Death By Black Hole Part 2

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designed to ensure that no one else could s.n.a.t.c.h prior credit for his radical and as-yet-unpublished discovery. When sorted out and translated from the Latin, the anagram becomes: "I have observed the highest planet to be triple-bodied." As the years went by, Galileo continued to monitor Saturn's companions. At one stage they looked like ears; at another stage they vanished completely.

In 1656 the Dutch physicist Christiaan Huygens viewed Saturn through a telescope of much higher resolution than Galileo's, built for the express purpose of scrutinizing the planet. He became the first to interpret Saturn's earlike companions as a simple, flat ring. As Galileo had done half a century earlier, Huygens wrote down his groundbreaking but still preliminary finding in the form of an anagram. Within three years, in his book Systema Saturnium, Systema Saturnium, Huygens went public with his proposal. Huygens went public with his proposal.

Twenty years later Giovanni Ca.s.sini, the director of the Paris Observatory, pointed out that there were two rings, separated by a gap that came to be known as the Ca.s.sini division. And nearly two centuries later, the Scottish physicist James Clerk Maxwell won a prestigious prize for showing that Saturn's rings are not solid, but made up instead of numerous small particles in their own orbits.

By the end of the twentieth century, observers had identified seven distinct rings, lettered A through G. Not only that, the rings themselves turn out to be made up of thousands upon thousands of bands and ringlets.

So much for the "ear theory" of Saturn's rings.

SEVERAL SATURN FLYBYS took place in the twentieth century: took place in the twentieth century: Pioneer 11 Pioneer 11 in 1979, in 1979, Voyager 1 Voyager 1 in 1980, and in 1980, and Voyager 2 Voyager 2 in 1981. Those relatively close inspections all yielded evidence that the ring system is more complex and more puzzling than anyone had imagined. For one thing, the particles in some of the rings corral into narrow bands by the so-called shepherd moons: teeny satellites that orbit near and within the rings. The gravitational forces of the shepherd moons tug the ring particles in different directions, sustaining numerous gaps among the rings. in 1981. Those relatively close inspections all yielded evidence that the ring system is more complex and more puzzling than anyone had imagined. For one thing, the particles in some of the rings corral into narrow bands by the so-called shepherd moons: teeny satellites that orbit near and within the rings. The gravitational forces of the shepherd moons tug the ring particles in different directions, sustaining numerous gaps among the rings.

Density waves, orbital resonances, and other quirks of gravitation in multiple-particle systems give rise to pa.s.sing features within and among the rings. Ghostly, s.h.i.+fting "spokes" in Saturn's B ring, for instance-recorded by the Voyager Voyager s.p.a.ce probes and presumed to be caused by the planet's magnetic field-have mysteriously vanished from close-up views supplied by the s.p.a.ce probes and presumed to be caused by the planet's magnetic field-have mysteriously vanished from close-up views supplied by the Ca.s.sini Ca.s.sini s.p.a.cecraft, sending images from Saturnian orbit. s.p.a.cecraft, sending images from Saturnian orbit.

What kind of stuff are Saturn's rings made of? Water ice, for the most part-though there's also some dirt mixed in, whose chemical makeup is similar to one of the planet's larger moons. The cosmo-chemistry of the environment suggests that Saturn might once have had several such moons. Those that went AWOL may have orbited too close for comfort to the giant planet and gotten ripped apart by Saturn's tidal forces.

Saturn, by the way, is not the only planet with a ring system. Close-up views of Jupiter, Ura.n.u.s, and Neptune-the rest of the big four gas giants in our solar system-show that each planet bears a ring system of its own. The Jovian, Uranian, and Neptunian rings weren't discovered until the late 1970s and early 1980s, because, unlike Saturn's majestic ring system, they're made largely of dark, unreflective substances such as rocks or dust grains.

THE s.p.a.cE NEAR a planet can be dangerous if you're not a dense, rigid object. As we will see in Section 2, many comets and some asteroids, for instance, resemble piles of rubble, and they swing near planets at their peril. The magic distance, within which a planet's tidal force exceeds the gravity holding together that kind of vagabond, is called the Roche limit-discovered by the nineteenth-century French astronomer edouard Albert Roche. Wander inside the Roche limit, and you'll get torn apart; your disa.s.sembled bits and pieces will then scatter into their own orbits and eventually spread out into a broad, flat, circular ring. a planet can be dangerous if you're not a dense, rigid object. As we will see in Section 2, many comets and some asteroids, for instance, resemble piles of rubble, and they swing near planets at their peril. The magic distance, within which a planet's tidal force exceeds the gravity holding together that kind of vagabond, is called the Roche limit-discovered by the nineteenth-century French astronomer edouard Albert Roche. Wander inside the Roche limit, and you'll get torn apart; your disa.s.sembled bits and pieces will then scatter into their own orbits and eventually spread out into a broad, flat, circular ring.

I recently received some upsetting news about Saturn from a colleague who studies ring systems. He noted with sadness that the orbits of their const.i.tuent particles are unstable, and so the particles will all be gone in an astrophysical blink of an eye: 100 million years or so. My favorite planet, shorn of what makes it my favorite planet! Turns out, fortunately, that the steady and essentially unending accretion of interplanetary and intermoon particles may replenish the rings. The ring system-like the skin on your face-may persist, even if its const.i.tuent particles do not.

Other news has come to Earth via Ca.s.sini' Ca.s.sini's close-up pictures of Saturn's rings. What kind of news? "Mind-boggling" and "startling," to quote Carolyn C. Porco, the leader of the mission's imaging team and a specialist in planetary rings at the s.p.a.ce Science Inst.i.tute in Boulder, Colorado. Here and there in all those rings are features neither expected nor, at present, explainable: scalloped ringlets with extremely sharp edges, particles coalescing in clumps, the pristine iciness of the A and B rings compared with the dirtiness of the Ca.s.sini division between them. All these new data will keep Porco and her colleagues busy for years to come, perhaps wistfully recalling the clearer, simpler view from afar.

FIVE.

STICK-IN-THE-MUD SCIENCE.

For a century or two, various blends of high technology and clever thinking have driven cosmic discovery. But suppose you have no technology. Suppose all you have in your backyard laboratory is a stick. What can you learn? Plenty.

With patience and careful measurement, you and your stick can glean an outrageous amount of information about our place in the cosmos. It doesn't matter what the stick is made of. And it doesn't matter what color it is. The stick just has to be straight. Hammer the stick firmly into the ground where you have a clear view of the horizon. Since you're going low-tech, you might as well use a rock for a hammer. Make sure the stick isn't floppy and that it stands up straight.

Your caveman laboratory is now ready.

On a clear morning, track the length of the stick's shadow as the Sun rises, crosses the sky, and finally sets. The shadow will start long, get shorter and shorter until the Sun reaches its highest point in the sky, and finally lengthen again until sunset. Collecting data for this experiment is about as exciting as watching the hour hand move on a clock. But since you have no technology, not much else competes for your attention. Notice that when the shadow is shortest, half the day has pa.s.sed. At that moment-called local noon-the shadow points due north or due south, depending which side of the equator you're on.

You've just made a rudimentary sundial. And if you want to sound erudite, you can now call the stick a gnomon (I still prefer "stick"). Note that in the Northern Hemisphere, where civilization began, the stick's shadow will revolve clockwise around the base of the stick as the Sun moves across the sky. Indeed, that's why the hands of a clock turn "clockwise" in the first place.

If you have enough patience and cloudless skies to repeat the exercise 365 times in a row, you will notice that the Sun doesn't rise from day to day at the same spot on the horizon. And on two days a year the shadow of the stick at sunrise points exactly opposite the shadow of the stick at sunset. When that happens, the Sun rises due east, sets due west, and daylight lasts as long as night. Those two days are the spring and fall equinoxes (from the Latin for "equal night"). On all other days of the year the Sun rises and sets elsewhere along the horizon. So the person who invented the adage "the Sun always rises in the east and sets in the west" simply never paid attention to the sky.

If you're in the Northern Hemisphere while tracking the rise and set points for the Sun, you'll see that those spots creep north of the east-west line after the spring equinox, eventually stop, and then creep south for a while. After they cross the east-west line again, the southward creeping eventually slows down, stops, and gives way to the northward creeping once again. The entire cycle repeats annually.

All the while, the Sun's trajectory is changing. On the summer solstice (Latin for "stationary Sun"), the Sun rises and sets at its northernmost point along the horizon, tracing its highest path across the sky. That makes the solstice the year's longest day, and the stick's noontime shadow on that day the shortest. When the Sun rises and sets at its southernmost point along the horizon, its trajectory across the sky is the lowest, creating the year's longest noontime shadow. What else to call that day but the winter solstice?

For 60 percent of Earth's surface and about 75 percent of its human inhabitants, the Sun is never, ever directly overhead. For the rest of our planet, a 3,200-mile-wide belt centered on the equator, the Sun climbs to the zenith only two days a year (okay, just one day a year if you're smack on the Tropic of Cancer or the Tropic of Capricorn). I'd bet the same person who professed to know where the Sun rises and sets on the horizon also started the adage "the Sun is directly overhead at high noon."

So far, with a single stick and profound patience, you have identified the cardinal points on the compa.s.s and the four days of the year that mark the change of seasons. Now you need to invent some way to time the interval between one day's local noon and the next. An expensive chronometer would help here, but one or more well-made hourgla.s.ses will also do just fine. Either timer will enable you to determine, with great accuracy, how long it takes for the Sun to revolve around Earth: the solar day. Averaged over the entire year, that time interval equals 24 hours, exactly. Although this doesn't include the leap-second added now and then to account for the slowing of Earth's rotation by the Moon's gravitational tug on Earth's oceans.

Back to you and your stick. We're not done yet. Establish a line of sight from its tip to a spot on the sky, and use your trusty timer to mark the moment a familiar star from a familiar constellation pa.s.ses by. Then, still using your timer, record how long it takes for the star to realign with the stick from one night to the next. That interval, the sidereal day, lasts 23 hours, 56 minutes, and 4 seconds. The almost-four-minute mismatch between the sidereal and solar days forces the Sun to migrate across the patterns of background stars, creating the impression that the Sun visits the stars in one constellation after another throughout the year.

Of course, you can't see stars in the daytime-other than the Sun. But the ones visible near the horizon just after sunset or just before sunrise flank the Sun's position on the sky, and so a sharp observer with a good memory for star patterns can figure out what patterns lie behind the Sun itself.

Once again taking advantage of your timing device, you can try something different with your stick in the ground. Each day for an entire year, mark where the tip of the stick's shadow falls at noon, as indicated by your timer. Turns out that each day's mark will fall in a different spot, and by the end of the year you will have traced a figure eight, known to the erudite as an "a.n.a.lemma."

Why? Earth tilts on its axis by 23.5 degrees from the plane of the solar system. This tilt not only gives rise to the familiar seasons and the wide-ranging daily path of the Sun across the sky, it's also the dominant cause of the figure eight that emerges as the Sun migrates back and forth across the celestial equator throughout the year. Moreover, Earth's...o...b..t about the Sun is not a perfect circle. According to Kepler's laws of planetary motion, its...o...b..tal speed must vary, increasing as we near the Sun and slowing down as we recede. Because the rate of Earth's rotation remains rock-steady, something has to give: the Sun does not always reach its highest point on the sky at "clock noon." Although the s.h.i.+ft is slow from day to day, the Sun gets there as much as 14 minutes late at certain times of year. At other times it's as much as 16 minutes early. On only four days a year-corresponding to the top, the bottom, and the middle crossing of the figure eight-is clock time equal to Sun time. As it happens, the days fall on or about April 15 (no relation to taxes), June 14 (no relation to flags), September 2 (no relation to labor), and December 25 (no relation to Jesus).

Next up, clone yourself and your stick and send your twin due south to a prechosen spot far beyond your horizon. Agree in advance that you will both measure the length of your stick shadows at the same time on the same day. If the shadows are the same length, you live on a flat or a supergigantic Earth. If the shadows have different lengths, you can use simple geometry to calculate Earth's circ.u.mference.

The astronomer and mathematician Eratosthenes of Cyrene (276194 B.C B.C.) did just that. He compared shadow lengths at noon from two Egyptian cities-Syene (now called Aswan) and Alexandria, which he overestimated to be 5,000 stadia apart. Eratosthenes' answer for Earth's circ.u.mference was within 15 percent of the correct value. The word "geometry," in fact, comes from the Greek for "earth measurement."

Although you've now been occupied with sticks and stones for several years, the next experiment will take only about a minute. Pound your stick into the ground at an angle other than vertical, so that it resembles a typical stick in the mud. Tie a stone to the end of a thin string and dangle it from the stick's tip. Now you've got a pendulum. Measure the length of the string and then tap the bob to set the pendulum in motion. Count how many times the bob swings in 60 seconds.

The number, you'll find, depends very little on the width of the pendulum's arc, and not at all on the ma.s.s of the bob. The only things that matter are the length of the string and what planet you're on. Working with a relatively simple equation, you can deduce the acceleration of gravity on Earth's surface, which is a direct measure of your weight. On the Moon, with only one-sixth the gravity of Earth, the same pendulum will move much more slowly, executing fewer swings per minute.

There's no better way to take the pulse of a planet.

UNTIL NOW YOUR stick has offered no proof that Earth itself rotates-only that the Sun and the nighttime stars revolve at regular, predictable intervals. For the next experiment, find a stick more than 10 yards long and, once again, pound it into the ground at a tilt. Tie a heavy stone to the end of a long, thin string and dangle it from the tip. Now, just like last time, set it in motion. The long, thin string and the heavy bob will enable the pendulum to swing unenc.u.mbered for hours and hours and hours. stick has offered no proof that Earth itself rotates-only that the Sun and the nighttime stars revolve at regular, predictable intervals. For the next experiment, find a stick more than 10 yards long and, once again, pound it into the ground at a tilt. Tie a heavy stone to the end of a long, thin string and dangle it from the tip. Now, just like last time, set it in motion. The long, thin string and the heavy bob will enable the pendulum to swing unenc.u.mbered for hours and hours and hours.

If you carefully track the direction the pendulum swings, and if you're extremely patient, you will notice that the plane of its swing slowly rotates. The most pedagogically useful place to do this experiment is at the geographic North (or, equivalently, South) Pole. At the Poles, the plane of the pendulum's swing makes one full rotation in 24 hours-a simple measure of the direction and rotational speed of the earth beneath it. For all other positions on Earth, except along the equator, the plane still turns, but more and more slowly as you move from the Poles toward the equator. At the equator the plane of the pendulum does not move at all. Not only does this experiment demonstrate that it's Earth, not the Sun, that moves, but with the help of a little trigonometry you can also turn the question around and use the time needed for one rotation of the pendulum's plane to determine your geographic lat.i.tude on our planet.

The first person to do this was Jean-Bernard-Leon Foucault, a French physicist who surely conducted the last of the truly cheap laboratory experiments. In 1851 he invited his colleagues to "come and see the Earth turn" at the Pantheon in Paris. Today a Foucault pendulum sways in practically every science and technology museum in the world.

Given all that one can learn from a simple stick in the ground, what are we to make of the world's famous prehistoric observatories? From Europe and Asia to Africa and Latin America, a survey of ancient cultures turns up countless stone monuments that served as low-tech astronomy centers, although it's likely they also doubled as places of wors.h.i.+p or embodied other deeply cultural meanings.

On the morning of the summer solstice at Stonehenge, for instance, several of the stones in its concentric circles align precisely with sunrise. Certain other stones align with the extreme rising and setting points of the Moon. Begun in about 3100 B.C B.C. and altered during the next two millennia, Stonehenge incorporates outsize monoliths quarried far from its site on Salisbury Plain in southern England. Eighty or so bluestone pillars, each weighing several tons, came from the Preseli Mountains, roughly 240 miles away. The so-called sa.r.s.en stones, each weighing as much as 50 tons, came from Marlborough Downs, 20 miles away.

Much has been written about the significance of Stonehenge. Historians and casual observers alike are impressed by the astronomical knowledge of these ancient people, as well as by their ability to transport such obdurate materials such long distances. Some fantasy-p.r.o.ne observers are so impressed that they even credit extraterrestrial intervention at the time of construction.

Why the ancient civilizations who built the place did not use the easier, nearby rocks remains a mystery. But the skills and knowledge on display at Stonehenge are not. The major phases of construction took a total of a few hundred years. Perhaps the preplanning took another hundred or so. You can build anything in half a millennium-I don't care how far you choose to drag your bricks. Furthermore, the astronomy embodied in Stonehenge is not fundamentally deeper than what can be discovered with a stick in the ground.

Perhaps these ancient observatories perennially impress modern people because modern people have no idea how the Sun, Moon, or stars move. We are too busy watching evening television to care what's going on in the sky. To us, a simple rock alignment based on cosmic patterns looks like an Einsteinian feat. But a truly mysterious civilization would be one that made no cultural or architectural reference to the sky at all.

SECTION 2.

THE KNOWLEDGE OF NATURE.

THE CHALLENGES OF DISCOVERING THE CONTENTS OF THE COSMOS.

SIX.

JOURNEY FROM THE CENTER OF THE SUN.

During our everyday lives we don't often stop to think about the journey of a ray of light from the core of the Sun, where it's made, all the way to Earth's surface, where it might slam into somebody's b.u.t.tocks on a sandy beach. The easy part is the ray's 500-second speed-of-light jaunt from the Sun to Earth, through the void of interplanetary s.p.a.ce. The hard part is the light's million-year adventure to get from the Sun's center to its surface.

In the cores of stars, beginning at about 10-million degrees Kelvin, but for the Sun, at 15-million degrees, hydrogen nuclei, long denuded of their lone electron, reach high enough speeds to overcome their natural repulsion and collide. Energy is created out of matter as thermonuclear fusion makes a single helium (He) nucleus out of four hydrogen (H) nuclei. Omitting intermediate steps, the Sun simply says:

4H He + + energy energyAnd there is light.

Every time a helium nucleus gets created, particles of light called photons get made. And they pack enough punch to be gamma rays, a form of light with the highest energy for which we have a cla.s.sification. Born moving at the speed of light (186,282 miles per second), the gamma-ray photons unwittingly begin their trek out of the Sun.

An undisturbed photon will always move in a straight line. But if something gets in its way, the photon will either be scattered or absorbed and re-emitted. Each fate can result in the photon being cast in a different direction with a different energy. Given the density of matter in the Sun, the photon's average straight-line trip lasts for less than one thirty-billionth of a second (a thirtieth of a nanosecond)-just long enough for the photon to travel about one centimeter before interacting with a free electron or an atom.

The new travel path after each interaction can be outward, sideways, or even backward. How then does an aimlessly wandering photon ever manage to leave the Sun? A clue lies in what would happen to a fully inebriated person who takes steps in random directions from a street corner lamppost. Curiously, the odds are that the drunkard will not return to the lamppost. If the steps are indeed random, distance from the lamppost will slowly acc.u.mulate.

While you cannot predict exactly how far from the lamppost any particular drunk person will be after a selected number of steps, you can reliably predict the average distance if you managed to convince a large number of drunken subjects to randomly walk for you in an experiment. Your data would show that on average, distance from the lamppost increased in proportion to the square root of the total number of paces taken. For example, if each person took 100 steps in random directions, then the average distance from the lamppost would have been a mere 10 steps. If 900 steps were taken, the average distance would have grown to only 30 steps.

With a step size of one centimeter, a photon must execute nearly 5 s.e.xtillion steps to "random walk" the 70-billion centimeters from the Sun's center to its surface. The total linear distance traveled would span about 5,000 light-years. At the speed of light, a photon would, of course, take 5,000 years to journey that far. But when computed with a more realistic model of the Sun's profile-taking into account, for example, that about 90 percent of the Sun's ma.s.s resides within only half its radius because the gaseous Sun compresses under its own weight-and adding travel time lost during the pit stop between photon absorption and re-emission, the total trip lasts about a million years. If a photon had a clear path from the Sun's center to its surface, its journey would instead last all of 2.3 seconds.

As early as the 1920s, we had some idea that a photon might meet some major resistance getting out of the Sun. Credit the colorful British astrophysicist Sir Arthur Stanley Eddington for endowing the study of stellar structure with enough of a foundation in physics to offer insight into the problem. In 1926 he wrote The Internal Const.i.tution of the Stars The Internal Const.i.tution of the Stars, which he published immediately after the new branch of physics called quantum mechanics was discovered, but nearly 12 years before thermonuclear fusion was officially credited as the energy source for the Sun. Eddington's glib musings from the introductory chapter correctly capture some of the spirit, if not the detail, of an aether wave's (photon's) tortured journey: The inside of a star is a hurly-burly of atoms, electrons and aether waves. We have to call to aid the most recent discoveries of atomic physics to follow the intricacies of the dance.... Try to picture the tumult! Dishevelled atoms tear along at 50 miles a second with only a few tatters left of their elaborate cloaks of electrons torn from them in the scrimmage. The lost electrons are speeding a hundred times faster to find new resting-places. Look out! A thousand narrow shaves happen to the electron in [one ten-billionth] of a second.... Then...the electron is fairly caught and attached to the atom, and its career of freedom is at an end. But only for an instant. Barely has the atom arranged the new scalp on its girdle when a quantum of aether waves runs into it. With a great explosion the electron is off again for further adventures. (p. 19) (p. 19) Eddington's enthusiasm for his subject continues as he identifies aether waves as the only component of the Sun on the move: As we watch the scene we ask ourselves, can this be the stately drama of stellar evolution? It is more like the jolly crockery-smas.h.i.+ng turn of a music-hall. The knockabout comedy of atomic physics is not very considerate towards our aesthetic ideals.... The atoms and electrons for all their hurry never get anywhere; they only change places. The aether waves are the only part of the population which do actually accomplish something; although apparently darting about in all directions without purpose they do in spite of themselves make a slow general progress outwards. (pp. 1920) (pp. 1920) In the outer one-fourth of the Sun's radius, energy moves primarily through turbulent convection, which is a process not unlike what happens in a pot of boiling chicken soup (or a pot of boiling anything). Whole blobs of hot material rise while other blobs of cooler material sink. Unbeknownst to our hardworking photons, their residential blob can swiftly sink tens of thousands of kilometers back into the Sun, thus undoing possibly thousands of years of random walking. Of course the reverse is also true-convection can swiftly bring random-walking photons near the surface, thus enhancing their chances of escape.

But the tale of our gamma ray's journey is still not fully told. From the Sun's 15-million-degree Kelvin center to its 6,000-degree surface, the temperature drops at an average rate of about one one-hundredth of a degree per meter. For every absorption and re-emission, the high-energy gamma-ray photons tend to give birth to multiple lower-energy photons at the expense of their own existence. Such altruistic acts continue down the spectrum of light from gamma rays to x-rays to ultraviolet to visible and to the infrared. The energy from a single gamma-ray photon is sufficient to beget a thousand x-ray photons, each of which will ultimately beget a thousand visible-light photons. In other words, a single gamma ray can easily sp.a.w.n over a million visible and infrared photons by the time the random walk reaches the Sun's surface.

Only one out of every half-billion photons that emerge from the Sun actually heads toward Earth. I know it sounds meager, but at our size and distance from the Sun it totals Earth's rightful share. The rest of the photons head everywhere else.

The Sun's gaseous "surface" is, by the way, defined by the layer where our randomly walking photons take their last step before escaping to interplanetary s.p.a.ce. Only from such a layer can light reach your eye along an unimpeded line of sight, which allows you to a.s.sess meaningful solar dimensions. In general, light with longer wavelengths emerges from within deeper layers of the Sun than light of shorter wavelengths. For example, the Sun's diameter is slightly smaller when measured using infrared than when measured with visible light. Whether or not textbooks tell you, their listed values for the Sun's diameter typically a.s.sume you seek dimensions obtained using visible light.

Not all the energy of our fecund gamma rays became lower-energy photons. A portion of the energy drives the large-scale turbulent convection, which in turn drives pressure waves that ring the Sun the way a clanger rings a bell. Careful and precise measurements of the Sun's spectrum, when monitored continuously, reveal tiny oscillations that can be interpreted in much the same way that geoseismologists interpret subsurface sound waves induced by earthquakes. The Sun's vibration pattern is extraordinarily complex because many oscillating modes operate simultaneously. The greatest challenges among helioseismologists lie in decomposing the oscillations into their basic parts, and thus deducing the size and structure of the internal features that cause them. A similar "a.n.a.lysis" of your voice would take place if you screamed into an open piano. Your vocal sound waves would induce vibrations of the piano strings that shared the same a.s.sortment of frequencies that comprise your voice.

A coordinated project to study solar oscillating phenomena was carried out by GONG (yet another cute acronym), the Global Oscillation Network Group. Specially outfitted solar observatories that span the world's time zones (in Hawaii, California, Chile, the Canary Islands, India, and Australia) allowed solar oscillations to be monitored continuously. Their long-antic.i.p.ated results supported most current notions of stellar structure. In particular, that energy moves by randomly walking photons in the Sun's inner layers and then by large-scale turbulent convection in its outer layers. Yes, some discoveries are great simply because they confirm what you had suspected all along.

Heroic adventures through the Sun are best taken by photons and not by any other form of energy or matter. If any of us were to go on the same trip then we would, of course, be crushed to death, vaporized, and have every single electron stripped from our body's atoms. Aside from these setbacks, I imagine one could easily sell tickets for such a voyage. For me, though, I am content just knowing the story. When I sunbathe, I do it with full respect for the journey made by all photons that hit my body, no matter where on my anatomy they strike.

SEVEN.

PLANET PARADE.

In the study of the cosmos, it's hard to come up with a better tale than the centuries-long history of attempts to understand the planets-those sky wanderers that make their rounds against the backdrop of stars. Of the eight objects in our solar system that are indisputably planets, five are readily visible to the unaided eye and were known to the ancients, as well as observant troglodytes. Each of the five-Mercury, Venus, Mars, Jupiter, and Saturn-was endowed with the personality of the G.o.d for which it was named. For example, Mercury, which moves the fastest against the background stars, was named for the Roman messenger G.o.d-the fellow usually depicted with small and aerodynamically useless wings on his heels or his hat. And Mars, the only one of the cla.s.sic wanderers (the Greek word planete planete means "wanderer") with a reddish hue, was named for the Roman G.o.d of war and bloodshed. Earth, of course, is also visible to the unaided eye. Just look down. But terra firma was not identified as one of the gang of planets until after 1543, when Nicolaus Copernicus advanced his Sun-centered model of the universe. means "wanderer") with a reddish hue, was named for the Roman G.o.d of war and bloodshed. Earth, of course, is also visible to the unaided eye. Just look down. But terra firma was not identified as one of the gang of planets until after 1543, when Nicolaus Copernicus advanced his Sun-centered model of the universe.

To the telescopically challenged, the planets were, and are, just points of light that happen to move across the sky. Not until the seventeenth century, with the proliferation of telescopes, did astronomers discover that planets were orbs. Not until the twentieth century were the planets scrutinized at close range with s.p.a.ce probes. And not until later in the twenty-first century will people be likely to visit them.

Humanity had its first telescopic encounter with the celestial wanderers during the winter of 160910. After merely hearing of the 1608 Dutch invention, Galileo Galilei manufactured an excellent telescope of his own design, through which he saw the planets as...o...b.., perhaps even other worlds. One of them, brilliant Venus, went through phases just like the Moon's: crescent Venus, gibbous Venus, full Venus. Another planet, Jupiter, had moons all of its own, and Galileo discovered the four largest: Ganymede, Callisto, Io, and Europa, all named for a.s.sorted characters in the life and times of Jupiter's Greek counterpart, Zeus.

The simplest way to explain the phases of Venus, as well as other features of its motion on the sky, was to a.s.sert that the planets revolve around the Sun, not Earth. Indeed, Galileo's observations strongly supported the universe as envisioned and theorized by Copernicus.

Jupiter's moons took the Copernican universe a step further: although Galileo's 20-power telescope could not resolve the moons into anything larger than pinpoints of light, no one had ever seen a celestial object revolve around anything other than Earth. An honest, simple observation of the cosmos, except that the Roman Catholic Church and "common" sense would have none of it. Galileo discovered with his telescope a contradiction to the dogma that Earth occupied the central position in the cosmos-the spot around which all objects revolve. Galileo reported his persuasive findings in early 1610, in a short but seminal work he t.i.tled Sidereus Nuncius Sidereus Nuncius ("the Starry Messenger"). ("the Starry Messenger").

ONCE THE COPERNICAN model became widely accepted, the arrangement of the heavens could legitimately be called a model became widely accepted, the arrangement of the heavens could legitimately be called a solar solar system, and Earth could take its proper place as one among six known planets. n.o.body imagined there could be more than six. Not even the English astronomer Sir William Herschel, who discovered a seventh in 1781. system, and Earth could take its proper place as one among six known planets. n.o.body imagined there could be more than six. Not even the English astronomer Sir William Herschel, who discovered a seventh in 1781.

Actually, the credit for the first recorded sighting of the seventh planet goes to the English astronomer John Flamsteed, the first British Astronomer Royal. But in 1690, when Flamsteed noted the object, he didn't see it move. He a.s.sumed it was just another star in the sky, and named it 34 Tauri. When Herschel saw Flamsteed's "star" drift against the background stars, he announced-operating under the unwitting a.s.sumption that planets were not on the list of things one might discover-that he had discovered a comet. Comets, after all, were known to move and to be discoverable. Herschel planned to call the newfound object Georgium Sidus ("Star of George"), after his benefactor, King George III of England. If the astronomical community had respected these wishes, the roster of our solar system would now include Mercury, Venus, Earth, Mars, Jupiter, Saturn, and George. In a blow to sycophancy the object was ultimately called Ura.n.u.s, in keeping with its cla.s.sically named brethren-though some French and American astronomers kept calling it "Herschel's planet" until 1850, several years after the eighth planet, Neptune, was discovered.

Over time, telescopes kept getting bigger and sharper, but the detail that astronomers could discern on the planets did not much improve. Because every telescope, no matter the size, viewed the planets through Earth's turbulent atmosphere, the best pictures were still a bit fuzzy. But that didn't keep intrepid observers from discovering things like Jupiter's Great Red Spot, Saturn's rings, Martian polar ice caps, and dozens of planetary moons. Still, our knowledge of the planets was meager, and where ignorance lurks, so too do the frontiers of discovery and imagination.

CONSIDER THE CASE of Percival Lowell, the highly imaginative and wealthy American businessman and astronomer, whose endeavors took place at the end of the nineteenth century and the early years of the twentieth. Lowell's name is forever linked with the "ca.n.a.ls" of Mars, the "spokes" of Venus, the search for Planet X, and of course the Lowell Observatory in Flagstaff, Arizona. Like so many investigators around the world, Lowell picked up on the late-nineteenth-century proposition by the Italian astronomer Giovanni Schiaparelli that linear markings visible on the Martian surface were of Percival Lowell, the highly imaginative and wealthy American businessman and astronomer, whose endeavors took place at the end of the nineteenth century and the early years of the twentieth. Lowell's name is forever linked with the "ca.n.a.ls" of Mars, the "spokes" of Venus, the search for Planet X, and of course the Lowell Observatory in Flagstaff, Arizona. Like so many investigators around the world, Lowell picked up on the late-nineteenth-century proposition by the Italian astronomer Giovanni Schiaparelli that linear markings visible on the Martian surface were ca.n.a.li. ca.n.a.li.

The problem was that the word means "channels," but Lowell chose to translate the word badly as "ca.n.a.ls" because the markings were thought to be similar in scale to the major public-works projects on Earth. Lowell's imagination ran amok, and he dedicated himself to the observation and mapping of the Red Planet's network of waterways, surely (or so he fervently believed) constructed by advanced Martians. He believed that the Martian cities, having exhausted their local water supply, needed to dig ca.n.a.ls to transport water from the planet's well-known polar ice caps to the more populous equatorial zones. The story was appealing, and it helped generate plenty of vivid writing.

Lowell was also fascinated by Venus, whose ever-present and highly reflective clouds make it one of the brightest objects in the night sky. Venus...o...b..ts relatively near the Sun, so as soon as the Sun sets-or just before the Sun rises-there's Venus, hanging gloriously in the twilight. And because the twilight sky can be quite colorful, there's no end of 9-1-1 calls reporting a glowing, light-adorned UFO hovering on the horizon.

Lowell maintained that Venus sported a network of ma.s.sive, mostly radial spokes (more ca.n.a.li ca.n.a.li) emanating from a central hub. The spokes he saw remained a puzzle. In fact n.o.body could ever confirm what he saw on either Mars or Venus. This didn't much bother other astronomers because everyone knew that Lowell's mountaintop observatory was one of the finest in the world. So if you weren't seeing Martian activity the way Percival was, it was surely because your telescope and your mountain were not as good as his.

Of course, even after telescopes got better, n.o.body could duplicate Lowell's findings. And the episode is today remembered as one where the urge to believe undermined the need to obtain accurate and responsible data. And curiously, it was not until the twenty-first century that anybody could explain what was going on at the Lowell Observatory.