Transcript Document
Welcome to Starry Monday at Otterbein Astronomy Lecture Series -every first Monday of the monthMay 5, 2007 Dr. Uwe Trittmann Today’s Topics • From Galileo to Newton - The Birth of Modern Astronomy, Part II • The Night Sky in May On the Web • To learn more about astronomy and physics at Otterbein, please visit – http://www.otterbein.edu/dept/PHYS/weitkamp.a sp (Observatory) – http://www.otterbein.edu/dept/PHYS/ (Physics Dept.) From Galileo to Newton - the Birth of Modern Astronomy, Part II 1610 1687 Sunspots • MPEG video from Galileo Project (June 2 – July 8, 1613) Saturn • Sketch of 1616 • Engraving in “The Assayer” (1623) Galileo and his Contemporaries • • • • • • • • • • Elizabeth I. (1533-1603) – Queen of England Tycho Brahe (1546-1601) – Danish Astronomer Francis Bacon (1561-1626) – English Philosopher Shakespeare (1564- 1616) – Poet & Playwright Galileo Galilei (1564-1642) – Italian PAM Johannes Kepler (1571-1630) – German PAM Rene Descartes (1596 - 1650) – French PPM Christiaan Huygens (1629-1695) – Dutch PAM Isaac Newton (1643-1727) – English PM Louis XIV (1638-1715) – French “Sun King” Epochs • Renaissance: 1450-1600 – “Rebirth”, back to the roots • Baroque: 1600-1715 – Epoch of the religious wars – Later: Louis XIV and Newton • Rococo: 1715-1775 Tycho Brahe Johannes Kepler Galileo Galilei Observations Data Experiment test predictions Phenomenology/Theory Predictions The Scientific Method • Systematized by Francis Bacon, Descartes and Galileo in the 17th century • Not the only way of knowing, but a very successful one • A method to yield conclusions that are independent of the individual • Conclusions are based on observation Francis Bacon (1561-1626) Contemporary of Shakespeare, Elizabeth I, Kepler and Galilei Rejects Aristotelianism and Scholasticism Major Work: • Novum Organum (1620) (“New Tools”, the old Organum was Aristotle’s) Francis Bacon (1561-1626) Novum Organum Title: Allegory on the daring mind: A Ship passes through the “pillars of Hercules” (the straight of Gibraltar, the “end of the world”), beyond old knowledge. Rene Descartes – The Rationalist Described the method to do science, known for his mind-body dualism Major Works: • Discourse (1637) [full title: Discours de la méthode pour bien conduire sa raison et chercher la vérité dans les sciences; Discourse on the Method of Rightly Conducting the Reason, and Seeking Truth in the Sciences] • Meditations on first Philosophy (1641) [6 Meditations: Of the Things that we may doubt; Of the Nature of the Human Mind; Of God: that He exists; Of Truth and Error; Of the Essence of Material Things; Of the Existence of Material Things; Of the Real Distinction between the Mind and the Body of Man] Rene Descartes (1596-1650) Rene Descartes’ Discourse Describes the method to do science in a straightforward way (see below) Major points: • Science must be based on correct reasoning (logic) • Science must be formulated in mathematical language Starting line: “Good sense is the most evenly distributed thing in the world, for all people suppose themselves so well provided with it that even those who are the most difficult to satisfy in every other respect never seem to desire more than they have.” A Classical Example • Aristotle observes that during lunar eclipses the Earth’s shadow on the moon is curved • He assumes it will be curved for all eclipses • A hypothesis that explains this: the earth is round • A prediction of this theory is that the location of the stars in the sky should be different for observers at different latitudes • This is confirmed by additional observations – E.g. Canopus is visible in Egypt but not further north Scientific Theories • Must be falsifiable – There must be some way the theory could fail • Should make predictions – The more, the better! • Theories that are very well tested and have the widest applicability are often known as “laws of nature” • Always subject to revision or modification • Occam’s razor: the simplest theory wins Common Misconceptions • Theories can be proven – Fact: they cannot! If an experiment agrees 10 billion times with predictions, maybe the next experiment disagrees – However: one disagreeing experiment can falsify the theory – Most probably, the sun will rise tomorrow – but no guarantees! Common Misconceptions • Theories are just “theories”, i.e. hypotheses – Fact: scientists call a hypothesis a theory if it is very well tested, e.g. Einstein’s theory of Relativity, Darwin’s theory of evolution – There may be hundreds of thousands of reviewed publications that independently lend evidence to the theory’s correctness Common Misconceptions • Scientists will defend their theories against any new and/or unconventional approach – Fact: scientists love new ideas that explain everything known to date – and make new predictions – However, since so much is known, it is very unlikely that someone comes up with a better description of Nature than the previous, well-tested one – Unless the new theory is simpler, or makes correct predictions at odds with the old theory, it is quite efficient to stay with the old theory Common Misconceptions • Scientists want to explain everything – Fact: scientists want to come up with an accurate description of Nature, i.e. of how the fundamental entities in the universe interact – Naturally, science cannot (and does not want to) say anything about topics like God, Religion, Freedom, Ethics, Art, etc. Common Misconceptions • Scientists can predict everything that will happen in the Universe (Laplace’s Demon) • Fact: even in principle (if you would know the position and position of every particle) this is not possible because of – Quantum Mechanics: the laws of QM are inherently stochastic; QM predicts the probability of what might happen, not what will actually happen – Possibility of chaotic behavior: very small differences in the initial conditions can have vastly different effects (a butterfly’s wing movements can stir up a hurricane, in principle) The New Physics & Astronomy in a Nutshell: Newton’s Principia • Newton’s key question: Why are things happening? • Invented calculus and physics while on vacation from college • His three Laws of Motion, together with the Law of Universal Gravitation, explain all of Kepler’s Laws (and more!) • Principia (1687) [Full title: Philosophiae naturalis principia mathematica] has his famous three laws on page 19 of 443. Isaac Newton (1642–1727) Newton’s Laws of Motion 1. Every body continues in a state of rest or in a state of uniform motion in a straight line unless it is compelled to change that state by forces acting on it (law of inertia) 2. The change of motion is proportional to the motive force impressed (i.e. if the mass is constant, F = ma) 3. For every action, there is an equal and opposite reaction (That’s where forces come from!) Newton’s Laws Always the same constant pull a) No force: particle at rest b) Force: particle starts moving c) Two forces: particle changes movement Gravity pulls baseball back to earth by continuously changing its velocity (and thereby its position) Law of Universal Gravitation Mman MEarth R Force = G Mearth Mman / R2 Orbital Motion Cannon “Thought Experiment” • http://www.phys.virginia.edu/classes/109N/more_stuff/Appl ets/newt/newtmtn.html Applications • From the distance r between two bodies and the gravitational acceleration a of one of the bodies, we can compute the mass M of the other F = ma = G Mm/r2 (m cancels out) – From the weight of objects (i.e., the force of gravity) near the surface of the Earth, and known radius of Earth RE = 6.4103 km, we find ME = 61024 kg – Your weight on another planet is F = m GM/r2 • E.g., on the Moon your weight would be 1/6 of what it is on Earth Applications (cont’d) • The mass of the Sun can be deduced from the orbital velocity of the planets: MS = rOrbitvOrbit2/G = 21030 kg – actually, Sun and planets orbit their common center of mass • Orbital mechanics. A body in an elliptical orbit cannot escape the mass it's orbiting unless something increases its velocity to a certain value called the escape velocity – Escape velocity from Earth's surface is about 25,000 mph (7 mi/sec) Objections to the Heliocentric Model Answered • If the Earth is moving, why do dropped objects appear to fall straight down? – Dropped objects start with the velocity of Earth (Galileo) • If the Earth rotates, why don't we get thrown off? – Earth's rotation isn't fast enough! • If the Earth revolves around the Sun, why don't we observe stellar parallax? – It's there, but very small, because the stars are so far away (Aristarchus) • Why don't we feel the wind of our motion? – The air moves along with the Earth Problems of Both Models • Lack of a fundamental explanation? – Provided by Newton (but what explains Newton?!) • Lack of direct evidence? – Proof that the Earth rotates: • Coriolis force (hurricanes are counterclockwise in the Northern Hemisphere) • Foucault pendulum – Proof that earth and other planets revolve around the sun: • Aberration of starlight observed 1729 • Stellar parallax observed 1838 • Phases of Venus (Galileo) The Night Sky in May • Nights still long, but EDT => later observing! • Spring constellations are up: Cancer, Leo, Big Dipper • Saturn dominates the evening, Jupiter early morning. Moon Phases • Today: Waning Gibbous Moon • 5 / 10 (Last quarter Moon) • 5 / 16 (New Moon) • 5 / 23 (First Quarter Moon) • 5 / 31 (Full Moon) Today at Noon Sun at meridian, i.e. exactly south 10 PM Typical observing hour, early May Jupiter Saturn Zenith Big Dipper points to the north pole South • Saturn near Praesepe (M44), an open star cluster South • Spring constellations: – Leo – Hydra – Crater – Sextans East • Canes Venatici: – M51 • ComaVirgo Cluster • Globular Star Clusters – M3, M5 East Virgo and Coma with the VirgoComa galaxy cluster VirgoComa Cluster • Lots of galaxies within a few degrees M87, M88 and M91 East – Hercules – Corona Borealis – Bootes Globular Star Clusters: •M3 • M 13 • M 92 M13: Globular Cluster Mark your Calendars! • Next Starry Monday: October 1, 2007, 7 (!!!) pm (this is a Monday • Observing at Prairie Oaks Metro Park: – Friday, May 25, 2007, 9:00 pm • Web pages: – http://www.otterbein.edu/dept/PHYS/weitkamp.asp (Obs.) – http://www.otterbein.edu/dept/PHYS/ (Physics Dept.) ) Mark your Calendars II • • • • Physics Coffee is every Wednesday, 3:30 pm Open to the public, everyone welcome! Location: across the hall, Science 256 Free coffee, cookies, etc.