HOW DID WE FIND OUT THE EARTH IS ROUND Isaac Asimov 1. Is the Earth Flat? FAR BACK in ancient times, everybody thought the earth was flat. This is because it looks flat. If you are in a boat way out in the middle of the ocean, the top of the water looks flat in every direction and the sky seems to fit over it like an upside-down bowl. The line where the sky and water meet is called the “horizon.” The horizon looks like a circle with you yourself at the center. If you are on land, the land stretches out to a horizon also. The horizon on land, however, is not even. It goes up and down because of houses, trees, hills, and other things. Some ancient people suspected the earth went on forever. They thought it might be a huge flat piece of land and sea with no end at all. But if this were the case, then what about the sun? The sun rose in the east in the morning. It traveled across the sky and set in the west in the evening. Then the next morning, it rose again in the east. Some ancient people tried to explain this by saying that every morning a brand-new sun was manufactured and rose. When it set, it was destroyed. Others said that the sun set in the ocean to the west. Then during the night, it was put in a boat and rowed to the east by morning, it was ready to rise again, the sun being rowed to the east Still others thought the sun was a golden, flaming chariot pulled by magic horses that could fly through the air, In the morning, the sun-god would get into the chariot in the east. Then he and his horses would climb through the air, reaching the top the sky at noon. They would race downwards, reaching the far western ground in the evening. Somehow the sun-god would get back to the east during the night when his golden, flaming chariot gave out no light. Stars that were very far from the North Star moved in such big circles that those circles dipped low the horizon. Those stars rose in the east and set in the west. The moon also traveled across the sky from east to west. So did the stars. These things had to be explained also. The ancient explanations just didn’t make sense. Suppose we have a flat earth stretching out in every direc- tion. How deep is it? Suppose you begin to dig a hole. Can you keep on digging forever, going down and down without end? Or is the earth just a slab of material, maybe a mile thick—or ten miles—or fifty miles? If it is just a slab of material, what keeps it from falling down? The people who lived in India in ancient times decided the earth didn’t fall because it was resting on huge elephants. But what were the elephants standing on? They said all the elephants were standing on the back of a gigantic turtle. And what was the turtle standing on? They said it was swim- ming in a tremendous ocean. Well, then, did the ocean stretch all the way down? There was no answer to that. So you see, while the earth looks flat, it may not be safe to decide that it is flat. There are problems to the flatness The first people who thought about the problems of the flat earth were certain Greeks who lived about twenty-five hundred years ago on what is now the western coast of the nation of Turkey. One of them was a man named Anaximander (uh- NAK-sih-MAN-der). He wasn’t satisfied with the tales of sun-gods and flaming chariot s and flying horses. Instead, he looked at the night sky and asked himself what he really saw. On a clear night, he saw the stars. During the night, they seemed to travel across the sky. One star, however, didn’t move. It was the North Star. It stayed in the same place in the northern sky all night long. It stayed there night after night. The stars near it moved in a circle around it. If the stars were very near it, they moved in small circles. If they were farther away, they moved in bigger circles. The most important thing about the night sky to Anaximander was that the stars traveled in patterns. They weren’t like a swarm of bees, in which each bee moves its own wav. Instead, all the stars moved together. Anaximander decided that the sky was a huge hollow ball, or “sphere.” The sphere of the sky turned around on an invisible line or “axis.” One end of the axis stuck through the sky where the North Star was situated. The other end was at the opposite side of the sphere where he couldn’t see it. Everyday the sphere of the sky turned around, or “rotated.” The stars were all stuck to the sky and turned with it. That’s why they kept the same pattern. The sun and moon were stuck to the sky, too, and that’s why they rose and set. Even though the sky was a sphere, it was still possible for the earth to be flat. Anaximander thought it was a flat slab that stretched across the sphere of the sky at its center. As the sphere of the sky turned, the sun rose in the east, trav- eled across the sky, and set in the west. The turning sky carried it along. Then as the sky kept turning, the sun was carried to the bottom part of the sphere. When the sun shone on the bottom side of the slab of the earth it was night. When the turning sky carried the sun around to the east, it rose and it was day again. The moon and the stars also moved in this was. Anaximander’s idea made more sense than the ideas of earlier thinkers. The sun was not de- stroyed each night nor was it rowed from west to east: Yet Anaximander wasn’t satisfied. He kept on thinking. 2. The Disappearing Stars If the earth were a flat slab that fitted tightly across the middle of the sphere of the sky, we could travel to the place where the earth and sky met. We could reach the place where the sun rose in the east and we could reach out and touch it (unless its heat killed us). If we traveled far enough to the west, we could reach the place where the sun set. Some people, centuries ago, really thought that could be done. They even drew pictures showing a man coming to the place where the sky touched the earth. The man could stick his head through the sky and see the machinery that kept the sphere of the sky turning. The ancient Greek thinkers however didn’t really believe that. After all, no matter how far to the east or west people traveled, they never seemed to get any closer to the sun, the moon or the stars. Perhaps, then, the earth didn’t stretch from one side of the sky to the other. Perhaps our eyes only fooled us when they showed the sky touching the earth at the ho- rizon, Maybe the earth was a flat disc that was quite large but was far smaller than the sphere of the sky, If this were the case, the sun, the moon, and the stars would be far away from the edge of the earth. No one on earth would be able to reach them or even get particularly close to them. But if the earth were a flat disc in the center of the sphere of the sky with the sky far away on all sides, then why didn’t travelers reach the end of the earth? Perhaps because the land portion of the earth was in the middle of the flat disc and was surrounded by water. Travelers always reached the ocean if they traveled far enough. It was the ocean, then, that stretched out to the end of the earth. People, in ancient times, didn’t travel far out of sight of the land. Maybe that was why thee never came to the end of tile earth. But then why didn’t the water of the ocean spill off the end of the earth? Maybe the end of the earth was turned up at every side, so that the water was held in. Maybe the earth wasn’t exactly a flat slab, but was a shallow bowl. In that case, why didn’t the whole earth simply fall? It was still hard to consider the earth as flat, even if the sky was a huge sphere and the problem of sunrise and sunset was explained. If the earth isn’t flat, what other shape can it be? Suppose we look at the sky again. In the sky, there are many shining objects, but most of them are stars. Stars are just little points of light to the eye and the ancient thinkers couldn’t tell anything about them. Two objects in the sky are different, however. They are the sun and the moon. The sun is a circle of light at all times, but the most isn’t. Sometimes it is a circle of light, but sometimes it is only half a circle. Sometimes it is in between a whole circle and half a circle. Sometimes it is just a thin curve of light called a “crescent. “ The Greeks who watched the moon, night after night, noticed that it changed its position in respect to the sun. They noticed that as it changed its position, it also changed its shape. When the moon and sun were on opposite sides of the earth, the moon was always a full circle of light. The sun shone past the earth onto the moon. It lit up the whole side of the moon. When the moon and sun were on the same side of the earth, they couldn’t see the moon at all. The sun shone on the other side of the moon, the side they couldn’t see. The side they could see received no sunlight and it was dark. The ancient scholars who observed this decided the sun had light of its own and the moon didn’t. The moon shone only because it was lit up by the sun. The moon shone by “reflected light.” The ancient Greeks had begun to work out the study of “geometry,” which deals with the shapes of objects. They considered the different shapes of the lighted part of the moon. They considered the half- moons, the crescent-moons, and other types. They could easily show that in order for the lighted part of the moon to take on the shapes it did, the moon had to be a round ball, or sphere. Then what about the shape of the sun? It shone on the moon and it did so equally well from all angles. Whether the moon and sun were on opposite sides of the earth, or on the same side, or anything in between, the moon received the same kind of light on the side that faced the sun. This could only have happened if the sun were a sphere. With all this in mind, Anaximander could see that there were three objects in the sky that had a particular shape. There was the sun, the moon, and the whole sky itself. All three were spheres. Did that mean the earth was also a sphere? Did that mean the earth was round instead of flat? Not necessarily. Maybe the rules were different for the sky and earth. Just because the sky and a few objects in the sky were spheres didn’t mean that the earth bad to be a sphere. After all, the sun was hot and blazing but the earth wasn’t. The moon moved through the sky, but the earth didn’t seem to. The sky itself was full of stars but the earth wasn’t. No, in order to decide upon the shape of the earth, people had to consider the earth itself, and not other objects. So let’s go back to the earth and ask ourselves the following question: Would we see the stars dif- ferently from different parts of the earth? We wouldn’t if the earth were flat. Suppose we looked at the sky at night. We would see all the stars in the sky above us if the night were clear. If we were anywhere else on a flat earth, we would still see those same stars. But that is not the way things really are! There were many people, in ancient times, who had to travel. People who traveled north would notice that the sky at night seemed a little different. Some stars that they used to see near the southern horizon when they were at home couldn’t be seen at all when they went north- ward. Then, when they returned home, those stars they hadn’t been able to see showed up again just above the southern horizon. People who traveled southward found things just the opposite. When they went south, they could make out stars just above the southern horizon that the never saw at home. When they returned, those stars disappeared again. This was true of the northern horizon, too. At home, some stars dipped just a little below the northern horizon as they turned with the sky. If one traveled north, those stars stayed just a little above it. If one traveled south, stars that stayed a little above the northern horizon when one was at home began to dip below it. The fact of the matter was that the stars were not seen from all parts of the earth, so the earth could not be flat. Maybe the earth was shaped like a tin can, or “cylinder.” This is exactly how Anaximander thought the earth might be shaped. He thought it was a cylinder lying in the center of the sphere of the sky. When you went north, you traveled along the curve of the cylinder. When you looked back, the curve hid some of the stars to the south. If you went south, you also traveled along curve of the cylinder. When you looked back, the curve hid some of the stars to the north. That explained the difference in the appearance of the sky as you went from place to place. 3. The Disappearing SHIPS ANAXIMANDER’S THEORY of a cylindrical earth raises some questions. First, if the earth is cylindrical, why does it look flat? That’s not hard to answer. The earth is so large and we’re so small that we can only see a tiny part of it as we look around. The curve is so slight on a very tiny part of the earth that the surface looks flat. To see this, imagine a large balloon blown up until it is a couple of feet across. Imagine a tiny circle on the balloon 1/ 8 inch across. If a tiny insect could see only that much of the balloon, the surface of the balloon would appear flat. Here is a second question that’s harder to answer. If you are traveling down a curve when you go northward or southward, why don’t you feel as though you are walking downhill? Why don’t you start to slide? You might think that the ground is so rough that it prevents you from sliding. But what if you’re not traveling over rough ground? What if you travel by ship northward or southward over the smooth ocean? Why doesn’t the ship start sliding forward? In fact, why doesn’t the whole ocean just slide downward and all its water pour right off the earth. Anaximander didn’t have a good answer for that. He just felt that there was no other way to explain the appearance of the night sky than to suppose the earth had a cylindrical shape. Why does the earth curve only if we go north and south? Suppose we go east and west. Does the appearance of the stars change? Do stars disappear be- hind the western horizon if we go east? Do they dis- appear behind the eastern horizon if we go west? That’s hard to tell. Since the sky turns, stars are always appearing at the eastern horizon and disappearing at the western horizon. Trav- eling east and west doesn’t change that. If the sky stopped rotating, it would be possible to tell whether stars were appearing and disappearing in the east and west. But the sky never stops rotating for even a mo- ment, so that’s no help. But it’s not very good to make a decision just because you have no evidence. To prove something, you need evidence. Perhaps there’s some other way of getting information about the shape of the earth, some way that doesn’t involve the sky and the way it turns. That kind of information can be obtained on the seashore and does not involve the sky. If the earth were flat, a ship would get smaller and smaller as it sailed farther and farther away from shore. Finally, it would become a dot and disappear. This, however, is not what happens. To begin with, some- one watching a ship sail away can see the whole ship. He or she sees the wooden hull of the ship below and the sails above. After a while, however, the hull disappears. The water seems to reach above it and all that is left are the sails. Then only the top of the sails, Then the whole ship disappears. Can it be that the ship is sinking? Is the water rising higher and higher, covering first the hull and then the sails? That can’t be, since it happens to every ship that sails away and most of them return safely. When a ship returned in ancient times, the sailors swore that at no time did the water rise above the hull. How else could this be explained? There was one way. Suppose the earth’s surface curved. The ship would then sail away over that curve and would gradually disappear behind it. Naturally, the bottom part would disappear first. The ships were hidden by the curve of the earth, just as the stars near the horizon were hidden. But there was one great difference. You could see the stars disappear behind the curve of the earth only if you traveled north and south. In other directions, the turning of the sky spoiled things. The ships, on the other hand, disappeared bottom- first, in whatever direction they went. They disappeared bottom-first, whether they went north, south, east, west, or any direction in between. What’s more, it always looked as though they dis- appeared at about the same rate. If they were two miles away, a certain amount of the hull was hidden, no mat- ter in what direction they were sailing. It looked as though the earth curved in every direction, and by the same amount in every direction. But the only shape that curves by the same amount in every direction is a sphere. If you make a point on a large ball and draw a line away from that point in any direction, you will see that all the lines will curve in the same way. Judging by what happens to ships, the earth is not a cylinder, but a sphere. It is a large sphere in the center of the much larger sphere of the sky. If the earth is a large sphere, then the tiny bit we can see at any one time looks flat. But that still leaves us with the question of why we don’t slip off the earth when we move around and why the air and ocean don’t slip off. Is there some- thing else that can give us proof of the earth’s shape? There is something, but for that we have to go back to the sky again. 4. The EARTH ‘S Shadow EVERY ONCE IN A WHILE, the moon loses its light. A black shadow moves across it until all one can see is a dim red glow. After a while, the shadow moves away and the moon is back again, shining as brightly as ever. When this happens, the moon is said to be ‘’ eclipsed.” In ancient times, an eclipse of the moon frightened people. They thought the moon might remain dark forever and they did not want to lose the moon’s helpful light at night. Those who studied the sky carefully were sure this would not happen. They noticed, for instance, that an eclipse took place only at the time of the full moon. It never took place at any other time. What’s more it only took place during certain full moons. The ancient Greeks who studied the sky knew that when the moon was full, it was on the opposite side of the earth from the sun. The sun shone past the earth onto the moon. The sun lit up the entire side facing them and that is why the Greeks saw the moon as a full, round circle of light. Suppose, however, that the earth were exactly in between the moon and sun. Then the sunlight would have to go through the earth to reach the moon. It couldn’t do that, of course, so no light would reach the moon.