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Men of science. Tell us that telescopes. Show the earth. To the a planet. Just like mars or venus. A planet rotating about the sun. But how could a telescope show that it is after all you can't stand off in space to look at the earth through one. Really then on such evidence is the earth a planet. After all. Well. I'm not so sure a telescope wouldn't help. To show that there is a planet. Speaking. It is to with hindsight. It seems to me that cooking at the planets with a telescope. We are apt to learn many things we didn't know before we had one. But how a telescope could teachers things about the earth is indeed hard to imagine. Still it wouldn't be the first time that science would have led us to believe things quite gone totally to expectations. And even to common
sense. So potent of forces science and. Transforming our material civilization. The very few of us question the statements made by scientists. Even when they plainly contradict. Our ordinary sensory experience. Nevertheless apparently absurd conclusions of scientology justified. In particular as this story unfolds. The dramatic effect on science. Of the introduction of a new instrument. That enables the scientists to transcend the limits of ordinary experience. Will be seen. Now. To say that the year. Is a planet. Is to establish a similarity between the year. And those star like bodies.
Mercury and venus mars jupiter and saturn. The stars that we see. On any given night. Appear to us to be a series of bright lights. Set out against the dock draperies of the heavens. These stars are usually arranged in groups and groups of names like the great bear the little bear. Orion's belt. Cassiopeia is chair. And so on the assignment. Of stars to groups of constellations. Is of course purely arbitrary. They're intended to serve as an aid to memory. For instance. There's nothing about the group of stars which is called the great bear. To suggest a bear rather than a pig or a beautiful maiden. But if we take the whole group of stars. And arrange them in constellations or groups. On an astronomical globe. Which is like a globe of the earth. Except that instead of
showing continents. We show. All the difference dawes. Arranged here. In constellations that we can represent on this globe. The way the constellations at night. Rise in the east. And set in the west. And as the year progresses through the seasons. New constellations will be seen in the night sky. Some of the other constellations. Will disappear. Now the amazing thing about these stars. Is that although they appear to move in the course of a night. Through the heavens as a whole the relation of each one of these daws to its neighbors is fixed and determined. It's just about the same today. On a star globe like this. As it was at the dawn of history. Now for this reason. The stars are known as fixed as the fixed with respect to one another in constellations. And each of these constellations. Is fixed with respect to the other constellations.
But what we call the planets. Mercury venus mars jupiter and saturn are brighter than the fixed stars. And their positions in the heavens are not fixed. Either with respect to one another or even with respect to those fixed stars in the constellations. Now the motion of a planet. With respect to the fixed stars. Can't be seen in the course of a single evening. But a month is long enough to show that a difference appears. Between the place where we see a planet in the stars. And where it will be seen at a later time. Here is a chart of the heavens. You observe on it that there is a series of stars that are shown in a single line. Which indicates the path along which mars moves from one month. To the next. So one of the features which distinguishes a planet from the fixed stars that we see in the heavens. Is not only the fact that a planet wanders with respect to the fixed stars. But
that sometimes it. Wanders toward the east. And sometimes toward the west. In fact the very name planet comes from the greek word planet those. Which means wander in. Other words. The primary distinction between a planet and any other star. That we see in the heavens. Is that the planets actually do wonder. While the other stars are fixed with respect to one another. Now if we turn to the historical record. We find that from ancient times say the period of the egyptians until about a little more than three hundred years ago. Most people thought that the earth was at the center of the universe. And that all apparent motions observed in the heavens. Were actual motions. It was believed that the earth was fixed in space that it was immobile. And that the heavens turn around the earth. As we see at night. When we watch the heavens rotate. We see the sun and moon rise and set. We see the planets rise and move through the heavens. Moving with respect to the earth.
Now the sun and moon. Are not fixed with respect to the stars. And so they used to be called planets too. So the ancient list of planets includes. First of all the moon. And the sun. And then mercury and venus mars jupiter and saturn. From these seven wanderers or ancient planets. We derive the names of the seven days of the week. Sunday is of course the sun's day. Moon is the moon's day and saturday is named after saturn. But this old system astronomy. In which the earth was considered to be at the center of the universe. Had the planets. Mercury and venus and mars jupiter and saturn. Which looked like stars to us when we go out at night. To be real stars. And like the other stars that exist in the heavens. These planets were thought to revolve around the sun. And to shine by some inner light. Just like the other
stars. And so there was a simple homogeneously in the conception of stars planets. And. Fix stars. Both rules around the earth. And the earth. Was different and was not a star. The sun it was held. Was like a star it shone by its own light. And so it was plainly reasonable. But the sun and the moon. Should move around the earth the moon however the nearest of all the astronomical bodies to the earth. Was conceived to be a little different. It was halfway between being earth like and star like the moon of course doesn't look like the fixed stars. Or the sun and the planets. It doesn't have any light of its own. And it was soon realized in greek times. That it was a huge smooth opaque ball. Which receives its light from the sun. Now if this is so. Half of the moon. Is always illuminated by the sun. While the other half. Must remain in darkness. For over here.
We have a model to show. Why the moon gives us the appearance is that it does over here. I have a large. Opaque. Ball and over here. I have a spotlight. Which can serve to eliminate that all play ball. Suppose i were to look at it from this direction. Then what i'd see is the whole disc of the moon illuminated by a condition that arises. When the moon is full. Suppose now. I want to look at this. Opaque ball and spot light. In this direction. Then the part of the moon which is illuminated and which i see is just one half. Which corresponds to the condition of the moon. When we see. The first quarter. If i move over here. And look at my spot light and. opaques. Fear from this direction. Then we see the appearance of the moon. In the last quarter.
And finally. If i look at the glow and spot light from this direction. What i see is simply the opaque or. Unlighted side of the moon. But i may be able to notice. A very tiny crescent on one side. Giving us the appearance of the moon. In its very last period of visibility. Or in its first period of visibility. This type of demonstration. Was actually conceived by the ancient greeks. And it was conceived by them to explain why the moon shows the phases that it does. But the fact is that even though people generally conceive that the moon. Was more earth like than the other stars. Or the planets or sun. It was still generally thought that the moon must be a perfect sphere. And that some on loan phenomenon. Some. Optical phenomenon of which people had no idea. Was responsible for the markings on the moon.
But this is discussed. In all the older literature. For instance in the third book of dantes divino com a.d.f.. There's a long discussion between dante and bad trade jay as to why the moon. Which is a perfect sphere. Should nevertheless look to us the way it does. Some parts being doc and some parts being light. And even having the appearance some time. That there's a face. To be visible. The face of the man in the moon. Melissa system. Was challenged in antiquity for there were a few people who had the idea that maybe the earth isn't at the center of the universe. Maybe the sun is located at the center. But of course there's an obvious objection. Because this scheme implies that the earth and the planet. It's venus mercury mars jupiter and saturn. Are all alike. Since all of them move in the same way around the sun. The idea. Was very repugnant.
One of the primary reasons that nobody liked it. Was that is implied that the abode of man. This solid earth on which we stand. That this was something. Which is rushing through space. How much more comforting to think that the earth is standing still. Then there's another problem. Because in view of what man consider the universe to be like. It was much more reasonable to think that the earth was unique. Different from the planets and not the same for the earth. Is the only place where man layers. And presumably. The only place where life had been created. So the earth. Was released. In most men's opinion. Different from the planets. And surely not the same. Because it was different. It had a special role. A special place in the order of heavens. And that was to be fixed and stationary. And to be located at the center. In the years fifteen hundred forty three. The polish astronomer copernicus.
Published a great book which is called. On the revolutions. Of the celestial spheres. In this book. Copernicus advanced the theory that all the phenomenon of the heavens. Can be explained by assuming that the sun is fixed at the center. And the planets move around it. Here is a diagram of the copernican system. As it appeared in print. In the first version of the copernican system. In the english language. Not the sun at the very center. And the planets moving about the sun. In a series of concentric. Orbits. Copernicus suggested that the earth was to be thought of as only another planet. Moving around the sun. Now the book that i hold here. Is actually a printed facsimile of the original manuscript of copernicus's book. Which survives today some four hundred years after it was written.
It was a very difficult book. It was hard to understand. Because it was based on complex mathematical arguments. And has many formidable calculations. Long series of tables. Copernicus in the preface said. Mathematics is for the mathematicians. And it was generally agreed that a reader would have to be a very skilled mathematician. To master the contents of this noble book. And one of the main reasons that this book of copernicus. Did not at once. Excite men's imaginations. And give rise to those latent controversies. Was the obviously absurd implication that the earth. Is only another planet. The mere fact that the planets have a wandering motion with respect to the other stars. Surely doesn't make them in our minds any the less stars. For a star is a bright luminous object and presumably it shines because it has some inner light of its own in the earth as in such a body.
It's made of earth and dirt and. Clay and living material. And no one would conceive that it shines. The way stars shine. I think we can see therefore how philosophically absurd it was and how contrary to every possible conclusion of common sense applied to experience. To take that statement of copernicus. The earth is merely another planet. Planet indeed. Why anyone has only to go out at night and look at the heavens. See what a planet was like to find out that it was like a star. And not like the earth. And when he looked at the earth on which he stood it was perfectly clear to him. That the conclusion of copernicus was absurd. The earth was not a planet. Then in the year sixteen hundred ten. Everything that i've just been talking about. Was upset. It was upset by the publication of. One of the most important forks. In the history of scientific thought. The book was called. The message of the stars. And it was written by the great
italian scientist. Galileo galilei. Here we have his picture. And up in the corner of this picture. Is shown the instrument. In a somewhat fanciful reproduction of cherub. Looking through a tube like device. In order to see what the heavens are like even more than a dozen years before this book of been published by galileo had become a convert to the ideas of copernicus. But he hadn't found any way of convincing anybody. That the new system was better than the old. And then. In sixteen hundred nine. One year before that book was published. Galileo heard is that he said. Rumors that a new instrument had been made by putting two lenses together. No one knows where this device was invented. When or by whom. Presumably the invention was in galileo's native italy. Since a telescope had been brought to holland. Which said on it that had been made in fifteen
hundred ninety. In italy. And by sixteen hundred nine. Number of such instruments. Had appeared in holland. They were all made by combining two lenses together. And one of these lenses. Was a positive lens. By positive lens we mean that it's very thick in the center. And rather thinner at the edges. This positive lens. Has the property of enlarging. And if we take a piece of cardboard. On which. Some writing appears. And place that piece of cardboard. In. Front of our optical bench. And then place. The lens in front of it. And now. Look through the positive lens. At that piece of paper with the right printing on it. We see that it's very much enlarged. The second lens. Gives a negative light and. It's the lens which is very thin at the center. And rather thick at the edge. And this lens has just the opposite property.
This lens makes things look small. Or as a reducing lens. And if we place this here. Then as you can see the letters look very very small. Now. In making the early telescope. To such lenses a human a positive lens placed here. The negative lens placed here. And defined how look through this combination with a negative lens near my eye. And the positive lens far away. We get an enormous magnification as you may see something that makes things look very large. While that makes us. Have the possibility for the first time. Of seeing what distant objects a like. This was the first telescope. Now the modern telescope is somewhat different. But the principles of this early one. Are still used. It still is often called the galileo and the telescope. It's used in the common opera glasses. In which we have a negative lens here.
Placed near the eye and a positive lens placed here. Just as in the combination of lenses which we showed a moment ago. Now when galileo. Heard the rumors of this new instrument. He went at once to his workshop and began to experiment with combinations of lenses and quickly produced such an instrument. When he had done so. He recognized that the one thing this instrument might really do was to answer the question as to whether the earth is unique. And different from the planets or whether those planets might be like the earth. And he thought that he might even demonstrate that the conclusions of copernicus were reasonable. And not absurd. The first thing he looked at was the most prominent thing in the heavens. The moon. And here is it would cut. Made from that early book the message from the stars. Which shows what galileo saw. Our telescope unable to us to see more detail than to us the moon looks something like this. What's most impressive. Is the way the moon really looks something like the earth. Here
obviously mountains sticking up. And hear what appear to be oceans. Or seas. And here is a kind of coral a tall like we would see in the pacific ocean. Here a continents. And here. Even our islands. With one view through the telescope. That all distinction between the earth and moon disappeared. The moon was obviously not a smooth opaque disk. It was certainly earth like later on galileo found that there really isn't any water on the moon. And so presumably there is no life. But it was plain from that very first view that the moon is current gated and rough. And full of mountains like the earth. Here we have a close up of the moon as seen through the modern telescope. There were little dots of light. That can be seen here and here. These are mountains. And galileo realized that these mountains were so high that they could still catch a little bit of the sun's light. Even though the lower portions were in shadow. In
the dark part of the moon. Galileo calculated what the distance would be from these bright spots. To the boundary between the dark and bright part of the moon. And from these data. The computer at the height of the mountains. They came to about four miles. Or about two thirds as high as the highest mountains on the earth. Like mount everest. Now it's certain times. Galileo found a telescope. Showed that a certain amount of light was being reflected into the dog part of the moon. A light which couldn't be seen easily with the naked eye but with the telescope in this picture. We see on one side the moon as it normally appears to us. A sort of crescent. And then the remainder of it shows us that careful. The same part. Shows through the telescope. In which we can see the fine markings. Indicating that a certain amount of light. Must be getting into the dog part of the moon. Where could it possibly
come from. Galileo studied this question. At great length. And finally he concluded. There was only one possible place. Sunlight. Light from the sun striking the earth. And being reflected into the dock part of the moon. Now this was a great discovery. We can't overestimate it. For it shows that even though the earth can't shine for many inner light. It does shine. And it shines. Because it reflects the light from the sun. Just as the moon. Reflects the light which it receives from the sun. So said galileo. If there were an observer on mars. He would see the earth shine. Just as we observers on earth. See mars shining. And one of the most interesting of all the discoveries that galileo made with his telescope. Was the fact that around the planet jupiter. There's a system of four moons. And these moons move around jupiter. Just as our own moon moves around the earth. Another similarity between a planet and the earth. Nobody could say anymore.
That only the earth has a satellite or companion a moon. And is unique. Jupiter has not only one moon but four of them. But probably. Of all the discoveries that galileo made the most important for the copernican system. Was the appearance of faces in venus. Here is a diagram. It shows us how venus would appear to an observer on earth. If venus. Like earth and moon. Is merely a dull piece of matter or a sphere. With no light of its own. But shining by reflected sunlight. Suppose we were to see venus here. Then we'd see a perfect circle. Just as before we demonstrated. The condition to be when we see a full moon. Over here. We would see half of moon and venus alumina ated. Just as we see half of the moon illuminated. In the same situation would obtain over here. At this point. We have the appearance of the moon. At first for her. And the
other. At last quarter. But when venus is here. We would expect to see practically none of the planet. Since the dark side is facing us. Except that maybe we'd see a very thin crescent on this edge. Now let me call your attention to one for the fact. When we see that full disk. When we see the complete illuminated circle. Then venus is much further away from us than at any other position. And when we would see the crescent here. Venus would be at its nearest position to the earth. Hence a venus does move around the sun and has no light of its own. We would expect that crescent which is near us to be very large. And the full disk which is far away from us. To be very small. And the half disk here. And here. To be intermediate between the two. Now let's look at what galileo actually saw. Here is a series of photographs. Made at the lowell observatory in new mexico. They show us the appearance of venus is seen through a telescope. At constant magnification. Here is the disk the position roughly resembling the last
quarter. And here is the very thin crescent. Note that the relative sizes. Accord perfectly with the prediction we made a moment ago. When we assume that the planet. Minas moves around the sun. But it's like the earth unlike the moon. And not like the stars. That it has no light of its own. But shines only by reflection. Now let's put all of these discoveries together. And let's make is our final assumption that what happens on one planet. Can be assumed to happen in most of the others. The system is one in which the earth shines like the moon. And presumably shines like the other planets. The moon is current gated and roughen like the earth and so presumably. All the other planets. Some of the planets like the earth of moons. The planets shine by reflected light from the sun their earth. They aren't stars at all and so you can see that it once. It wasn't philosophically absurd any longer. To think of the earth as a planet. Or if you like to think of the planets
as being earth's. Rather than stars. And with the publication of that book of galileo. In sixteen hundred ten. The whole climate of opinion in regard to the discussion of the copernican system. Was radically altered. The effect of the new instrument. Had been to take that statement. The earth there's another planet. Which had been contrary to all sensory experience showing the planets to be stars rather than earths. And to transform that statement into one. Which agreed with a new kind of experience. Since the telescope had shown that the planets. Earth's. Surely mars jupiter saturn mercury and venus being like the earth. Should behave like the earth and how reasonable now. That all of them should move around the sun. The copernican hypothesis therefore. Was no longer merely a mathematical device for computing. It was a reasonable description of the universe. And so we can understand why galileo's message from the stars created such a sensation. All of the traditional ideas of the universe as
a small tiny place. The fixed and movable center had to be abandoned. And not only. Scientists the philosophers. And men of affairs and statesmen poets and artists were shaken by the frightening ideas. Of the vastness of space. And the new system of the world. The perplexity of those thinking man who now had to consider that the earth was really a planet. Twirling around with no fixed location in space and insignificant speck in the vast starry reaches of heaven. This feeling. Is perhaps best expressed for us. By a contemporary poet. Here john donne. A new philosophy calls all in doubt. The element of fire. Is quite put out. The sun is lost on the earth. And no man's wit can well direct him. Where to look for it too is all in pieces. All coherence gone. All just supply all relations.
You the you you know who. This is.
Of Science and Scientists
Episode Number
Is the Earth a Planet?
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WGBH Educational Foundation
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WGBH (Boston, Massachusetts)
Library of Congress (Washington, District of Columbia)
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Episode Description
Take a piece of glass, thicker in the center than at the edges. Add it to a piece thicker at the edges that in the center. Use the two to create a revolutionary view of the earth. Holding such lenses, I. Bernard Cohen (Associate Professor of the History of Science and of General Education, Harvard University) shows how, pointed at the stars, they destroy a commonsense notion that the earth is the center of the universe. The revolution in thought which described the earth as a wanderer through space, like Mars or Venus, challenged the old tenets of common sense as has no other revelation of science. And yet, how can a telescope, two lenses, do this? After all, you cannot stand off in space to look at the wandering of the earth. Really then, on such evidence, is the earth a planet after all? Cohen invites you to inspect the evidence: phases of the moon and Venus, enormous shadows cast by mountains of the moon, the notebooks of Copernicus. (Description adapted from documents in the NET Microfiche)
Series Description
Americans tend to think of the sciences as potentially useful (air-conditioning) or potentially troublesome (Strontium-90). We accept or marvel at the revolutionary products of science while giving little thought to the basic ideas, concepts, techniques and logic that have gone into exploring, understanding and explaining our universe or in building our technical civilization. Such an understanding of science does not come easily. Limited by time and opportunity, scientists do not often explain themselves to non-scientists. Also, the quality of science is most difficult to the layman to understand is its indirect approach to the discovery of truths. Robert Frost summed up the problem by commenting that to his mind all science rested on the question, "How she differs from what she's like." Without attempting to teach physics or chemistry or geology, these programs suggest the qualities and outlook of science. By analogy and demonstration, they reveal the ideas which guide scientific research and the truths that research uncovers. They give an appreciation of what the scientist can and cannot do. As one speaker says, "The important thing about science is not merely that it gives rise to technological miracles, but that it provides us with one of many guidebooks we need to find our way in this universe." Today, as non-scientists are called upon to make decisions or concur in decisions that may affect the future of scientific research and even the future of life, a knowledge of "how she differs from what she's like," may not only be useful, but necessary. By explaining and demonstrating the guiding principles of science and scientists, these programs attempt to convey that increasingly necessary knowledge. Produced by WGBH-TV, Boston, the producer-director was David Walker and the executive producer Lawrence Kreshkoff. Dr. Phillipe Le Corbeiller is Professor of Applied Physics and of General Education at Harvard University. He is host and program editor for Harvard in the television series, "Of Science and Scientists." A leader in Harvard's general education program since its beginning in 1946, when he started the course, "Principles of Physical Science," Dr. Le Corbeiller was the first to be appointed professor of General Education in 1949, in addition to being professor of Applied Physics. Throughout the series Dr. Le Corbeiller is joined by other scientists. (Description adapted from documents in the NET Microfiche)
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Director: Walker, David
Executive Producer: Kreshkoff, Lawrence
Guest: Cohen, I. Bernard
Host: Le Corbeiller, Philippe
Producer: Walker, David
Producing Organization: WGBH Educational Foundation
AAPB Contributor Holdings
Identifier: 283610 (WGBH Barcode)
Format: Betacam
Generation: Master
Duration: 00:00:30
Identifier: 283601 (WGBH Barcode)
Format: Digital Betacam
Generation: Master
Duration: 00:00:30
Identifier: 01483A (WGBH Item ID)
Format: 16mm film
Generation: Kinescope
Identifier: c9a0c912af615f507740395dd49b130204013d14 (ArtesiaDAM UOI_ID)
Format: video/quicktime
Color: B&W
Duration: 00:29:56:00
Library of Congress
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Format: 16mm film
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Chicago: “Of Science and Scientists; 4; Is the Earth a Planet?,” 1957-00-00, WGBH, Library of Congress, American Archive of Public Broadcasting (GBH and the Library of Congress), Boston, MA and Washington, DC, accessed June 12, 2024,
MLA: “Of Science and Scientists; 4; Is the Earth a Planet?.” 1957-00-00. WGBH, Library of Congress, American Archive of Public Broadcasting (GBH and the Library of Congress), Boston, MA and Washington, DC. Web. June 12, 2024. <>.
APA: Of Science and Scientists; 4; Is the Earth a Planet?. Boston, MA: WGBH, Library of Congress, American Archive of Public Broadcasting (GBH and the Library of Congress), Boston, MA and Washington, DC. Retrieved from