1	Chapter 3 The Science of Astronomy Chapter Outline Everyday Science Ancient Observations The Modern Lineage Modern Science and the Scientific Method
2	Synodic and Sidereal Periods Synodic period – time interval between successive occurrences of the same planetary configuration Sidereal period – time interval for 360^o revolution of planet about the Sun measured relative to the stars
3	Synodic and Sidereal Month Sidereal month - 360^o revolution measured relative background stars Approximately 27.3^d Synodic month - 386^o revolution measured relative to Earth/Sun line New moon to new moon Approximately 29.5^d
4	Planetary Motions Observed by the Ancients Babylonians and Egyptians observed planetary motions Sun, Moon, and 5 naked-eye planets, all wander among stars Significance of number 7 as in 7 days of the week Sun and Moon move eastward on celestial sphere Mostly uniform over synodic period Planets move mostly eastward on celestial sphere, but not uniformly over synodic period Brief period of retrograde motion near opposition for superior planets Planets move through different angles in same interval of time over synodic period Move swiftly some times and slowly at other times Planets display variations in brightness over synodic period
5	What geometrical relationships for planetary motion did the Greeks devise? They devised a “two-sphere” geometry between the terrestrial and the celestial spheres Several Greek natural philosophers contributed concepts to what would become known as the Ptolemaic System, which lasted over 1500 years as the reigning cosmology
6	Ptolemaic System Planet moves uniformly on small circle called epicycle Epicycle moves uniformly on the circumference of large circle called a deferent Appropriate sizes and rates of uniform motion on epicycle and deferent, produce mostly direct motion, occasional retrograde
7	The Ptolemaic System
8	Nicolas Copernicus (1473-1543) University of Krakow in Poland, studied Mathematics Philosophy Astronomy and astrology Universities of Bologna and Padua in Italy studied Law Medicine Elected Canon of Ecclesiastical Law by the Church 1543, published, On the Revolutions of the Heavenly Orbs
9	Copernican System Orbits perfect circles Earth and Moon move on circles Earth about Sun Moon about Earth Planets move on epicycles, which move on deferents (fewer epicycles than Ptolemaic system) Predictive accuracy no greater than Ptolemaic system, but no worse
10	Tycho Brahe (1546-1601) Extensive naked-eye observations Basis for acceptance of heliocentric cosmology Observations accurate to 1 minute of arc Note: 1 degree of arc = 60 minutes of arc = 3600 seconds of arc Published a cosmology Planets orbit Sun Sun and Moon orbit Earth
11	Johannes Kepler (1571-1630) Influenced by Pythagorean and "neo-Platonic" tradition Convinced mathematical relations existed that made sense of planetary motion Acquired and analyzed Brahe's observations of planets, primarily Mars 1609, New Astronomy, contains 1st and 2nd laws of planetary motion 1619, The Harmonies of the World, contains 3rd law Extended laws derived for Mars to all planets, regarded laws as universal Anticipated Newton's cause and effect concept
12	Kepler could not get the circular orbit model for Mars’ to agree with Brahe’s observations. “If I had believed that we could ignore these eight minutes [of arc], I would have patched up my hypothesis accordingly. But, since it was not permissible to ignore, those eight minutes pointed the road to a complete reformation in astronomy.”
13	What are Kepler’s “Laws of Planetary Motion?” Kepler’s laws are empirical laws (deduced from observations) describing planetary motion (kinematics)
14	Kepler’s Laws First Law: Planet moves on elliptic orbit around Sun Second Law: Imaginary line connecting any planet to Sun sweeps over equal areas in equal intervals of time Third Law: Square of planet's orbital period (sidereal) is proportional to cube of its mean distance from Sun (semi-major axis), P² µ a³
15	Ellipse
16	Galileo Galilei (1564-1642) Galileo brings together strands of medieval thought regarding space, time, and motion Devised telescope in 1609 Discoveries and observations with telescope Jupiter's four large satellites Craters and mountains on Moon Phases of Venus Milky Way composed of individual faint stars Observes sunspots (not new) 1609, published discoveries in Starry Messenger
17	What is science? Science is about making measurements and observations of the physical world Science is about the collection of facts, their organization, and dissemination Science is about formulating laws and theories articulating the physical world Science is how and what we humans think about the nature of our physical existence
18	What is it that scientists believe they are doing in the practice of science? They are identifying a “pre-existing order” in the Universe and then organizing that order into some logical representation of our experiences as human beings
19	Scientific Laws Hypotheses - propositions assumed to be true, but are not verifiable except through possibly falsification Scientific laws - rules, preferably mathematical, by which nature operates Empirical laws - general statement identifying regularity in observations with no cause-effect explanation Example - Kepler's laws Definitional laws - definition of fundamental concept Example - Newton's 2nd law of motion (F = ma) Derived laws - derived from some underlying theory Example - Newton's law of gravitation (derived from Kepler's laws, Newton's laws of motion, and concept of action-at-a-distance)
20	Scientific Theory A theory is a conceptual scheme which we invent or postulate in order to explain to ourselves, and to others, observed phenomena and the relationships between them. Thus the theory brings together in one structure the observations, concepts, hypotheses, principles, and laws from often very widely different fields.
21	The Big Picture The basic ingredients of scientific thinking—observation and trial-and-error testing—are a part of common experience. The modern scientific method provides a way of organizing this thinking. Each new piece of knowledge rests upon foundations of older discoveries. These foundations reach far back into history and are intertwined with the development of human culture. The Copernican revolution, which replaced the Ptolemaic universe, did not occur instantaneously. It unfolded over more than a century and involved observational, experimental, and theoretical work by many different people. Making conceptual models of nature lies at the heart of modern science. Models are created by generalizing from many specific facts. They then yield further predictions that allow the models to be tested.