The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars. There was a time when the stars seemed an impenetrable mystery. Today we have begun to understand them. In our personal lives also we journey from ignorance to knowledge. Our individual growth reflects the advancement of the species. The exploration of the cosmos is a voyage of self-discovery.

When I was a child I lived here, in the Bensonhurst section of Brooklyn in the city of New York. I knew my immediate neighborhood intimately. Every candy store, front stoop, back yard, empty lot, and wall for playing Chinese handball. It was my whole world. But more than a few blocks away, north of the raucous traffic and elevated railway on 86 ^th Street, was an unknown territory—off-limits to my wanderings. It could have been Mars for all I knew.

Even with an early bedtime, in the winter you could occasionally see the stars. I would look up at them and wonder what they were. I’d ask other kids and adults, and they would answer: “They’re lights in the sky, kid.” Well, I could tell they were lights in the sky. But what were they? There had to be some deeper answer.

I remember I was issued my first library card—I think it was some library over there on 85^th Street; anyway, it was in alien territory—and I asked the librarian for a book on stars. She gave me a funny kind of picture book with portraits of men and women with names like Veronica Lake and Alan Ladd. I explained that wasn’t what I wanted at all. And for some reason then obscure to me, she smiled and got me another book—the right kind of book. I was so excited to know the answer that I opened the book breathlessly right there in the library, and the book said something astonishing. A very big thought. “Stars,” it said, “were suns, but very far away. The sun was a star, but close-up.”

How, I wondered, could anybody know such things for sure? How did they figure it out? Where did they even begin? I was ignorant of the idea of angular size. I didn’t know a thing about the inverse square law of the propagation of light. I didn’t have a ghost of a chance of calculating the distance to the stars. But I could tell that if the stars were suns, they had to be awfully far away—further away than 86^th Street, further away than Manhattan, further away, probably, than New Jersey. The universe had become much grander than I had ever guessed.

And then I read another astonishing fact. The Earth (which includes Brooklyn) was a planet. It went around the sun. There were other planets. They also went around the sun; some closer to the sun, some further from the sun. But planets didn’t shine by their own light the way the sun does. No, planets simply reflected the little bit of light that shines on them from the sun back to us. If you were a great distance away from the sun, you wouldn’t be able to see the Earth or the other planets at all. Well, then, it stood to reason, I thought, that those other stars ought to have their own planets. And some of those planets ought to have life. Why not? And that life ought to be pretty different from life as we know it; life here in Brooklyn.

As a child, it was my immense good fortune to have parents and a few teachers who encouraged my curiosity. This was my sixth-grade classroom. I came back here one day to remember what it was like. I brought some of the breathtaking pictures of other worlds that had been radioed back by the Voyager spacecraft in their encounters with Jupiter and its moons.

Every one of us begins life with an open mind, a driving curiosity, a sense of wonder.

I thought it might be fun if we now had some questions.

07:16

Sagan

That’s a good question. I like that question. That’s a question I’ve asked myself. And the answer has to do with gravity. The Earth has a strong gravity. If you were to make a mountain very high—higher than Everest; you know, it’s the biggest mountain on Earth—it would be crushed by its own weight. You see, gravity pulls everything towards the center. So any really big bump on the Earth is crushed. But if you had a small object, a tiny world, the gravity is very low, and then it can be very different from a sphere. I think I have here a world that isn’t a sphere. Here, look at this one. See? It’s lumpy. It’s a lumpy world. It looks like a potato, right? There’s a large potato orbiting the planet Mars. This is one of the moons of Mars. And that’s a perfect example. You can have big departures from a sphere if your gravity is low.

Now, the question in the front, okay?

08:25

Sagan

Sure. You are considered part of the Milky Way galaxy. Everything except other galaxies is part of the Milky Way galaxy. The sun is one star. There is a few hundred billion stars in the Milky Way. And around each star, maybe, is a whole bunch of planets. And on one of those planets is life. And one of the life forms on that planet is you. So you’re a part of the Milky Way galaxy, too.

Sometimes I think how lucky we are to live in this time, the first moment in human history when we are, in fact, visiting other worlds and engaging in a deep reconnaissance of the cosmos. But if we had been born in a much earlier age, no matter how great our dedication, we could not have understood what the stars and planets are. We would not have known that there were other suns and other worlds. This is one of the great secrets wrested from nature through a million years of patient observation and courageous thinking.

Human beings have always asked questions about the stars. It’s as natural as breathing. But imagine a time before science had found out the answers. Imagine what it was like, say, hundreds of thousands of years ago, soon after the discovery of fire. We were just as smart and just as curious then as we are now. Sometimes it seems to me that there were people then who thought like this.

We are wandering hunter folk. Fire keeps us warm. Its light makes holes in the darkness. It keeps hungry animals away. In the darkness we can see each other and talk. We take care of the flame, the flame takes care of us. The stars are not near to us. When we climb a hill or a tree, they are no closer. They flicker with a strange, cold, white, faraway light. Many of them, all over the sky, but only at night. I wonder what they are. One night I thought: the stars are flames. They give a little light at night, as fire does. Maybe the stars are campfires which other wanderers light at night. The stars give a much smaller light than campfires, so they must be very far away. I wonder if our campfires look like stars to the people in the sky. But why don’t those campfires and the wanderers who made them fall down at our feet? Why don’t strange tribes drop from the sky? Those beings in the sky must have great powers.

I don’t suppose that every hunter-gatherer had such thoughts about the stars. But we know from contemporary hunter-gatherer communities that very imaginative ideas arise. The ǃKung Bushmen of the Kalahari Desert in the Republic of Botswana have an explanation of the Milky Way. At their latitude it’s often overhead. They call it the “backbone of night.” They believe it holds the sky up. They believe that if not for the Milky Way, pieces of sky would come crashing down at our feet. So the Milky Way, in their view, has some practical value. The backbone of night.

Later on, metaphors about campfires or backbones or holes through which the flame could be seen were replaced in most human communities by another idea. The powerful beings in the sky were promoted to gods. They were given names and relatives and special responsibilities for the cosmic services they were expected to perform. There was a god for every human concern. Gods ran nature. Nothing happened without the direct intervention of some god. If the gods were happy, there was plenty of food and humans were happy. But if something displeased the gods—and it didn’t take much—the consequences were awesome: droughts, floods, storms, wars earthquakes, volcanic eruptions, epidemics. The gods had to be propitiated. And a vast industry of priests arose to make the gods less angry. But because the gods were capricious you couldn’t be sure what they would do. Nature was a mystery. It was hard to understand the world.

Our ancestors groped in darkness to make sense of their surroundings. Powerless before nature, they invented rituals and myths—some desperate and cruel, others imaginative and benign. The ancient Greeks explained that diffuse band of brightness in the night sky as the milk of the goddess Hera, squirted from her breast across the heavens. We still call it the Milky Way. In gratitude for the many gifts of the gods, our ancestors created works of surpassing beauty. This is all that remains of the ancient temple of Hera, queen of heaven: a single marble column standing in a vast field of ruins on the Greek island of Samos. It was one of the wonders of the world, built by people with an extraordinary eye for clarity and symmetry.

Those who thronged to that temple were also the architects of a bridge from their world to ours. We were moving once again in our voyage of self-discovery, on our journey to the stars. Here, 25 centuries ago, on the island of Samos and in the other Greek colonies which had grown up in the busy Aegean Sea, there was a glorious awakening. Suddenly there were people who believed that everything was made of atoms, that human beings and other animals had evolved from simpler forms, that diseases were not caused by demons or the gods, that the Earth was only a planet going around a sun which was very far away. This revolution made cosmos out of chaos. Here, in the sixth century B. C., a new idea developed; one of the great ideas of the human species. It was argued that the universe was knowable. Why? Because it was ordered. Because there are regularities in nature which permitted secrets to be uncovered. Nature was not entirely unpredictable. There were rules which even she had to obey.

This ordered and admirable character of the universe was called cosmos. And it was set in stark contradiction to the idea of chaos. This was the first conflict of which we know between science and mysticism, between nature and the gods. But why here? Why in these remote islands and inlets of the eastern Mediterranean? Why not in the great cities of India, or Egypt, Babylon, China, Mesoamerica? Because they were all at the center of old empires. They were set in their ways, hostile to new ideas. But here in Ionia were a multitude of newly colonized islands and city-states. Isolation, even if incomplete, promotes diversity. No single concentration of power could enforce conformity. Free inquiry became possible. They were beyond the frontiers of the empires. The merchants and tourists and sailors of Africa, Asia, and Europe met in the harbors of Ionia to exchange goods and stories and ideas. There was a vigorous and heady interaction of many traditions, prejudices, languages and gods. These people were ready to experiment. Once you are open to questioning rituals and time-honored practices, you find that one question leads to another.

What do you do when you’re faced with several different gods, each claiming the same territory? The Babylonian Marduk and the Greek Zeus were each considered king of the gods, master of the sky. You might decide, since they otherwise had different attributes, that one of them was merely invented by the priests. But if one, why not both? And so it was here that the great idea arose: the realization that there might be a way to know the world without the god hypothesis. That there be principles, forces, laws of nature, through which the world might be understood without attributing the fall of every sparrow to the direct intervention of Zeus. This is the place where science was born. That’s why we’re here.

This Greek revolution happened between 600 and 400 B.C. It was accomplished by the same practical and productive people who made the society function. Political power was in the hands of the merchants, who promoted the technology on which their prosperity depended. The earliest pioneers of science were merchants and artisans and their children.

The first Ionian scientist was named Thales. He was born over there, in the city of Miletus, across this narrow strait. He had traveled in Egypt and was conversant with the knowledge of Babylon. Like the Babylonians, he believed that the world had once all been water. To explain the dry land the Babylonians added that their god, Marduk, had placed a mat on the face of the waters and piled dirt on top of it. Thales had a similar view, but he left Marduk out. Yes, the world had once been mostly water, but it was a natural process which explained the dry land. Thales thought it was similar to the silting up he had observed at the delta of the river Nile.

Whether Thales’ conclusions were right or wrong is not nearly as important as his approach. The world was not made by the gods, but instead was the result of material forces interacting in nature. Thales brought back from Babylon and Egypt the seeds of new sciences: astronomy and geometry—sciences which would sprout and grow in the fertile soil of Ionia.

Anaximander of Miletus, over there, was a friend and colleague of Thales—one of the first people that we know of to have actually done an experiment. By examining the moving shadow cast by a vertical stick, he determined accurately the lengths of the year and seasons. For ages, men had used sticks to club and spear each other. Anaximander used a stick to measure time.

In 540 B.C. (or thereabouts), on this island of Samos, there came to power a tyrant named Polycrates. He seems to have started as a caterer and then went on to international piracy. His loot was unloaded on this very breakwater. But he oppressed his own people, he made war on his neighbors. He quite rightly feared invasion. So Polycrates surrounded his capital city with an impressive wall whose remains stand till this day.

To carry water from a distant spring through the fortifications, he ordered this great tunnel built. A kilometer long, it pierces a mountain. Two cuttings were dug from either side which met almost perfectly in the middle. The project took some fifteen years to complete. It is a token of the civil engineering of its day, and an indication of the extraordinary practical capability of the Ionians. The enduring legacy of the Ionians is the tools and techniques they developed, which remain the basis of modern technology.

This was the time of Theodorus, the master engineer of the age—a man who is credited with the invention of the key, the ruler, the carpenter’s square, the level, the lathe, bronze casting. Why are there no monuments to this man? Those who dreamt and speculated and deduced about the laws of nature talked to the engineers and the technologists. They were often the same people. The practical and the theoretical were one.

This new hybrid of abstract thought and everyday experience blossomed into science. When these practical men turned their attention to the natural world they began to uncover hidden wonders and breathtaking possibilities. Anaximander studied the profusion of living things and saw their interrelationships. He concluded that life had originated in water and mud, and then colonized the dry land. “Human beings,” he said, “must have evolved from simpler forms.” This insight had to wait 24 centuries until its truth was demonstrated by Charles Darwin.

Nothing was excluded from the investigations of the first scientists. Even the air became the subject of close examination by a Greek from Sicily named Empedocles. He made an astonishing discovery with a household implement that people had used for centuries. This is the so-called water thief. It’s a brazen sphere with a neck and a hole at the top, and a set of little holes at the bottom. It was used as a kitchen ladle. You fill it by immersing it in water. If, after it’s been in there a little bit, you pull it out with the neck uncovered, then the water trickles out the little holes, making a small shower. Instead, if you pull it out with the neck covered, the water is retained. Now try to fill it with the neck covered with my thumb. Nothing happens. Why not? There’s something in the way. Some material is blocking the access of the water into the sphere. I can’t see any such material. What could it be? Empedocles identified it as air. What else could it be? A thing you can’t see can exert pressure, can frustrate my wish to fill this vessel with water, if I were dumb enough to leave my thumb on the neck. Empedocles had discovered the invisible. “Air,” he thought, “must be matter in a form so finely divided that it couldn’t be seen.”

This hint, this whiff of the existence of atoms, was carried much further by a contemporary named Democritus. Of all the ancient scientists, it is he who speaks most clearly to us across the centuries. The few surviving fragments of his scientific writings reveal a mind of the highest logical and intuitive powers. He believed that a large number of other worlds wander through space, that worlds are born and die, that some are rich and living creatures and others are dry and barren. He was the first to understand that the Milky Way is an aggregate of the light of innumerable faint stars. Beyond campfires in the sky, beyond the milk of Hera, beyond the backbone of night, the mind of Democritus soared. He saw deep connections between the heavens and the Earth. “Man,” he said, “is a microcosm”—a little cosmos.

Democritus came from the Ionian town of Abdera on the northern Aegean shore. In those days, Abdera was the butt of jokes. If, around the year 400 B.C., in the equivalent of a little outdoor restaurant like this, you told a story about someone from Abdera, you were guaranteed a laugh. It was, in a way, the Brooklyn of its time. For Democritus, all of life was to be enjoyed and understood. In fact, for him, understanding and enjoyment were pretty much the same thing. He said, “A life without festivity is a long road without an inn.” Democritus may have come from Abdera, but he was no dummy.

Democritus understood that the complex forms changes and motions of the material world all derived from the interaction of very simple moving parts. He called these parts atoms. All material objects are collections of atoms intricately assembled—even we. When I cut this apple, the knife must be passing through empty spaces between the atoms, Democritus argued. If there were no such empty spaces, no void, then the knife would encounter some impenetrable atom and the apple wouldn’t be cut. Let’s compare the cross sections of the two pieces. Are the exposed areas exactly equal? No, said Democritus, the curvature of the apple forces this slice to be slightly shorter than the rest of the apple. If they were equally tall, then we’d have a cylinder and not an apple. No matter how sharp the knife, these two pieces have unequal cross sections. But why? Because, on the scale of the very small, matter exhibits some irreducible roughness. And this fine scale of roughness Democritus of Abdera identified with the world of the atoms. His arguments are not those we use today. But they’re elegant and subtle and derived from everyday experience. And his conclusions were fundamentally right.

Democritus believed that nothing happens at random, that everything has a material cause. He said, “I would rather understand one cause than be king of Persia.” He believed that poverty in a democracy was far better than wealth in a tyranny. He believed that the prevailing religions of his time were evil and that neither souls nor immortal gods existed. There is no evidence that Democritus was persecuted for his beliefs. But then again, he came from Abdera.

However, in his time, the brief tradition of tolerance for unconventional views was beginning to erode. For instance, the prevailing belief was that the moon and the sun were gods. Another contemporary of Democritus, named Anaxagoras, taught that the moon was a place made of ordinary matter and that the sun was a red-hot stone far away in the sky. For this, Anaxagoras was condemned, convicted, and imprisoned for impiety, a religious crime. People began to be persecuted for their ideas. A portrait of Democritus is now on the Greek 100-drachma note. But his ideas were suppressed and his influence on history made minor. The mystics were beginning to win.

You see, Ionia was also the home of another quite different intellectual tradition. Its founder was Pythagoras, who lived here on Samos in the sixth century B.C. According to local legend this cave was once his abode. Maybe that was once his living room. Many centuries later, this small Greek Orthodox shrine was erected on his front porch. There’s a continuity of tradition from Pythagoras to Christianity. Pythagoras seems to have been the first person in the history of the world to decide that the Earth was a sphere. Perhaps he argued by analogy with the moon or the sun. Maybe he noticed the curved shadow of the Earth on the moon during a lunar eclipse. Or maybe he recognized that when ships leave Samos, their masts disappear last.

Pythagoras believed that a mathematical harmony underlies all of nature. The modern tradition of mathematical argument essential in all of science owes much to him. And the notion that the heavenly bodies move to a kind of music of the spheres was also derived from Pythagoras. It was he who first used the word “cosmos” to mean a well-ordered and harmonious universe; a world amenable to human understanding.

For this great idea we are indebted to Pythagoras. But there were deep ironies and contradictions in his thoughts. Many of the Ionians believed that the underlying harmony and unity of the universe was accessible—through observation and experiment, the method which dominates science today. However, Pythagoras had a very different method. He believed that the laws of nature can be deduced by pure thought. He and his followers were not basically experimentalists, they were mathematicians, and they were thoroughgoing mystics.

They were fascinated by these five regular solids—bodies whose faces are all polygons: triangles or squares or pentagons. There can be an infinite number of polygons, but only five regular solids. Four of the solids were associated with earth, fire, air, and water. The cube, for example, represented earth. These four elements, they thought, make up terrestrial matter. So the fifth solid they mystically associated with the cosmos. Perhaps it was the substance of the heavens. This fifth solid was called the dodecahedron. Its faces are pentagons; twelve of them. Knowledge of the dodecahedron was considered too dangerous for the public.

Ordinary people were to be kept ignorant of the dodecahedron. In love with whole numbers, the Pythagoreans believed that all things could be derived from them, certainly all other numbers. So a crisis in doctrine occurred when they discovered that the square root of two was irrational. That is, the square root of two could not be represented as the ratio of two whole numbers, no matter how big they were. Irrational originally meant only that: that you can’t express a number as a ratio. But for the Pythagoreans it came to mean something else, something threatening—a hint that their worldview might not make sense; the other meaning of irrational. Instead of wanting everyone to share and know of their discoveries, the Pythagoreans suppressed the square root of two and the dodecahedron. The outside world was not to know.

The Pythagoreans had discovered in the mathematical underpinnings of nature one of the two most powerful scientific tools. The other, of course, is experiment. But instead of using their insight to advance the collective voyage of human discovery, they made of it little more than the hocus-pocus of a mystery cult. Science and mathematics were to be removed from the hands of merchants and the artisans. This tendency found its most effective advocate in a follower of Pythagoras named Plato. He preferred the perfection of these mathematical abstractions to the imperfections of everyday life. He believed that ideas were far more real than the natural world. He advised the astronomers not to waste their time observing stars and planets. It was better, he believed, just to think about them. Plato expressed hostility to observation and experiment. He taught contempt for the real world and disdain for the practical application of scientific knowledge. Plato’s followers succeeded in extinguishing the light of science and experiment that had been kindled by Democritus and the other Ionians.

Plato’s unease with the world as revealed by our senses was to dominate and stifle Western philosophy. Even as late as 1600, Johannes Kepler was still struggling to interpret the structure of the cosmos in terms of Pythagorean solids and Platonic perfection. Ironically, it was Kepler who helped re-establish the old Ionian method of testing ideas against observations. But why had science lost its way in the first place? What appeal did these teachings of Pythagoras and Plato have for their contemporaries? They provided, I believe, an intellectually respectable justification for a corrupt social order.

The mercantile tradition which had led to Ionian science also led to a slave economy. You could get richer if you owned a lot of slaves. Athens, in the time of Plato and Aristotle, had a vast slave population. All of that brave Athenian talk about democracy applied only to a privileged few. Plato and Aristotle were comfortable in a slave society. They offered justifications for oppression. They served tyrants. They taught the alienation of the body from the mind—a natural enough idea, I suppose, in a slave society. They separated thought from matter. They divorced the Earth from the heavens—divisions which were to dominate Western thinking for more than twenty centuries. The Pythagoreans had won.

In the recognition by Pythagoras and Plato that the cosmos is knowable, that there is a mathematical underpinning to nature, they greatly advanced the cause of science. But in the suppression of disquieting facts, the sense that science should be kept for a small elite, the distaste for experiment, the embrace of mysticism, the easy acceptance of slave societies—their influence has significantly set back the human endeavor.

The books of the Ionian scientists are entirely lost. Their views were suppressed, ridiculed, and forgotten—by the Platonists and by the Christians, who adopted much of the philosophy of Plato. Finally, after a long, mystical sleep in which the tools of scientific inquiry lay moldering, the Ionian approach was rediscovered. The Western world reawakened. Experiment and open inquiry slowly became respectable once again. Forgotten books and fragments were read once more. Leonardo and Copernicus and Columbus were inspired by the Ionian tradition.

The Pythagoreans and their successors held the peculiar notion that the Earth was tainted, somehow nasty, while the heavens were pristine and divine. So the fundamental idea that the Earth is a planet, that we’re citizens of the universe, was rejected and forgotten. This idea was first argued by Aristarchus, born here on Samos, three centuries after Pythagoras. He held that the Earth moves around the sun. He correctly located our place in the solar system. For his trouble he was accused of heresy.

From the size of the Earth’s shadow on the moon during a lunar eclipse he deduced that the sun had to be much, much larger than the Earth, and also very far away. From this he may have argued that it was absurd for so large an object as the sun to be going around so small an object as the Earth. So he put the sun, rather than the Earth, at the center of the solar system. And he had the Earth and the other planets going around the sun. He also had the Earth rotating on its axis once a day. These are ideas that we ordinarily associate with the name Copernicus. But Copernicus seems to have gotten at least some hint of these ideas by reading about Aristarchus. In fact, in the manuscript of Copernicus’ book he referred to Aristarchus, but in the final version he suppressed the citation.

Resistance to Aristarchus, a kind of geocentrism in everyday life, is with us still. We still talk about a sun rising and the sun setting. It’s 2,200 years since Aristarchus, and the language still pretends that the Earth does not turn, that the sun is not at the center of the solar system.

Aristarchus understood the basic scheme of the solar system, but not its scale. He knew that the planets move in concentric orbits about the sun, and he probably knew their order out to Saturn. But he was much too modest in his estimates of how far apart the planets are. In order to calculate the true scale of the solar system you need a telescope. It wasn’t until the seventeenth century that astronomers were able to get even a rough estimate of the distance to the sun. And once you knew the distance to the sun, what about the stars? How far away are they?

There is a way to measure the distance to the stars, and the Ionians were fully capable of discovering it. Aristarchus had toyed with the daring idea that the stars were distant suns. Now, if a star were as near as the sun, it should appear as big and as bright as the sun. Everyone knows that the farther away an object is, the smaller it seems. This inverse proportionality between apparent size and distance is the basis of perspective in art and photography. So the further away we are from the sun, the smaller and dimmer it appears. How far from the sun would we have to be for it to appear as small and dim as a star? Or equivalently, how small a piece of sun would be as bright as a star?

An experiment to answer this question was performed in seventeenth-century Holland by Christiaan Huygens, and is very much in the Ionian tradition. Huygens drilled a number of holes in a brass plate, and held the plate up to the sun. He asked himself: which hole seemed as bright as he remembered the bright star Sirius to have been the previous evening? Well, the hole that matched was effectively ¹⁄_28,000^th the apparent size of the sun. So Sirius, he reasoned, must be 28,000 times further away than the sun, or about half a light-year away. It’s hard to remember just how bright a star is hours after you’ve looked at it, but Huygens remembered very well. In fact, if he had known that Sirius was intrinsically brighter than the sun, he would’ve gotten the answer exactly right. Sirius is 8.8 light-years away from us.

Between Aristarchus and Huygens, people had answered that question which had so excited me as a young boy growing up in Brooklyn—the question: what are the stars? And the answer is that the stars are mighty suns, light-years away in the depths of interstellar space. And around those suns, are there other planets? And on those other worlds, are there beings who wonder as we do?

Here is a light bulb which is supposed to represent a nearby star. And next to it—and very hard to see because of the bright light—is a planet. Now, we’ll need a volunteer. Who would like to come up, please? Ordinarily, you would have a hard time seeing the planet, because it’s so close that the star washes out the planet. But if we’re able to put something in front of the star to make an artificial eclipse, then we might be able to see the planet. So I’m gonna stand over here. Imagine that I’m a telescope somewhere near the Earth. And, Tab, if you’d slowly move the disc across. Good. A little faster would be nice. Now you’re just beginning to cover over the star. I really can’t see the planet at all. Keep going. Good! Now, right there I can’t see the star at all, and I see the planet lit by the light of the star. Now, that is a method for looking for planets around nearby stars. And that method uses a spacecraft to hold the disc and scan the sky for another telescope to see if there are any planets. So Tab, you have successfully accomplished your mission to look for planets around other stars. Thank you for being our interplanetary spacecraft!

So this is one way. And there are spaceships that will be able to do this in the next ten years or so. And there’s another way. This has already been tried from the Earth. Imagine that there’s a nearby star that you can see. It’s bright. And it has a dark companion, a planet, shining only by reflected light near it—so dim you can’t see it. But imagine that this planet and its star are going around each other, like that. You can see the star, you can’t see the planet. So now I’m gonna need two volunteers. You two. Just to save time because they’re in the front row.

Now, I need one of you to turn the star and the planet, and another person to pull the star and planet along. And what you will see is that the star you can make out will be moving in a funny, wiggly pattern, which will be the clue, the evidence, for the existence of the dark planet. Okay, let’s have a spin. Good. And a pull. And you see this funny motion that the star makes because of the planet. Thank you very much. So that’s another way of finding out the existence of a planet that you couldn’t see directly.

Well, both of these methods are being used. And by the time that you people are as old as I am, we should know for all the nearest stars whether they have planets going around them or not. We might know dozens or even hundreds of other planetary systems, and see if they’re like our own, or very different, or no other planets going around other stars at all. That will happen in your lifetime, and it’ll be the first time in the history of the world that anybody found out, really, if there are planets around the other stars.

Now, the nearby stars—the ones you can see with the naked eye—those are all in what’s called the solar neighborhood. That’s really what astronomers call it: the neighborhood. But it’s a very tiny place in the Milky Way galaxy. The Milky Way is that band of light that you see across the sky on a clear night. I can’t tell if there are any more clear nights in Brooklyn. But you must’ve seen the Milky Way, right? See? A faint band of light at night. Well, that’s just a hundred billion stars all seen together, edge on, as in this picture. If you could get out of the Milky Way galaxy and look down on it, it would look like that picture. If we did look down on the Milky Way galaxy, where would the sun and nearby stars be? Would it be in the center where things look important, or at least well-lit? No. We would be way out here in the suburbs, in the countryside of the galaxy. We’re not in any important place. All the stars you could see would be in a little, little place like that. And the Milky Way would be this band of light; a hundred billion stars all together.

The fact that we live in the outskirts of the galaxy was discovered a long time ago, towards the end of the First World War, by a man named Harlow Shapley, who was mapping the position of these clusters of stars. See, every one of these is a bunch of maybe 10,000 stars all together. It’s called a globular cluster. And you can see that they are centered around the middle, the center of the galaxy. People used to think that the sun was at the center of the galaxy—something important about our position. That turns out to be wrong. We live in the outskirts. The globular clusters are centered around the marvelous middle of the Milky Way galaxy.

And then it turned out that this isn’t the only galaxy. We live in this one, but there are many others. And as this picture reminds us, there are many different kinds of galaxies, of which ours might be just this one. There are, in fact, a hundred billion other galaxies, each of which contains something like a hundred billion stars. Think of how many stars and planets and kinds of life there may be in this vast and awesome universe!

As long as there have been humans, we have searched for our place in the cosmos. Where are we? Who are we? We find that we live on an insignificant planet of a humdrum star, lost in a galaxy tucked away in some forgotten corner of a universe in which there are far more galaxies than people. We make our world significant by the courage of our questions and by the depth of our answers.

We embarked on our journey to the stars with a question first framed in the childhood of our species, and in each generation asked anew with undiminished wonder: “What are the stars?” Exploration is in our nature. We began as wanderers, and we are wanderers still. We have lingered long enough on the shores of the cosmic ocean. We are ready at last to set sail for the stars.