Pale Blue Dot: A Vision of the Human Future in Space - Carl Sagan, Ann Druyan (1997)


I implore you, you do not hope to be able to give the reasons for the number of planets, do you?

This worry has been resolved …

BOOK 4 (1621)

Before we invented civilization, our ancestors lived mainly in the open, out under the sky. Before we devised artificial lights and atmospheric pollution and modern forms of nocturnal entertainment, we watched the stars. There were practical calendrical reasons, of course, but there was more to it than that. Even today, the most jaded city dweller can be unexpectedly moved upon encountering a clear night sky studded with thousands of twinkling stars. When it happens to me, after all these years, it still takes my breath away.

In every culture, the sky and the religious impulse are intertwined. I lie back in an open field and the sky surrounds me. I’m overpowered by its scale. It’s so vast and so far away that my own insignificance becomes palpable. But I don’t feel rejected by the sky. I’m a part of it—tiny, to be sure, but everything is tiny compared to that overwhelming immensity. And when I concentrate on the stars, the planets, and their motions, I have an irresistible sense of machinery, clockwork, elegant precision working on a scale that, however lofty our aspirations, dwarfs and humbles us.

Most of the great inventions in human history—from stone tools and the domestication of fire to written language—were made by unknown benefactors. Our institutional memory of long-gone events is feeble. We do not know the name of that ancestor who first noted that planets were different from stars. She or he must have lived tens, perhaps even hundreds of thousands of years ago. But eventually people all over the world understood that five, no more, of the bright points of light that grace the night sky break lockstep with the others over a period of months, moving strangely—almost as if they had minds of their own.

Sharing the odd apparent motion of these planets were the Sun and Moon, making seven wandering bodies in all. These seven were important to the ancients, and they named them after gods—not any old gods, but the main gods, the chief gods, the ones who tell other gods (and mortals) what to do. One of the planets, bright and slow-moving, was named by the Babylonians after Marduk, by the Norse after Odin, by the Greeks after Zeus, and by the Romans after Jupiter, in each case the king of the gods. The faint, fast-moving one that was never far from the Sun the Romans named Mercury, after the messenger of the gods; the most brilliant of them was named Venus, after the goddess of love and beauty; the bloodred one Mars, after the god of war; and the most sluggish of the bunch Saturn, after the god of time. These metaphors and allusions were the best our ancestors could do: They possessed no scientific instruments beyond the naked eye, they were confined to the Earth, and they had no idea that it, too, is a planet.*

When it got to be time to design the week—a period of time, unlike the day, month, and year, with no intrinsic astronomical significance—it was assigned seven days, each named after one of the seven anomalous lights in the night sky. We can readily make out the remnants of this convention. In English, Saturday is Saturn’s day. Sunday and Mo[o]nday are clear enough. Tuesday through Friday are named after the gods of the Saxon and kindred Teutonic invaders of Celtic/Roman Britain: Wednesday, for example, is Odin’s (or Wodin’s) day, which would be more apparent if we pronounced it as it’s spelled, “Wedn’s Day”; Thursday is Thor’s day; Friday is the day of Freya, goddess of love. The last day of the week stayed Roman, the rest of it became German.

In all Romance languages, such as French, Spanish, and Italian, the connection is still more obvious, because they all derive from ancient Latin, in which the days of the week were named (in order, beginning with Sunday) after the Sun, the Moon, Mars, Mercury, Jupiter, Venus, and Saturn. (The Sun’s day became the Lord’s day.) They could have named the days in order of the brightness of the corresponding astronomical bodies—the Sun, the Moon, Venus, Jupiter, Mars, Saturn, Mercury (and thus Sunday, Monday, Friday, Thursday, Tuesday, Saturday, Wednesday)—but they did not. If the days of the week in Romance languages had been ordered by distance from the Sun, the sequence would be Sunday, Wednesday, Friday, Monday, Tuesday, Thursday, Saturday. No one knew the order of the planets, though, back when we were naming planets, gods, and days of the week. The ordering of the days of the week seems arbitrary, although perhaps it does acknowledge the primacy of the Sun.

This collection of seven gods, seven days, and seven worlds—the Sun, the Moon, and the five wandering planets—entered the perceptions of people everywhere. The number seven began to acquire supernatural connotations. There were seven “heavens,” the transparent spherical shells, centered on the Earth, that were imagined to make these worlds move. The outermost—the seventh heaven—is where the “fixed” stars were imagined to reside. There are Seven Days of Creation (if we include God’s day of rest), seven orifices to the head, seven virtues, seven deadly sins, seven evil demons in Sumerian myth, seven vowels in the Greek alphabet (each affiliated with a planetary god), Seven Governors of Destiny according to the Hermetists, Seven Great Books of Manichaeism, Seven Sacraments, Seven Sages of Ancient Greece, and seven alchemical “bodies” (gold, silver, iron, mercury, lead, tin, and copper—gold still associated with the Sun, silver with the Moon, iron with Mars, etc.). The seventh son of a seventh son is endowed with supernatural powers. Seven is a “lucky” number. In the New Testament’s Book of Revelations, seven seals on a scroll are opened, seven trumpets are sounded, seven bowls are filled. St. Augustine obscurely argued for the mystic importance of seven on the grounds that three “is the first whole number that is odd” (what about one?), “four the first that is even” (what about two?), and “of these … seven is composed.” And so on. Even in our time these associations linger.

The existence even of the four satellites of Jupiter that Galileo discovered—hardly planets—was disbelieved on the grounds that it challenged the precedence of the number seven. As acceptance of the Copernican system grew, the Earth was added to the list of planets, and the Sun and Moon were removed. Thus, there seemed to be only six planets (Mercury, Venus, Earth, Mars, Jupiter, and Saturn). So learned academic arguments were invented showing why there had to be six. For example, six is the first “perfect” number, equal to the sum of its divisors (1 + 2 + 3). Q.E.D. And anyway, there were only six days of creation, not seven. People found ways to accommodate from seven planets to six.

As those adept at numerological mysticism adjusted to the Copernican system, this self-indulgent mode of thinking spilled over from planets to moons. The Earth had one moon; Jupiter had the four Galilean moons. That made five. Clearly one was missing. (Don’t forget: Six is the first perfect number.) When Huygens discovered Titan in 1655, he and many others convinced themselves that it was the last: Six planets, six moons, and God’s in His Heaven.

The historian of science I. Bernard Cohen of Harvard University has pointed out that Huygens actually gave up searching for other moons because it was apparent, from such arguments, that no more were to be found. Sixteen years later, ironically with Huygens in attendance, G. D. Cassini* of the Paris Observatory discovered a seventh moon—Iapetus, a bizarre world with one hemisphere black and the other white, in an orbit exterior to Titan’s. Shortly after, Cassini discovered Rhea, the next Saturnian moon interior to Titan.

Here was another opportunity for numerology, this time harnessed to the practical task of flattering patrons. Cassini added up the number of planets (six) and the number of satellites (eight) and got fourteen. Now it so happened that the man who built Cassini’s observatory for him and paid his salary was Louis XIV of France, the Sun King. The astronomer promptly “presented” these two new moons to his sovereign and proclaimed that Louis’s “conquests” reached to the ends of the Solar System. Discreetly, Cassini then backed off from looking for more moons; Cohen suggests he was afraid one more might now offend Louis—a monarch not to be trifled with, who would shortly be throwing his subjects into dungeons for the crime of being Protestants. Twelve years later, though, Cassini returned to the search and found—doubtless with a measure of trepidation—another two moons. (It is probably a good thing that we have not continued in this vein; otherwise France would have been burdened by seventy-some-odd Bourbon kings named Louis.)

WHEN CLAIMS OF NEW WORLDS WERE MADE in the late eighteenth century, the force of such numerological arguments had much dissipated. Still, it was with a real sense of surprise that people heard in 1781 about a new planet, discovered through the telescope. New moons were comparatively unimpressive, especially after the first six or eight. But that there were new planets to be found and that humans had devised the means to do so were both considered astonishing, and properly so. If there is one previously unknown planet, there may be many more—in this solar system and in others. Who can tell what might be found if a multitude of new worlds are hiding in the dark?

The discovery was made not even by a professional astronomer but by William Herschel, a musician whose relatives had come to Britain with the family of another anglified German, the reigning monarch and future oppressor of the American colonists, George III. It became Herschel’s wish to call the planet George (“George’s Star,” actually), after his patron, but, providentially, the name didn’t stick. (Astronomers seem to have been very busy buttering up kings.) Instead, the planet that Herschel found is called Uranus (an inexhaustible source of hilarity renewed in each generation of English-speaking nine-year-olds). It is named after the ancient sky god who, according to Greek myth, was Saturn’s father and thus the grandfather of the Olympian gods.

We no longer consider the Sun and Moon to be planets, and—ignoring the comparatively insignificant asteroids and comets—count Uranus as the seventh planet in order from the Sun (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto). It is the first planet unknown to the ancients. The four outer, Jovian, planets turn out to be very different from the four inner, terrestrial, planets. Pluto is a separate case.

As the years passed and the quality of astronomical instruments improved, we began to learn more about distant Uranus. What reflects the dim sunlight back to us is no solid surface, but atmosphere and clouds—just as for Titan, Venus, Jupiter, Saturn, and Neptune. The air on Uranus is made of hydrogen and helium, the two simplest gases. Methane and other hydrocarbons are also present. Just below the clouds visible to Earthbound observers is a massive atmosphere with enormous quantities of ammonia, hydrogen sulfide, and, especially, water.

At depth on Jupiter and Saturn, the pressures are so great that atoms sweat electrons, and the air becomes a metal. That does not seem to happen on less massive Uranus, because the pressures at depth are less. Still deeper, discovered only by its subtle tugs on Uranus’ moons, wholly inaccessible to view, under the crushing weight of the overlying atmosphere, is a rocky surface. A big Earthlike planet is hiding down there, swathed in an immense blanket of air.

The Earth’s surface temperature is due to the sunlight it intercepts. Turn off the Sun and the planet soon chills—not to trifling Antarctic cold, not just so cold that the oceans freeze, but to a cold so intense that the very air precipitates out, forming a ten-meter-thick layer of oxygen and nitrogen snows covering the whole planet. The little bit of energy that trickles up from the Earth’s hot interior would be insufficient to melt these snows. For Jupiter, Saturn, and Neptune it’s different. There’s about as much heat pouring out from their interiors as they acquire from the warmth of the distant Sun. Turn off the Sun, and they would be only a little affected.

But Uranus is another story. Uranus is an anomaly among the Jovian planets. Uranus is like the Earth: There’s very little intrinsic heat pouring out. We have no good understanding of why this should be, why Uranus—which in many respects is so similar to Neptune—should lack a potent source of internal heat. For this reason, among others, we cannot say we understand what is going on in the deep interiors of these mighty worlds.

Uranus is lying on its side as it goes around the Sun. In the 1990s, the south pole is heated by the Sun, and it is this pole that Earthbound observers at the end of the twentieth century see when they look at Uranus. It takes Uranus 84 Earth years to make one circuit of the Sun. So in the 2030s, the north pole will be sunward (and Earthward). In the 2070s the south pole will be pointing to the Sun once again. In between, Earthbound astronomers will be looking mainly at equatorial latitudes.

All the other planets spin much more upright in their orbits. No one is sure of the reason for Uranus’ anomalous spin; the most promising suggestion is that sometime in its early history, billions of years ago, it was struck by a rogue planet, about the size of the Earth, in a highly eccentric orbit. Such a collision, if it ever happened, must have worked much tumult in the Uranus system; for all we know, there may be other vestiges of ancient havoc still left for us to find. But Uranus’ remoteness tends to guard its mysteries.

In 1977 a team of scientists led by James Elliot, then of Cornell University, accidentally discovered that, like Saturn, Uranus has rings. The scientists were flying over the Indian Ocean in a special NASA airplane—the Kuiper Airborne Observatory—to witness the passage of Uranus in front of a star. (Such passages, or occultations as they’re called, happen from time to time, precisely because Uranus slowly moves with respect to the distant stars.) The observers were surprised to find that the star winked on and off several times just before it passed behind Uranus and its atmosphere, then several times more just after it emerged. Since the patterns of winking on and off were the same before and after occultation, this finding (and much subsequent work) has led to the discovery of nine very thin, very dark circum-planetary rings, giving Uranus the appearance of a bull’s-eye in the sky.

Surrounding the rings, Earthbound observers understood, were the concentric orbits of the five moons then known: Miranda, Ariel, Umbriel, Titania, and Oberon. They’re named after characters in Shakespeare’s A Midsummer Night’s Dream and The Tempest, and in Alexander Pope’s The Rape of the Lock. Two of them were found by Herschel himself. The innermost of the five, Miranda, was discovered as recently as 1948, by my teacher G. P. Kuiper.* I remember how great an achievement the discovery of a new moon of Uranus was considered back then. The near-infrared light reflected by all five moons subsequently revealed the spectral signature of ordinary water ice on their surfaces. And no wonder—Uranus is so far from the Sun that it is no brighter there at noontime than it is after sunset on Earth. The temperatures are frigid. Any water must be frozen.

A REVOLUTION IN OUR UNDERSTANDING of the Uranus system—the planet, its rings, and its moons—began on January 24, 1986. On that day, after a journey of 8½ years, the Voyager 2 spacecraft sailed very near to Miranda, and hit the bull’s-eye in the sky. Uranus’ gravity then flung it on to Neptune. The spacecraft returned 4,300 close-up pictures of the Uranus system and a wealth of other data.

Uranus was found to be surrounded by an intense radiation belt, electrons and protons trapped by the planet’s magnetic field. Voyager flew through this radiation belt, measuring the magnetic field and the trapped charged particles as it went. It also detected—in changing timbres, harmonies, and nuance, but mainly in fortissimo—a cacaphony of radio waves generated by the speeding, trapped particles. Something similar was discovered on Jupiter and Saturn and would be later found at Neptune—but always with a theme and counterpoint characteristic of each world.

On Earth the magnetic and geographical poles are quite close together. On Uranus the magnetic axis and the axis of rotation are tilted away from each other by some 60 degrees. No one yet understands why: Some have suggested that we are catching Uranus in a reversal of its north and south magnetic poles, as periodically happens on Earth. Others propose that this too is the consequence of that mighty, ancient collision that knocked the planet over. But we do not know.

Uranus is emitting much more ultraviolet light than it’s receiving from the Sun, probably generated by charged particles leaking out of the magnetosphere and striking its upper atmosphere. From a vantage point in the Uranus system, the spacecraft examined a bright star winking on and off as the rings of Uranus passed by. New faint dust bands were found. From the perspective of Earth, the spacecraft passed behind Uranus; so the radio signals it was transmitting back home passed tangentially through the Uranian atmosphere, probing it—to below its methane clouds. A vast and deep ocean, perhaps 8,000 kilometers thick, of super-heated liquid water floating in the air is inferred by some.

Among the principal glories of the Uranus encounter were the pictures. With Voyager’s two television cameras, we discovered ten new moons, determined the length of the day in the clouds of Uranus (about 17 hours), and studied about a dozen rings. The most spectacular pictures were those returned from the five larger, previously known moons of Uranus, especially the smallest of them, Kuiper’s Miranda. Its surface is a tumult of fault valleys, parallel ridges, sheer cliffs, low mountains, impact craters, and frozen floods of once-molten surface material. This turmoiled landscape is unexpected for a small, cold, icy world so distant from the Sun. Perhaps the surface was melted and reworked in some long-gone epoch when a gravitational resonance between Uranus, Miranda, and Ariel pumped energy from the nearby planet into Miranda’s interior. Or perhaps we are seeing the results of the primordial collision that is thought to have knocked Uranus over. Or, just conceivably, maybe Miranda was once utterly destroyed, dismembered, blasted into smithereens by a wild careening world, with many collision fragments still left in Miranda’s orbit. The shards and remnants, slowly colliding, gravitationally attracting one another, may have reaggregated into just such a jumbled, patchy, unfinished world as Miranda is today.

For me, there’s something eerie about the pictures of dusky Miranda, because I can remember so well when it was only a faint point of light almost lost in the glare of Uranus, discovered through great difficulty by dint of the astronomer’s skills and patience. In only half a lifetime it has gone from an undiscovered world to a destination whose ancient and idiosyncratic secrets have been at least partially revealed.

*There was one moment in the last 4,000 years when all seven of these celestial bodies were clustered tightly together. Just before dawn on March 4, 1953 B.C., the crescent Moon was at the horizon. Venus, Mercury, Mars, Saturn, and Jupiter were strung out like jewels on a necklace near the great square in the constellation Pegasus—near the spot from which in our time the Perseid meteor shower emanates. Even casual watchers of the sky must have been transfixed by the event. What was it—a communion of the gods? According to the astronomer David Pankenier of Lehigh University and later Kevin Pang of JPL, this event was the starting point for the planetary cycles of the ancient Chinese astronomers.
    There is no other time in the last 4,000 years (or in the next) when the dance of the planets around the Sun brings them so close together from the vantage point of Earth. But on May 5, 2000, all seven will be visible in the same part of the sky—although some at dawn and some at dusk and about ten times more spread out than on that late winter morning in 1953 B.C. Still, it’s probably a good night for a party.

*After whom the European-American mission to the Saturn system is named.

*He so named it because of the words spoken by Miranda, the heroine of The Tempest: “O brave new world, That has such people in’t.” (To which Prospero replies, “Tis new to thee.” Just so. Like all the other worlds in the Solar System, Miranda is about 4.5 billion years old.)