Chapter 1: Ancient Astronomy PDF

Chapter 1: Ancient Astronomy Physics 100 Lecture Note 1  What is Science? Science is a body of knowledge; a collection of unified insights about nature, the evidence for which is an array of facts. Why Study Science? An understanding of science can help you take better care of your he...

Chapter 1: Ancient Astronomy Physics 100 Lecture Note 1  What is Science? Science is a body of knowledge; a collection of unified insights about nature, the evidence for which is an array of facts. Why Study Science? An understanding of science can help you take better care of your health, be a wiser consumer, and become a better-informed citizen. Science can also be used to improve the world around you. Develop social values appropriate to the scientific age Physics 100 Lecture Note 2 What is science? The word Science derives from the Latin word Scientia which means knowledge Dictionary Definition: Intellectual and practical activity encompassing the systematic study of the structure and behavior of the physical and natural world through observation and Experiment. Systematic: means doing something or acting according to a fixed plan or system; Methodical Observation: the action or process of observing something or someone carefully or in order to gain information Experiment: a scientific procedure undertaken to make a discovery, test a hypothesis, or demonstrate a known fact. Physics 100 Lecture Note 3 What is science...? What type of knowledge is science? Organized Explains Capable of making predictions Testable What does scientific knowledge involve? Collection of information Analysis of such information in a logical way Puts forth explanations Makes predictions based on what is understood Includes subject areas: Astronomy, physics, chemistry, geology and biology, physiology, and more. Physics 100 Lecture Note 4 Science as a Body of Knowledge Science can also be defined as a way of learning; a process of coming to understand the natural world through observation and the testing of hypotheses. Physics 100 Lecture Note 5 Science as a Body of Knowledge Physics 100 Lecture Note 6 Physics Physics: From Ancient Greek phusike which means knowledge of nature. The second oldest branch of science (after Astronomy). Physics deals with HOW things happen? Exp: How can a car move It involves the study of matter and its motion through space and time. It is concerned with the general analysis of nature, conducted in order to ❖ understand how the universe behaves. Tries to explain properties of objects such as: Solids, liquids, gases, conduction of electricity, atoms and their structures, particles and the ultimate structure of matter. ❖Heat, Waves and radiation. ❖The universe in its large scale: Planets, stars and properties, galaxies, etc. Physics 100 Lecture Note 7 Observing the night sky ❑ Celestial objects: The sun The moon Stars Planets ❑ Celestial objects are in motion The sun takes a day to come around The moon takes shorter or longer? Planets Stars Physics 100 Lecture Note 8 Observing the Universe... consequences Consequences of Celestial object’s motion 1- Day and night Earth turns around its axis, an imaginary line that goes through its center. This motion is called rotation. Earth takes about 24 hours to rotate around its axis once. When a place on Earth faces the sun, it has daytime. When it faces away from the sun, it has a night 2- High tide and low tide The high and low tides are caused by the gravitational forces between the earth and the moon. Physics 100 Lecture Note 9 Observing the Universe... Consequences 3- Seasons: The earth is tilted (wonky) as it makes its yearly journey around the sun. The Earth's axis is tilted at an angle of 23.5 degrees. This means that the Earth is always "pointing" to one side as it goes around the Sun. The Earth takes 365 days to orbit the sun. Physics 100 Lecture Note 10 The moon orbits the Earth The Earth orbits the sun Physics 100 Lecture Note 11 Night and Day Night and Day Day Night Light Physics 100 Lecture Note 12 Unit 5E: Orbits The moon orbits the Earth It takes 28 days to orbit the Earth once Physics 100 Lecture Note 13 Orbits Orbits The Earth orbits the sun It takes 1 year to orbit the sun once Physics 100 Lecture Note 14 Aristarchus Aristarchus : lived between 310 and 230 BC (2345 and 2245 years ago) in Samos (a Greek island near Turkey) What Aristarchus findings: Aristarchus figured out how to measure the distances between the earth and the moon and between the earth and the sun. He also figured out sizes of the Sun and the Moon He found that the sun was much larger than the earth. Physics 100 Lecture Note 15 Aristarchus Aristarchus concluded The earth must revolve around the sun. The earth rotates about itself (to produce the day and the night) Aristarchus explanation was rejected mainly because people thought that the earth was too heavy to revolve around the sun Physics 100 Lecture Note 16 Ptolemy's theory Observations: The sun, moon, planets and stars all appear to revolve around earth. The Earth seems to be at the center of these motions. The earth seems to be stable and motionless (still) The sun takes a day (24 hours) The moon takes slightly longer than a day Planets take much longer than a day The stars take even longer (1 year) Theory (explanation): The earth is motionless at the center of the universe The sun, moon, planets and stars all revolves around the earth All move in circular orbits with unchanging speeds (uniform motion) Physics 100 Lecture Note 17 Ptolemy's theory Physics 100 Lecture Note 18 Ptolemy's theory Observation of retrograde motion Physics 100 Lecture Note 19  Observation of retrograde motion Observations Retrograde motion means the backward motion. Planets were observed to be move backwards occasionally. During a planet's retrograde motion, the planet appears brighter than other times as though it was closer to us This change in the brightness of planets coincided with the time when the planet is making a retrograde motion Ptolemy's explanation How could this be explained, if the planets move in circular orbits? Ptolemy's explanation: ▪ Both of these observations can be explained if each planet moves in a circle within a circle. Physics 100 Lecture Note 20 Planets’ retrograde motion appeared brighter than other times Deferent – inner circle centered on Earth Epicycle – small outer circle Physics 100 Lecture Note 21 Summary of Ptolemy's modified theory Physics 100 Lecture Note 22 Ptolemy's theory Ptolemy’s Earth-centered Epicycle Theory (100 CE) – requires more than 80 epicycles to agree with observations. Hypothesis is tentative scientific idea, Theory is confirmed by evidence. Physics 100 Lecture Note 23  Summary of Ptolemy's theory The earth is rounded and motionless and is located at the center of the Universe. The sun, the moon, the planets, and the stars all revolve around the Earth The moon is closest to the earth and the stars are farthest According to this theory, everything in the universe revolves around the earth. The theory explains retrograde motion in terms of double circular (epicyclic) motion This theory is said to be earth-centric, which means that the earth is at the center of the universe. Physics 100 Lecture Note 24 Rounded Earth (aside): How did the ancient people found out that the earth was rounded? How ships disappear when they leave the port The shadow of the earth on moon The elevation of the summer sun in the sky decreases when one travels northerly. Physics 100 Lecture Note 25 Rounded Earth (aside): The universe according to the Ancient Greeks The universe was composed of four substances Earth: Solid matter with the highest density Water: Liquid substance with less density than Earth Air: Gaseous substance with much less density than water Fire: The rarest (least dense) substance Planets and stars were made of the least dense of these substances (i.e., Fire) These substances arranged themselves according to their Density: The substance with the highest density (Earth) fills the center of the universe The substance with next highest density (Water), filling around the earth... This kind of understand was in the background of Ptolemy's thinking. This why ancient Greeks thought the earth to be special and that planets and stars stayed away from the center of the universe. Physics 100 Lecture Note 26  Better Understanding of The Universe Consequences of changing understanding With time people learn more about the universe... The planets were very solids that were like the earth The sun is much bigger than the earth Stars were like the sun, but were only too far away Some planets were much bigger than the earth With this change in how we understand what celestial objects are made of, there were no reason to place the earth at the center of the universe. Also, Ptolemy's theory keep getting more complicated with time as a result of retrograde motion. Physics 100 Lecture Note 27 Copernicus' Theory This made Copernicus wonder: Whether there could perhaps be found a more reasonable arrangement of circles, from which everything would move uniformly about its proper center, as the rule of absolute motion requires. Physics 100 Lecture Note 28 Copernicus' theory: A sun-centered universe... Copernicus motivation: The earth is no longer unique or special (there bigger things out there) He finds Ptolemy's theory complicated. He asks if the motion of celestial objects can be explained in a simpler way. Copernicus likes the simplicity of uniform circular motion. Copernicus theory: The sun is at the center of the universe, motionless The earth, other planets and stars all move around the sun The earth revolves around itself (day and night) The moon goes around the earth These things move in uniform circular motion Physics 100 Lecture Note 29 Graphical summary of Copernicus theory Physics 100 Lecture Note 30 Copernicus theory 1543– Nicolaus Copernicus, On the Revolutions of the Heavenly Spheres Egocentrism refers to a cosmological and planetary theory, in which the Earth occupies the central position of the world system Ptolemy’s Theory is complicated Physics 100 Lecture Note 31 Copernicus' Explanation of the retrograde motion Physics 100 Lecture Note 32 Copernicus' Explanation for the Retrograde Motion Physics 100 Lecture Note 33 Physics 100 Lecture Note 34 Tycho Brahe's observations (1546-1601 AD) ▪ Built an accurate measuring devices ▪ Was able to repeatedly measure the locations of planets ▪ Used the device shown on the right to make accurate measurements of planetary locations Physics 100 Lecture Note 35 Tycho Brahe's observations ▪ When he began his project, there were two competing theories of the universe. Despite their great dissimilarity, both theories agreed with the data. ▪ Tycho Brahe measured the locations of the planets and stars with enough accuracy to determine whether the Ptolemaic or Copernican system was more valid in describing the heavens. ▪ For about 2o years, Brahe collected accurate data on the positions on the positions of the sun, moon, and planets. ▪ And soon afterward, it became obvious that both theories disagreed with Brahe’s observations! Physics 100 Lecture Note 36  Kepler & Copernicus' theory ▪ A student of Tycho Brahe ▪ Accepted Copernicus’ theory ▪ Compares Tycho Brahe’s measurements against the predictions of Copernicus’s theory ▪ No success for 16 years of trying ▪ Gets a new idea: orbits are elliptical and things fall in place ▪ Concludes: Planets move in ellipses with the sun at a focus Physics 100 Lecture Note 37 Physics 100 Lecture Note 38  The Solar System Physics 100 Lecture Note 39  The Solar System vs. The Universe ▪ Our solar system consists of the sun and 9 planets, namely, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus & Neptune. ▪ Stars are not part of our solar system. They are other far away suns of their own. ▪ Each of these stars can possible be a solar system of its own. ▪ Some of these stars are much larger than our sun. Hence, our sun is not that special and our solar system isn’t that special and there is no reason to assume that were are located at the center of the universe. Physics 100 Lecture Note 40  Summary Ptolemy Copernicus Tycho Brahe Kepler Earth is at the Sun is at the center Built a pointing Uses Brahe’s center of universe Earth moves around device accurate Earth is the suns Makes accurate measurements motionless The earth spins measurements of Make a Planets orbit the about itself planetary comparison earth The moon orbit the positions with The moon orbits earth Does not fully Copernicus's the earth Orbits are circles accept Copernican theory The sun orbits the Speeds are theory Finds earth unchanging disagreements Orbits are circular Retrograde is an Introduces Speed are illusion elliptical orbits constant And variable Retrograde speeds to make motion is real agreement Physics 100 Lecture Note 41  Conclusions ▪ In this chapter we studied two theories that try to explain the same thing: The structure of the universe. ▪ Ptolemy’s theory placed the earth at the center whereas Copernicus placed the sun at the center. This is their most essential disagreement. ▪ Both theories are capable of explaining retrograde motion. ▪ Both theories assume planets move in circular orbits. ▪ Tycho Brahe made very accurate measurements of planet locations. Both of Ptolemy’s and Kepler’s theories disagreed with Tycho Brahe’s measurements. ▪ At this time though, Copernicus’s theory was becoming more popular than Ptolemy’s. ▪ Brahe’s student, Johannes Kepler, found that if one assumed that planets moved in elliptical orbits, Brahe’s data and Copernicus’s theory can be reconciled Physics 100 Lecture Note 42 GLOSSARY Physics 100 Lecture Note 43