the scientific community: a Mars-sized object collided with Earth some four billion years ago, and the remains of this colossal explosion-the Big Splat-came together to form the Moon. Beginning with notions of the Moon in ancient cosmologies, Mackenzie relates the fascinating history of lunar speculation, moving from Galileo and Kepler to George Darwin (son of Charles) and the Apollo astronauts, whose trips to the lunar surface helped solve one of the most enigmatic mysteries of the night sky: who hung the Moon?
Dana Mackenzie (Santa Cruz, CA) is a freelance science journalist. His articles have appeared in such magazines as Science, Discover, American Scientist, The Sciences, and New Scientist.
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The Big Splat, or How Our Moon Came to Be
By Dana Mackenzie
John Wiley & Sons, Inc.Copyright © 2003 Dana Mackenzie
All right reserved.
Chapter OneA Highly Practical Stone
The Moon that Neil Armstrong and Buzz Aldrin walked on was already very different from the one our ancestors had worshiped. It was hard for people watching on television to feel a spiritual connection to this new Moon-seemingly a place of desolation, not of spirituality. Some of the astronauts, to their credit, tried to express the new and alien sense of grandeur they felt, and some came home deeply changed by their experience. But for those who watched at home-and those who didn't watch-the Moon had become just a place, and not a terribly inviting one at that.
But in truth, our disconnection with the Moon began long before 1969. It has been going on for centuries. The Moon used to be more than a religious symbol and more than just a pretty ball in the sky: it was an integral part of daily life. Farmers used it to guide them in planting crops, and sailors watched the tides it produced. Travelers used it to find their way at night, and everybody used it to keep track of the passage of time. But over the centuries, all of these practical uses for the Moon have become weakened or obsolete as people developed a more sophisticated understanding of the natural world.
"The Parish Lantern"
It is hard for us to imagine today how utterly different the world of night used to be from the daylit world. Of course, we still can recreate something of that lost mystique. When we sit around a campfire and tell ghost stories, our goose bumps (or our children's) remind us of the terrors that night used to hold. But it is all too easy for us to pile in the car at the end of our camping trip and return to the comfort of our incandescent, fluorescent, floodlit modern world. Two thousand years ago, or even two hundred, there was no such escape from the darkness. It was a physical presence that gripped the world from sunset until the cock's crow.
"As different as night and day," we say today. But in centuries past, night and day really were different. The social order was inverted at night. At night the robber and the hoodlum were king, while law-abiding citizens cowered at home and the night watchman patrolled uneasily by torchlight. Night was, of course, the time for supernatural terrors as well, ghosts and werewolves and fairies and witches. Even whistling at night was dangerous-you risked inviting the devil.
In a time when every scrap of light after sunset was desperately appreciated, when travelers would mark the road by piling up light stones or by stripping the bark off trees to expose the lighter wood underneath, the Moon was the traveler's greatest friend. It was known in folklore as "the parish lantern." It was steady, portable, and-unlike a torch-entailed no risk of fire. It would never blow out, although it could, of course, hide behind a cloud. According to the Bible, that was why God gave us the Moon. "And God made two great lights; the greater light to rule the day, and the lesser light to rule the night."
Nowadays we don't need the Moon to divide the light from the darkness because electric lights do it for us. Many of us never even see a truly dark sky. According to a recent study on light pollution, 97 percent of the U.S. population lives under a night sky at least as bright as it was on a half-moon night in ancient times. Many city-dwellers live their entire lives under the equivalent of a full moon.
Although it took Thomas Edison's invention of the electric light-bulb to sever our dependence on moonlight, its influence was waning even before that. As early as the 1660s, Louis XIV (the "Sun King") installed the first permanent streetlights in Paris, to discourage the bands of vandals who had until then roamed unchecked at night. In 1784, a Swiss physicist named Aime Argand discovered that he could increase the brightness of an oil lamp tenfold by making the wick hollow, so that the flame could be fed by air from the inside as well as the outside. Argand lamps were the first artificial lights that could compete with the full Moon, but another invention soon outshone them. The gas lamp, invented in 1799 by Englishman William Murdoch, was ten times more powerful. Suddenly the nighttime was transformed: shops could remain open after dark, and factories could run all night. "Noctambulism" became a new fad in Paris, which became the "City of Light."
In 1879, when Thomas Edison finally invented the electric light-bulb, some people even thought its brightness was overkill. "Such a light as this should shine only on murderers and public crime, or along the corridors of lunatic asylums, a horror to heighten horror," wrote the author Robert Louis Stevenson in 1893. "To look at it once is to fall in love with gas." Nevertheless, the public hungered for more light. The gaslight faded into technological oblivion, and the Moon, a hundred times fainter than Edison's light, now circled the sky in forgotten splendor.
A similar tale, though a much older one, can be told about the Moon's other great service to humanity and how we eventually outgrew it.
The Greek philosopher Plato expressed a different view of the Moon's purpose from the one in Genesis. "Time came into being with the heavens in order that, having come into being together, they should also be dissolved together if they are ever dissolved," he wrote in the dialogue Timaeus. "As a result of this plan and purpose of god for the birth of time, the sun and moon and five planets ... came into being to define and preserve the measures of time."
Of the celestial bodies Plato mentioned, the Moon certainly was the first one that humans turned to for timekeeping for time scales longer than a day. The most celebrated prehistoric site in Europe, the cave paintings in Lascaux, France, include a probable lunar calendar that is fifteen thousand years old. Among the vibrant animal pictures in the Chamber of the Bulls, one also can find some curious patterns of dots. Some of these may represent constellations, such as the Pleiades. But others are too regular, such as a straight row of thirteen dots terminated by an empty square. What could thirteen dots in a row mean? According to Michael Rappenglueck, a historian of astronomy at the University of Munich, they might represent half of a lunar cycle: the time from the new moon to the full moon (or vice versa), with the empty square representing the day when the Moon disappears.
Deeper in the cave is an even more striking example: a dappled brown horse with a black mane, and a meandering row of black dots trailing from its muzzle. To the modern eye it looks as if the horse has broken free and is galloping along with its reins trailing behind it. But that naive interpretation is historically impossible: In the era of the Lascaux paintings, the horse was still a wild animal, and reins did not exist. Again, says Rappenglueck, the clue to the dots' meaning is their number. There are twenty-nine of them-this time, the number of days in a complete lunar cycle. "It was a rhythm of nature that was important to these people," says Rappenglueck. Evidently, the Moon's motions in the sky meant as much to them as the motions of their prey on the ground.
Although it takes a little bit of creative mind reading to discern a lunar calendar in a fifteen-thousand-year-old cave painting, there is no doubt that humans were attuned to the cycles of the Moon by the time the first civilizations emerged five thousand years ago. Throughout the world-in Egypt, Sumer, Central America, and Asia-the first calendars were lunar. In Asia and the Muslim world, lunar calendars remain in use to this day.
To appreciate the convenience of a lunar calendar, imagine yourself in a world with no wristwatches to keep track of the hours and days. Imagine no pinup calendars, no desk organizers with witty cartoons. Imagine no paper to print a calendar on, and no printing press to print it with. You have nothing to tell time with but your own wits. How might you do it?
If you insisted on using a solar calendar, you would have to count out 365 days from one winter solstice to the next. Counting to 365 is harder, of course, than counting to 29. Even if you kept count by making marks on a stick, or a cave wall, you would have that many more chances to make a mistake. Most likely you would entrust the matter to the village priest or medicine man. But even if he kept good records, it wouldn't be so easy for him to determine exactly when the beginning of the year is. The solstice is the time of year when "the Sun stands still"-that's what "solstice" means in Latin. In other words, the Sun's highest point in the sky that day is almost the same as its highest point the next day, and the day after that. The very thing that makes the day unique also makes it hard to identify. It's like trying to time the moment when the stock market hits its peak.
Of course, one might argue that the winter solstice is the shortest day of the year, so the medicine man would only need to measure which day has the shortest time between sunrise and sunset. But keep in mind that your medicine man would have no way to measure hours and minutes accurately. In the ancient world there were no stopwatches, nor even any absolute measurements of time. When the hour was invented by the ancient Egyptians, it was simply one-twelfth part of a day or one-twelfth part of a night. So every day was twelve hours long-there was no "shortest day"!
All right, then, why not erect some kind of very visible monument, so that when the tip of that monument's shadow touches a certain point, or when the Sun is just visible through a crack in the stone, you would know that it is midwinter or midsummer? That is exactly what several ancient cultures did, from the builders of Stonehenge to the carvers of petroglyphs in Mesoamerica. But this solution to the solstice problem requires, at a minimum, a high degree of social organization. Moreover, it is not very portable.
All in all, a solar calendar is beginning to seem like a very expensive and inconvenient proposition. Contrast this with the simplicity of a lunar calendar. Instead of counting to 365, you only need to count to 28 at most, the number of days between new Moons. It's much easier to remember that today is the 11th day since the new Moon than to remember that it's the 164th day since the winter solstice. And even if you forget, the Moon itself can remind you. It changes its shape visibly in the sky from night to night. With a little experience, you can easily tell an 11-day-old Moon from a 9-day-old one.
For all these reasons, many early societies were tied to the rhythm of the Moon. The days of the month were more than numbers on a page. Each day and each time of the month acquired its own meaning. Hesiod wrote a book called Works and Days, which might be called the Farmer's Almanac of ancient Greece. Along with all sorts of choice advice on matters large and small, from when to marry to which direction to face when urinating, Hesiod also tells which days of the month are propitious for which actions: "On the fourth of the month bring home your bride, but choose the omens that are best for this business. Avoid fifth days: they are unkindly and terrible.... On the great twentieth, in full day, a wise man should be born. Such a one is very soundwitted. The tenth is favorable for a male to be born; but, for a girl, the fourth day of the midmonth."
No explanation is given for most of these admonitions, but the Greeks must have taken them seriously, as Hesiod's work is one of the oldest texts preserved to modern times. We still can see an echo of this tradition in the nursery rhyme "Monday's child is fair of face, Tuesday's child is full of grace." In ancient times these characteristics would have been ascribed to the days of the month, not the days of the week.
For cultures that were advanced enough to use either kind of calendar, the choice between lunar and solar calendars depended on the culture's priorities and the use it made of that calendar. The fact that the solar calendar is used in most of the Western world does not make it "better."
In Islam, each month begins when two reliable witnesses sight the crescent Moon. According to some traditions, the observers must be local, which means that the month may begin on different days in different parts of the world. (The first crescent may not yet be visible when the Sun sets in Tajikistan, but several hours later, when the Sun sets in New York, the crescent may be wide enough to become visible to the naked eye.) Because a physical observation of the Moon may depend on atmospheric conditions, the first day of the month cannot necessarily be predicted in advance. To a non-Muslim, this unpredictability might seem like a nuisance, but consider also the spiritual importance of this custom. Every Islamic community is in charge of its own time, and cannot rely on the learned pronouncement of some distant authority. Every community is joined intimately to the sky, and through the sky to Allah.
Another possible inconvenience of a lunar calendar stems from the fact that twelve lunar months add up to a little over 354 days. This means that the Islamic year is 11 days shorter than a solar year, and hence significant religious events such as Ramadan gradually migrate from season to season. Of course, Islamic astronomers could have produced solar calendars if they had wanted to; they were the world's most proficient astronomers until at least the European Renaissance. It simply wasn't important to them to synchronize their calendar to the seasons.
However, many other cultures have tried to devise calendars that synchronized both the lunar cycle and the solar cycle-a "luni-solar" calendar. They named moons after events that occurred at a specific time of year: harvest moon, hunter's moon, honey moon. But to make such a scheme work, it was necessary to add, or "intercalate," an extra moon in some years. Otherwise the harvest moon would slowly drift into spring.
An imperfect way to bring the lunar and solar cycles into harmony was instituted by the Babylonians as long ago as 528 B.C. Since 12 lunar cycles fall 11 days short of a solar year, it stands to reason that over an 8-year period 96 (8 x 12) moons would fall 88 (8 x 11) days short of 8 complete solar years. But 88 days is very close to three lunar cycles, so the lunar and solar calendars could be kept in step by adding three months at prescribed times every eight years.
A century later, an Athenian astronomer named Meton found an even better match: 19 years are almost exactly the same as 235 months. Using modern values for the length of the tropical year (365.2422 days) and the synodic month (29.53059 days), we can calculate just how good the match is. In 432 B.C.
Excerpted from The Big Splat, or How Our Moon Came to Be by Dana Mackenzie Copyright © 2003 by Dana Mackenzie . Excerpted by permission.
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Table of ContentsIntroduction: Genesis Revised.
1. A Highly Practical Stone.
2. The Stone Star.
3. Kepler Laughed.
4. The Clockwork Solar System.
5. Daughter Moon.
6. Captive Moon.
7. Sister Moon.
8. Renaissance and Controversy.
9. “A Little Science on the Moon”.
10. When Worlds Collide.
11. The Kona Consensus.
12. Introducing Theia.
Appendix: Did We Really Go to the Moon?
What People are Saying About This
Ace science writer Mackenzie's account of humanity's long relationship with Earth's only natural satellite, from a probable lunar calendar found in the Lascaux caves to the new "giant impact" theory of the moon's origin, is magnetically readable, preternaturally clear, and amazingly concise.
— Booklist Editors' Choice '03