Earth: Prove that it is Round ```Name: Ann Status: student Age: 14 Location: N/A Country: N/A Date: 1999-2001 ``` Question: Can you give one undisputable answer to: "Is the earth round, and how do we prove it." Because we are having a debate in my class, and the teacher kept saying 'the earth is flat' And I have to give him one undisputable answer. Replies: I may be wrong about this, but I think your teacher is trying to demonstrate to you that anything and everything is open to dispute. In fact, a rather famous theorem in the philosophy of science, the Duhem-Quine theorem, states that it is impossible to conclusively disprove any scientific hypothesis. That includes such hypotheses as the earth is flat, photography steals the soul of the object being photographed, and apples are highly toxic to humans. To hold on to such hypotheses, it may be necessary to put forth some rather farfetched explanations of what we can observe, but it can always be done. In fact, science does not work by conclusively and indisputably refuting bad hypotheses. Instead, hypotheses stand or fail on their ability to explain known observations, and to predict the outcome of observations that haven't been performed yet. Let's take the flat earth versus round earth hypotheses as an example. A round earth is the most stable shape according to gravitational theory; a flat earth would tend to collapse toward the center, and people standing near the edges would feel a gravitational pull toward the center of the flat surface instead of perpendicular to the surface. Your teacher could counter that gravitational theory is wrong; magic holds us to the earth. This may be true, but it doesn't explain why gravitational theory works so well at explaining how things fall, and the observed trajectories of the planets and their moons. You could tell your teacher that photographs of the earth from space show that it is round, just like the globe model in your classroom. He could counter that those photographs are government fakes. Perhaps so, but we can see that other planets and moons are round; wouldn't it make more sense to say that the earth is round, too? Your teacher could say that the earth is special and different. You could ask how numerous people have sailed or flown around the earth if the earth is flat. He would have to say that every one of these people, separated as they are in time and space, belongs to the same great conspiracy aimed at brainwashing us into supposing that the earth is round. You could ask him about the communications networks that are linked by satellite - how can you have satellites orbiting a flat earth? He might then tell you that there really aren't any satellite links - everything is connected by wires, or a close-packed array of relay antennas. We could go on and on, and I bet that in your classroom you HAVE gone on and on. As you see, there is no single fact you could tell your teacher that he couldn't dispute in some way. If you step back and look at it, though,what kind of world would it have to be if your teacher were right? Gravity doesn't function here like it does everywhere else in the solar system, there's a huge conspiracy reaching across hundreds of years to convince us (for no apparent reason) that the earth is round instead of flat, the progression of the seasons, and even of day and night, are controlled by the gods. Eventually, a reasonable person would have to concede that it would make a lot more sense to agree that the earth is indeed round. There are some other theories to explain things that do not meet this test of scientific reasonableness, yet are still put forth and defended by many people in society. This example of the flat earth hypothesis lets you see how difficult it is to change someone's mind when they're set on some strange or unrealistic theory. It also lets you see that just if a theory "can't be disproven" doesn't necessarily mean that it is correct, or that it should be taken seriously for very long. No matter what the Nebraska School Board says. Richard Barrans Jr., Ph.D. Three arguments come to mind. All of these date back to Greek times or earlier. 1. During lunar eclipses, the projected shadow of the earth on the moon is always round. If the earth were flat, then this projection will not always be circular (it could degenerate to a line!). But this is never observed, regardless of the time of the lunar eclipse. Aristotle (384-322 BCE) wrote about this. The third point, below, is also credited to Aristotle. 2. Eratosthanes (276-195(?) BCE) did a famous experiment on measuring the angle of the sun at noon in Alexandria, Egypt, and in Syrene. Finding the difference in the angles and knowing the distance between the two points, the circumference can be calculated. He calculated the circumference in stadia, which we do not know the conversion to our present day measure. 3. If you travel north or south a signigicant distance, you will see a different set of stars at night. This will not happen on a flat surface. Introductory astronomy texts deal with this topic. A good reference is George Abell _Exploration of the Universe_ The next question is: "Prove that the earth is moving." Nathan A. Unterman I presume your teacher won't accept hearsay evidence (evidence collected by someone else and told to you), such as testimony from astronauts or folks who have traveled around the world by boat, airplane or balloon. So here are the standard facts observable by anyone which your teacher would find difficult to explain with a flat Earth hypothesis: (1) When ships put out to see, they sink below the horizon gradually. First the hull goes down, then the masts, like so: ``` o -+- | . / \ . . . . . / . here you can still see the mast . |\ / but the hull is below the horizon \ \/ \ \ ``` This suggests that the Earth's surface curves away and downward from wherever you stand. It's hard to observe this effect on land, because mountains and trees and so forth get in the way when something moves away from you before it dips below the horizon. (2) If you climb higher, you can see further. On top of a mountain or lighthouse, or in an airplane, you can see things that are invisible -- below the horizon -- when you are on the ground. For example, if you watch the Sun set, and at the very moment when the Sun is just below the horizon you climb quickly up a hundred feet, you will see the Sun again. It is hard to explain why you can see further when you climb higher unless the Earth's surface curves downward away from you wherever you stand. (3) The Earth casts a shadow on the Moon during a lunar eclipse. The shadow is round. (4) When the Sun is directly overhead in any place, it is NOT directly overhead at the same time in any place a few hundred miles away: Sun's rays are parallel because the Sun is very far away. ``` | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | V V V V V V V V V V V V V V V this stick || casts no || shadow || this stick || casts a shadow . . . . // . . //. . . //... //.... . EARTH .... ... ``` If you were to put a stick in the ground sticking straight up at noon in New York City, then telephone a friend in Chicago to ask him to also put a stick in the ground sticking straight up, he would see a shadow, and you would not. This is hard to explain unless ``straight up'' (away from the Earth's surface) points in different directions when you are at different places on the Earth's surface. That is, the Earth is not flat. (5) If you travel North, you can see stars at night which you never saw before. Since there are so many stars, you only need to travel a hundred miles or so. The new stars are stars that were hidden below the horizon before you walked North. How could this be true if the Earth were flat? (6) There are tides in the oceans, which you can see at the shore, that repeat every 24 hours or so. If the Earth were flat, then there would be no tides, because the tides depend on their being a substantial distance between the near and far sides of the Earth, from the point of view of the Moon. This is kind of complicated, however. Grayce That is easy. Observation of the Earth from outside the planet shows it to be round, nearly spherical. I am sure that your teacher is trying to motivate you to think. By the way, if you look at the stars, the sun, and the moon, you do not see any of them to be non-round. Since you observe them at different times and from different angles, it seems plausible to say that they are round. Why should the Earth be an exception? Dr. Ali Khounsary Advanced Photon Source Argonne National Laboratory Argonne, IL 60439 Click here to return to the General Topics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs