3. Concepts and History of Astronomy and Physics

Transcript: Newton’s master work is the universal law of gravity. Newton’s law of gravity states that every object in the universe, every particle, every planet, every star, every galaxy, attracts each other with a force that is proportional to each of the masses of two objects and inversely proportional to the square of the distance between them. The constant in this equation is the gravitational constant. Thus, if you have two objects in space and double the distance between them the gravity force reduces by a factor of four. If you triple the distance, the gravity force reduces by a factor of nine. If you double the mass of one or other object the gravity force doubles. The gravity force applies to everything in the universe, and it is a wonderful approximation to the way objects move through space.

Transcript: Tycho Brahe is one of the most colorful characters in the history of astronomy. He was born into an extremely poor family and at a young age was essentially bought by his uncle, a nobleman, and so he grew up in wealth. Then, as a teenager he was lucky enough to rescue the king of his country and so received a handsome stipend with which he set up a fantastic observatory on an island off the coast of what is now Sweden. Brahe’s careful and meticulous observations over twenty years refined our understanding of the motions of the planets. They were the most accurate observations obtained in the time before telescopes. Brahe observed two supernovae during his life. And his careful observation of the motions of a comet he used to demonstrate that the crystalline spheres could not exist, and so he was able to destroy one more of the old Greek ideas. Ironically, Brahe did not accept the Copernican motion, but the quality of his data which he passed on to Johannes Kepler at the end of his life helped cement the Copernican revolution.

Transcript: We tend to think of the Renaissance in terms of developments in art and sculpture in the 15th century, mostly in Italy. But the Renaissance had its start several hundred years earlier in the 13th century, and it really started in Spain with the rediscovery of ancient Greek manuscripts and the Greek philosophers’ knowledge. Renaissance in fact means rebirth or rediscovery. In Toledo, in Spain, around 1100 AD translators working in large schools were recovering the Greek knowledge that had been lost for over a millennium. In addition to a rebirth in art and culture new ways of looking at the natural world, epitomized by Leonardo da Vinci in his natural way of drawing the human form, scientists were beginning to look anew at the natural world and see things that they had not seen before. The Renaissance marks the birth in a new phase of astronomy.

Transcript: With the decline of Greek civilization also came the decline of science. The next dominant civilization were the Romans, and the Romans were little interested in pure science, being more interested in practical matters such as agriculture and creating the bureaucracy of their huge empire. Several events from the fourth century AD symbolized the decline of science. One was the sacking of the great library at Alexandria and the horrible loss of knowledge represented by that event. The second was the death of Archimedes. Perhaps one of the most famous inventers and mathematicians, he has used the Greek cosmology to suppose a universe filled with ten to the power eighty particles as a demonstration to his king of his power in mathematics. Archimedes lived in Syracuse, and when that city fell under a Roman siege he used his ingenuity to create machines of war to keep the Romans at bay. When the Romans finally broke the siege and entered the city, he was cut down by a Centurion and killed, the last major mathematician or scientist of the golden era of Greek science.

Transcript: Looking at the distances of the planets from the sun, it’s easy to see that they are neither uniformly spaced nor randomly spaced. In fact the distance of the planets from the sun follow a geometric progression where each is one and a half to two times further out than the one before. This was noticed in the late 18th century by Bode and Titius, and it forms what is called Bode’s rule. At the time of the discovery of Uranus it fitted the progression by being roughly twice the distance of the next innermost planet. Astronomers realized that Bode’s rule predicted a planet between the orbits of Mars and Jupiter, and German and Italian astronomers set out to look for such a planet. They were called the celestial police. They didn’t find a planet. However, they did in the year 1800 discover Ceres, the first asteroid, and over the succeeding years a number of other asteroids at just the distance predicted for the missing planet. Kepler’s laws provide no explanation for why the planets should have this form of spacing. The details could only be understood when we understood the theory of how planets form.

Transcript: Too often science is treated in isolation from other human pursuits. However the broad history of ideas from the time of Copernicus to the time of Newton parallels a similar evolution in the arts in Europe at that time period. The popular cliché goes that science is pure analysis, or reductionism, or taking the rainbow to pieces, whereas art is pure synthesis, or putting the rainbow together. This makes an unnatural division between science and the arts. Thinkers as great as Jacob Bronowski have shown that the creative acts of the artist and scientist are similar. During the Renaissance the rediscovery of Greek ideas was just a first step towards a more realistic and naturalistic understanding of the world and the universe. In art it corresponded to a rejection of the static world order of medieval times, and often a rejection of the way of viewing the world proposed by the Catholic Church, and an appreciation of beauty and naturalism in nature, this epitomized perhaps by the work of Michelangelo and Leonardo da Vinci. In science the static Aristotelian world view is replaced by exploration and active observation as a way of understanding nature, and the universe becomes a dynamic place.

Transcript: Ptolemy was a scholar at the Alexandrian library in Egypt. Little is known about him. He wrote around the year 140 AD a vast encyclopedia of astronomy called the Almagest which means “the greatest.” This was the compendium that would carry forward through the next millennium as astronomy moved through Europe. He created a star catalog with over one thousand stars, and he propagated the geocentric model of Aristotle that was the standard cosmology for his day and for another thousand years.

Transcript: The Greek geocentric cosmology of Aristotle, as propagated by Ptolemy, was highly complex in trying to explain the motions within the solar system. Because the planets do not have uniform motion, the model needed the centers of their motion to be displaced from the Earth. Because Mars, Jupiter, and Saturn display retrograde motion, or occasional backward motion on the sky, they had to have epicycles inserted on their orbits. All in all, the Ptolemaic model was extremely complex and not very efficient. In the thirteenth century King Alfonso of Spain said he would have consulted the creator to come up with a better arrangement. One of the fundamental principles of science called Ockham's razor after William of Ockham in the fourteenth century is the idea that the best explanation for a natural phenomenon is the simplest explanation. By the thirteenth century it was clear that the Ptolemaic model was not very simple and not very elegant, so people were seeking new solutions.

Transcript: The Copernican revolution and the invention of the telescope opened up people’s ways of thinking about the universe. The universe now became a very large place in the heliocentric model because the stars must be very far away compared to the distance between the Earth and the sun. In addition, Galileo’s observations of mountains on the moon and of planets themselves that had objects in orbit around them, the moons of Jupiter, showed that there were worlds within space and led people to think about the possibility of other worlds, other creatures, possible life forms even. This idea is called the plurality of worlds. Giordano Bruno, a mystic, decided to take this idea to the limit and speculated in writing about an infinity of worlds or planets orbiting other stars and species on those other planets, some of whom may rival in intelligence humans. This was too much for the Catholic Church, and Bruno was burned at the stake in the Campo de' Fiori in Rome by the Catholic Church as an impenitent heretic.

Transcript: Science proceeds by making observations that discriminate between rival hypotheses. Galileo’s observations of the phases of Venus were decisive evidence in favor of the heliocentric model. In the geocentric model the fact that Venus has always seemed close in the sky to the sun is explained in terms of an epicycle. As Venus moves on its epicycle it is always between the sun and the Earth, thus the phases of Venus do not change substantially since Venus always has its sunlight face pointing back towards the sun. Nor does the apparent size of Venus change because it always lies between the Earth and the sun. In the heliocentric model Venus is sometimes between the Earth and the sun and sometimes on the far side of the sun from the Earth. When Venus is between the sun and the Earth we see its dark face. And so it is barely illuminated as seen by us, and it is larger in the sky because it is close to the Earth. When it is on the far side of the sun from the Earth we see its fully illuminated half so it has a full phase, and its angular size is smaller because it is far from the Earth. The changing and dramatic phases of Venus and the change of its apparent angular size were observed by Galileo and were decisive evidence in favor of Copernicus’s model.

Transcript: Parallax is the shift of angle when something is observed from two different perspectives. If you hold your finger out in front of your face and observe it with one eye and then the other you will notice the shift, or the parallax shift, relative to a distant backdrop. This angular shift becomes a way of measuring distances in astronomy. The parallax angle depends on distance. If you take the finger in front of your face and move it to a larger distance the parallax angle goes down. Parallax was major argument against the heliocentric model at the time of the ancient Greeks because if the Earth were in motion around the sun then surely the stars should show varying angles between them and also varying brightnesses as the Earth moved nearer and further away from the stars through a cycle of the seasons. The only explanation for no observed parallax in the heliocentric model is that the stars are enormously far away compared to the distance between the Earth and the sun.

Transcript: Newton’s third law of motion says that for every force there is an equal and opposite force. This is sometimes called the principle of action and reaction. For example, when you sit on a chair you exert a downward force due to gravity, but if the chair does not move or break the chair is exerting an upward force to support you. A rocket is another example of action and reaction. The forward motion of the rocket is countered by an equal force backwards that propels the exhaust gasses from the nozzle of the rocket. The recoil of a gun is a similar situation. When you punch a wall, if you are foolish enough to do so, the force you exert on the wall is countered by an equal and opposite force on your fist which will cause you much pain. If you jump from a boat onto the pier the boat will move backwards, the reaction force to your force that moves you forward towards the pier. The principle of action and reaction has many examples in everyday life, and it’s a fundamental statement of the raw rules of mechanics.

Transcript: Newton’s second law of motion mathematically relates a force to the change of motion that it causes. Newton’s law says that an object is accelerated when a force is applied to an object with an acceleration that is proportional to the force and in the direction of the force and inversely proportion to the mass: F equals M A. Imagine you push on a shopping cart. The mass is low, and so the inertia or resistance to a change in motion is low, and thus the shopping cart will suffer a large acceleration. If you apply the same force to a car with the brake off the mass and the resistance to motion is enormous, and the change in its motion, or acceleration, will be proportionately smaller.

Transcript: One of the things Newton is famous for is his understanding of the laws of mechanics and his development of three laws of motion. The first law is that an object continues in its present state of motion unless acted on by an external force. This is the concept of inertia. The second law is that an object is accelerated when an unbalanced force is applied to it, with acceleration proportional to the force and inversely proportional to the mass: F equals M A. The final law is for every force or action there is an equal and opposite force, reaction. These laws of motion are the basis of our understanding of how objects move on the Earth and how objects move in space.

Transcript: Newton’s first law of motion says that an object will stay at rest or in uniform motion unless an unbalanced force acts on it. Newton’s first law is a major departure from Greek physics which stated that rest was the natural state of an object. Newton realized that when an object subject to friction, such as a light thing falling through the Earth’s atmosphere or an object rolling on the surface, when such an object slowed down it was because it was subject to a force that decelerated it. Newton realized that in an ideal situation, no air resistance or no friction, an object in uniform motion would continue its uniform motion. Conversely, when an object changed its uniform motion, either a change of speed or a change of direction, this is because a force is acting. So when an object is in orbit, even though the circular velocity is constant the direction of motion is constantly changing. An object in an orbit is subject to the force of gravity. Similarly, other forces, electric or magnetic, could cause changes to uniform motion. In this way Newton produced an idea that unified large areas of knowledge.

Transcript: Perhaps the greatest scientist who ever lived, Isaac Newton was born just after the death of Galileo. Lonely and moody as a child, his early education was unremarkable, but when he went to university at Cambridge his true intelligence came forth. During a torrent of creativity between the years of the plague years when he was age 23 to 25 Newton developed the theory of calculus, the science of optics, developed an understanding of the properties of light, and the properties of gravity. He also perfected the design of the reflecting telescope. Of all his work Newton’s towering achievement is the universal theory of gravity. A difficult person relative to his peers and colleagues, Newton was humbled before nature. Near the end of his life he said, “If I have seen any further than others it is because I have stood on the shoulders of giants.” He also said, “I feel as if I have been turning over a few bright pebbles on the sea shore while the vast sea of knowledge lay undiscovered before me.”

Transcript: Newton’s work was central not only to the history of physics and astronomy but to the history of ideas of Europe in the last 400 years. Newton’s innovations in mechanics led to ways of harnessing energy and power in machines, and this within a few generations led to major inventions of the industrial revolution in England. Newton’s idea of gravity led to the notion of a clockwork universe where the law of gravity could be used to predict the motion of objects in space. This idea of determinism took root and even and received a reaction by romantic poets against it as squeezing out all humanism in the idea of modern science. Newton was a leader of the Royal Society, the first scientific society devoted to the free publication and open publication of scientific information, the basis of how modern science works. Newton was famous within his own lifetime. As master of the Royal Mint he was a big public figure in England, and he was the first scientist ever to be knighted. His ideas truly changed the way people thought.

Transcript: The idea of space exploration is a direct development from the ideas of Newton. Nearly 300 years before the invention of the rocket Newton had speculated as to how an object might be launched into Earth orbit. This is how he did it. In his book the Principia he imagined being positioned on top of a high mountaintop, high enough to be above the Earth’s atmosphere and so not subject to friction or air resistance. He imagined a cannon pointed sideways from the mountaintop. As the velocity of the projectile from the cannon is increased the cannonball falls in an arching trajectory. At a certain velocity the rate of fall of the projectile equals the rate of curvature of the Earth falling away. At this point you have created an orbit. The circular velocity of the orbit can be calculated directly from Newton’s law of gravity.

Transcript: Newton was the first person to develop a basic understanding of the nature of light. In a famous experiment, he admitted sunlight through a hole in the shutters in his bedroom and dispersed the light with a prism into the colors of the rainbow. But Newton wondered, were the colors fundamental or not? So he inserted a second prism on one of the colors and tried to spread those rays of light out even further but was unable to do so thereby proving that the individual colors could not be turned into something else by further dispersion. In the time of Newton the role of optics was not clear, and so people wondered if optics themselves could not introduce colors into light. To counter this Newton inverted a second prism and placed it behind the first prism and was able to recombine the colors into white light thereby showing that the colors were not created by the glass of the prism itself. In this elegant series of experiments, Newton was able to demonstrate that light is composed of a spectrum of colors and that the colors themselves represent a fundamental quantity.

Transcript: A basic concept in mechanics is momentum. Momentum is a measure of inertia and is proportional to velocity of an object. Momentum is calculated by multiplying the mass times the velocity. Momentum is conserved in any interaction between objects. We can see examples of this in a rocket at work where the forward momentum of the rocket represents a modest velocity of a large amount of mass, and that equals the huge velocity of the exhaust gasses representing a smaller amount of gas in the opposite direction. The momentum of each is equal and opposite. The recoil of a gun is a similar situation where the bullet, a small mass, travels forward at a very high velocity while the gun itself, a large mass, recoils backwards with a small velocity. In interactions momentum is always conserved.

Transcript: In the thirteenth century Aristotle’s work was rediscovered and merged with Christian thought by Thomas Aquinas in a synthesis that would define cultural world view of that period of European history. In the Aquinas view the world had a fixed and static social order in which humans were the pinnacle of creation, and the celestial sphere was the realm of God. The two were not related. Scientific inquiry of the heavens was not a high priority under this theology.

Transcript: Some of the most important ancient advances in astronomical knowledge came within Central and South America, in particular from the Mayan culture. Around the year 400 AD, when Europe was slipping into the dark ages, Mayan astronomy was at its peak. Mayan records recorded the phases of the moon, eclipses, motions of the planets, and they had an extremely accurate calendar. Mayan astronomy was well regulated and supported by the state. In the Mayan calendar the year starts on July 26th, and Venus, the brightest planet in the night or morning sky, is the most important astronomical object. Mayan astronomy and mythology presented a rich brew. Unfortunately, most of the evidence of this culture has been lost. In the 15th and 16h centuries when the Spanish conquerors invaded South and Central America, they destroyed most of the Mayan knowledge. One of the very few manuscripts that remains describing astronomical knowledge of the Mayans is the famous Dresden Codex.

Transcript: People use the words interchangeably, but there is a big difference between the concept of mass and the concept of weight. Mass is the amount of stuff in an object, or the number of atoms, measured in units of kilograms. Weight however is the response of an object to a gravitational field, and it depends on your location in space or in the universe. On the surface of the Earth the acceleration due to gravity is 9.8 meters per second per second, and that gives objects their weight on the surface of the Earth. On the surface of the moon for instance, the acceleration is only 1.6 meters per second per second, one-sixth that of on the Earth, so your weight on the moon would be one-sixth of your weight on the Earth. Your mass however would not change because the number of atoms in your body has not changed. If you are in orbit in a spacecraft you would appear to be weightless because in that situation both you and the spacecraft have forces acting on them due to gravity, but they are the same force. There is no difference, and so you are weightless. Once again, the mass in your body would not change. Mass is therefore a universal property depending on the number of atoms or the amount of stuff in an object, whereas the weight of an object depends on the local situation of gravitational force.

Transcript: Kepler deduced three laws of planetary motion that are applied to all the objects in the solar system. The first law says that the planets move in elliptical orbits with the sun at one focus. There is nothing at the other focus of the ellipse. We characterize an ellipse in terms of the semi-major axis. When the ellipticity, or amount by which the ellipse is squashed, goes to zero the ellipse turns into a circle and the semi-major axis into the radius. The second law says that in terms of the line between the planet and the sun planets sweep out equal areas in equal times. In an elliptical orbit this means that a planet moves faster when it is closer to the sun than when it is further away. An extreme version of this would be a comet that sweeps through the inner solar system rapidly and spends the large majority of its orbit in the distant reaches of space far from the sun. The third law of planetary motion relates the semi-major axis, or mean distance of a planet from the sun, to the orbital period and states that the cube of the semi-major axis is proportional to the square of the orbital period. This mathematical relationship accurately describes all the planets in the solar system.

Transcript: Johannes Kepler inherited Tycho Brahe’s meticulous observations and as a skilled mathematician knew exactly what to do with them. After eight years and thousands of pages of analysis he reluctantly concluded that the planets could not be explained with circular orbits but only by elliptical orbits. It was a reluctant conclusion because Kepler was in awe of the Greeks and was well aware that the sphere and the circle were the most perfect figures. However, he listened to what the data was saying to him, and he came up with the idea of elliptical orbits within the solar system. He also deduced the three fundamental laws of planetary motion. He even added the idea of a force that could somehow operate invisibly across the range of space that somehow bound the planets to the sun. Kepler was a mystic, and many of his writings do not sound scientific to a modern ear. But his work in making a decisive break with the circular orbits and the perfect spheres of the Greeks amounted to a second stage in the Copernican revolution.

Transcript: The key to Newton’s realization of the universal law of gravity was the understanding that gravity is an inverse square law. That is, the force of gravity diminishes with the square of the distance between two objects. Using this understanding, Newton was able to develop a formalism that showed why the planets should move in elliptical orbits with the sun at one focus. In fact, he made such a calculation and lost it on his desk for a period of some months before Edmond Halley, after whom the comet was named, visited him and persuaded him to write it up. Newton’s master work was the Principia, a summation of all his work on gravity published near the time of his death and one of the greatest books in science ever written.

Transcript: Galileo made other deductions based on the idea of inertia. He realized that an object dropped from the mast of a tall ship did not fall behind the ship, but its inertia gave it the forward motion of the ship and so it fell directly down the mast. By the same reasoning he realized that if the rotation of the Earth carried the atmosphere with it, we might not be aware of the rotation of the Earth even though most people are traveling at close to a thousand miles an hour as the Earth rotates. Similarly, Galileo argued that someone confined to the cabin of a ship traveling on smooth waters might be able to drop or throw or roll objects and be unaware of their motion. By such arguments Galileo was able to overcome one of the strongest objections to the heliocentric model, the fact that we do not feel ourselves moving through space.

Transcript: India was another center of ancient knowledge. The earliest astronomical practices in India date back to 1500 BC. Around 600 BC, before the golden age of Greek philosophy, astronomical texts from India could be found with motions of the planets and eclipse tables. Perhaps the greatest contribution of Indian scientists and mathematicians was the invention of zero, the concept of zero which is the basis of the decimal counting system. Unfortunately, a series of invasions of the Indian sub-continent in the 12th century destroyed much of the evidence of the golden age of Indian science.

Transcript: Hipparchus was a major astronomer of the second century BC. He had an observatory on the island of Rhodes. From him we have the first use of celestial coordinates and the first star catalog. He also invented the magnitude system for measuring the relative brightness of objects in the sky, and he was aware of the delicate and subtle motion of precession.

Transcript: Living just after the time of Aristotle, Aristarchus boldly proposed the heliocentric cosmology. In the heliocentric model the sun is stationary at the center of the solar system, and the Earth and the other planets and the stars move in circular orbits around it. Aristarchus used geometric reasoning to argue that the sun was larger than the Earth and the moon. Why in the history of ideas do the correct ideas not always prevail? Usually the answer is that there is no compelling evidence to support the hypothesis. Aristarchus had no way of proving that the Earth was in motion around the sun, and in fact people who supported the geocentric view argued strongly that the Earth was not moving because no motion was felt.

Transcript: Galileo was a giant of astronomy and perhaps the first physical scientist. A populist born in Italy, he used the newly discovered telescope to make observations that cemented the Copernican revolution. He made many experiments on the motions of objects. He invented the concepts of inertia, and mass, and acceleration. Although he did not invent the telescope, he used it to make profound observations in the sky from the craters of the moon, to the Milky Way turning into the pinpoint light of many stars, to the moons of Jupiter, to the phases of Venus, and the observation of sunspots. He wrote in Italian so that his work would be understood by the common people rather than using the scholarly language of Latin, and his observations in support of the Copernican world view brought him into direct and unfortunate confrontation with the Catholic Church.

Transcript: Galileo was a populist. He published in Italian so as to reach the widest possible audience. It was clear that Galileo’s strong support of the heliocentric model would throw him into conflict with the Catholic Church. The Catholic Church supported the geocentric picture and had folded it into the theology of the time. Galileo was called in front of Cardinal Bellarmine, the Pope’s right hand man, to explain why he was supporting the Copernican model as proved hypothesis when in fact the evidence was ambiguous. In this Galileo did overstate the case, but clearly he was supporting the right science. Galileo continued to propose the heliocentric model as being the truth and was supported by scientists around Europe, but in 1633, he was forced in front of the inquisition and had to recant his support of Copernican principle in writing. Thereafter, he spent the last years of his life under house arrest. He completed the last major work on mechanics but died a somewhat sad figure at the age of 78 having been made sunblind by careless observations of the sun over many years. However he ended his life, Galileo had changed the course of science forever with his firm observational support of the heliocentric model.

Transcript: Even at the time of Aristotle, the geocentric cosmology was becoming cumbersome. To explain the observed motions of the planets and the stars 55 crystalline spheres were needed. The reason is that the planets do not follow uniform motions in the sky. At the time of Ptolemy he used the device of an epicycle to explain the non-uniform motion. With an epicycle the planet orbits the Earth but is offset on its own circle within a circle. This epicyclic motion was able to explain the non-uniform angular motion of the planets at the expense of increasing complexity of the geocentric model.

Transcript: Holland was the center of the craft of glassmaking in Europe. Eyeglasses had been in use since the fourteenth century. Around the year 1600 both the telescope and the microscope were invented in Holland. Almost certainly the telescope was first invented by a Dutch optician called Hans Lippershey. Although he did not invent the telescope, Galileo used it for the first systematic observations of the sky. The things he discovered with his simple telescope, the mountains on the moon, the moons of Jupiter, sunspots, stars in the Milky Way galaxy, transformed our view of the heavens. Galileo’s first telescope only had a magnification of a factor of three. Even his best only had a magnification of ten and would be exceeded by a modest pair of binoculars today. However, because of the care he took with his observations, Galileo was able to convince other people of what he saw and of the reality of the Copernican model.

Transcript: In the geocentric cosmology of Aristotle the sun, the moon, the planets, and the stars were all carried about the Earth on crystalline spheres. The uniform motion was explained by each sphere being in a different motion centered on the Earth. The Greeks speculated about the nature of the crystalline spheres, but this was a substance unknown on Earth, a translucent substance on which were attached the celestial objects. Even though the geocentric cosmology was created to avoid having the Earth in motion, the outer most crystalline sphere that carried the stars was moving very fast in the Greek model, perhaps at one million miles per hour.

Transcript: The man responsible for the Copernican revolution was born in Poland. He was widely read, a master of Greek and Latin, a doctor of law, and a doctor of philosophy, and a doctor of medicine. This highly educated man was also a minor official in the Catholic Church. Copernicus adapted the heliocentric system, and he placed the sun at the center of a series of circular orbits of the Earth and all the other planets. Ironically, because the planets do not move in circular orbits of the sun, Copernicus’s basic system no more accurately predicted the motions of the planets than the Ptolemaic model. It was however revolutionary because it naturally explained the phenomenon of retrograde motion. In the Copernican model all the planets moved directly in the same motion and sense around the sun. It’s an ordered and simple model where the nearer planets to the sun move faster in their orbits. A necessary consequence of the Copernican model is that the stars must be extremely far away because no parallax is observed. Thus Copernicus’s model is revolutionary in two ways: one, because it displaces the Earth from the center of creation, and two, because it hypothesizes a vastly larger universe than the Greeks had considered.

Transcript: Very few books ever changed the world, but in 1543 Copernicus published such a book called On the Revolution of the Celestial Spheres. This book was the first presentation for many centuries of the idea of a heliocentric solar system, the sun at the center, and the Earth and all the other planets in motion around it. When we use the word revolution we tend to think of social and political upheaval and rapid change, but our use of this word to describe a social and political context actually dates back to and is referenced to Copernicus’s revolutionary book which changed our view of our place in space. With the Copernican revolution we were finally displaced from our centrality in the scheme of things. The Earth was just one celestial body moving amongst other celestial bodies with the sun at the center of the arrangement.

Transcript: Chinese astronomers were predicting eclipses over three thousand years ago. There is ample evidence that Chinese astronomy flourished before the great age of Greek philosophy and science. Chinese astronomers left detailed observations of the recurrence of Halley’s Comet, of fireballs in the sky, and of the appearance of guest stars which we know today as either novae or supernovae. Chinese astronomers left a continuous record serving in the court of the emperors for over two thousand years. There is even evidence that some Chinese astronomers were able to understand the fact that the Earth could be in motion without such motion being felt or apparent, something that the great Aristotle stumbled over. Chinese astronomy is a vital element in the scientific tradition, but China was isolated as a country, so much of this knowledge was retained within the country.

Transcript: During much of the dark ages and the medieval era the flame of science was carried by Arab astronomers and mathematicians. They refined the measurement of the Earth’s circumference. Arab opticians realized the way that the eye worked and developed advanced theories of optics. The Arabs made no cosmological models because the Qur’an forbids pictorial representation of the heavens. However many star names date from Arab times, and many of the terms for mathematical and chemical symbols and terminology are Arab words. By 1000 AD the great senders of learning in Europe had spread under the influence of the Islamic empire to Spain.

Transcript: Angular momentum is a concept related to linear momentum, only in this case the situation of an orbit. Any physical or mechanical object either in rotation or in an orbit can have its angular momentum calculated. The angular momentum is the product of the mass times the distance from the center of the system times the velocity. This product of these three quantities is a constant and is always conserved. This is the law of conservation of angular momentum. You can see this at work in the rotation of a skater when they go into their final spin. As the skater brings their arms and legs close to the center of their body their spin rate increases. What is happening is that their mass is being redistributed to be closer to the center of gravity, and as a result the velocity, or their spin rate, increases. Conservation of angular momentum can also be seen in the orbits within a solar system or a planetary system when the objects that are closer to the gravitating object of a star move faster in their orbits, or it can be seen in the individual motion of an object in an elliptical orbit. Comets for example move faster when they are close to the sun in their orbits and slower when they are furthest from the sun in their orbits.

Transcript: Accurate observations were decisive in discriminating between the heliocentric and geocentric cosmologies. Neither the heliocentric nor the geocentric cosmology produces an accurate representation of planetary motions because the planets are in elliptical, not circular, orbits and so display non-uniform motion on the plane of the sky. It was Tycho Brahe’s careful observations accumulated over several decades that were able to prove the non-uniform motions. Brahe’s observations had an accuracy of about one or two arcminutes, three or four times better than any previous observations. With Brahe’s observations, there could be no doubt that the planets showed non-uniform motion. Given that, the heliocentric model is a more elegant arrangement for the solar system because the planets all move in the same direction around the sun with the nearer planets moving faster and the more distant planets moving slower.

Transcript: Galileo did major work on mechanics, or the science of motion. Aristotle had taught that rest was the natural state of an object and that a heavier object would fall faster than a lighter one. Galileo was able to demonstrate that both of these ideas were wrong. Rest is not necessarily the natural state of an object. Galileo realized that friction, or two surfaces rubbing against each other, would slow down a rolling object. If friction could be minimized then an object would continue in a state of uniform motion. Galileo also did experiments of dropping lighter and heavier objects. The story that he did so from the leaning tower of Pisa is apocryphal, written up by one of his students after his death. However, he did use inclined planes in the laboratory to slow down the falling motion, and he timed the observations using his pulse. With such observations he was able to show that massive and less massive objects move at exactly the same rate, and each accelerate under the operation of gravity. Acceleration is a rate of change of motion caused by a force.