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Astronomy Chapter 3 notes

by: Lisa Render

Astronomy Chapter 3 notes PHYS 1350

Marketplace > University of Nebraska at Omaha > Physics > PHYS 1350 > Astronomy Chapter 3 notes
Lisa Render
GPA 3.578

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3.1 The Ancient Roots of Science 3.2 Ancient Greek Science 3.3 The Copernican Revolution 3.4 The Nature of Science
Principles of Astronomy
Charles St Lucas
Class Notes
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This 6 page Class Notes was uploaded by Lisa Render on Wednesday October 5, 2016. The Class Notes belongs to PHYS 1350 at University of Nebraska at Omaha taught by Charles St Lucas in Fall 2016. Since its upload, it has received 3 views. For similar materials see Principles of Astronomy in Physics at University of Nebraska at Omaha.


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Date Created: 10/05/16
3.1 The Ancient Roots of Science Wednesday, September 7, 2016 12:56 PM In what ways do most humans use scientific thinking? • Scientific thinking is based on everydayideas of observationand trial-and-error experiments • Carefully organized, though How did astronomical observations benefit ancient societies? • In keeping track of time and seasons ○ For practical purposes, including agriculture ○ For religious and ceremonialpurposes ○ (sun dials, medicine wheels, Stonehenge) • Meridiem ○ Ante meridiem - AM ○ Post meridiem - PM How did astronomical observations benefit ancient societies? • In aiding navigation • Days of the week were named for the Sun, Moon, and visible planets ○ Sunday (sun-day) ○ Monday (moon-day) What did ancient civilizationsachieve in astronomy? • Daily timekeeping • Tracking the seasons and calendar • Monitoring lunar cycles • Monitoring planets and stars • Predicting eclipses • And more… • Conjunction - same location longitudinally as the as the Sun • Opposition - 180 degrees away in longitude along the Sun's path 3.2 Ancient Greek Science Wednesday, September 7, 2016 1:24 PM Why does modern science trace its roots to Greece? • (centrally located, Alexander the Great, Alexandria, Aristotle, Socrates) • Library of Alexandria ○ Half a million books/scrolls ○ Burned down • Greeks were the first people credited with making modelsof nature • They tried to explain patterns in nature without resorting to myth or the supernatural How did the Greeks explain planetary motion? • Underpinnings of the Greek geocentric model: ○ Earth at the center of the universe ○ Heavens must be "perfect"  Objects move on perfect spheres or in perfect circles • Eratosthenesmeasure Earth circumference240 BC ○ Calculated using trigonometry ○ 240 BC and they were only off by about 4%  Knew the earth wasn’t flat at least • The most sophisticated geocentric model was that of Ptolemy(AD 100-170)--thePtolemaicmodel ○ Sufficiently accurate to remain in use for 1500 years ○ Arabic translation of Ptolemy'swork named Almagest "the greatest compilation ○ Mathematician,astronomer,geographer, astrologer • Aristotle's CosmologicalSystem ○ 55 celestial spheres that fit inside each other ○ Earth at center, air, fire, moon,etc. • So how does the Ptolemaicmodel explain retrograde motion? ○ Planets really do go backward in this model ○ But this geocentric model made it difficult to explain to apparent retrograde motionof planets 3.3 The Copernican Revolution Wednesday, September 7, 2016 1:39 PM How did Copernicus, Tycho, and Kepler challenge the Earth-centered idea? • Nicolaus Copernicus ○ Born in 1473(around same time as Tesla) ○ Realized heliocentric solar system around ○ Aristarchus theorized about the heliocentric ○ Proposedthe sun-centered model (published in 1543) ○ He used the model to determine the layout of the solar system (planetary distances in AU (astronomicalunits (distance between the earth and the sun))) ○ The model was no more accurate than the Ptolemaicmodel in predicating planetary positions, because it still used perfect circles • Geocentricvs. heliocentric ○ Earth is stationary in the geocentric model but moves around the Sun in Sun-centered model ○ Retrograde motionis real (planets really go backward) in the geocentric model but only apparent (planets don’t really turn around) in the sun-centered model ○ Stellar parallax is expected in the Sun-centered model but not in the Earth-centered model ○ First models both didn’t work regardless of center • Tycho Brahe (1546-1601 ○ Brahe compiled the most accurate (1 arcminute) naked eye measurements ever made of planetary positions ○ He still could not detect stellar parallax and thus still thought Earth must be at the center of the solar systems(but recognized that other planets go around the sun) ○ He hired Kepler, who used Tycho's observationsto discover the truth about planetary motion • Johannes Kepler (1571-1630) ○ Works for Tycho ○ Kepler first tried to match Tycho's observationswith circular orbits ○ But an 8-arcminute discrepancy led him eventually to ellipses (NOT perfect circles) ○ One of the first professorsto believe Copernicus What are Kepler's three laws of planetary motion • 1st law: Planets orbit the sun in ellipticalpaths ○ Not perfect circles ○ Sun lies at one focus, nothing lies at the other focus • 2nd law: the line joining the sun and a planet sweeps through equal areas in equal times ○ Replaced the concept of planets moving at a constant rate to total area swept by planet at constant rate ○ Especially noticeable by comets  Heat up and speed up when they closer to the sun ○ Movesfaster when it's closer to the sun ○ Movesslowest when it's further from the sun ○ Equal areas • 3rd law: the square of the period of revolution is proportional to the cube of the planet's distance from the sun ○ Ergo, the further an object is from the body it is orbiting, the less of its orbit it covers per hour ○ 1 AU = distance from Earth to the Sun ○ More distant planets orbit the sun at slower average speeds, obeying the relationship    p= period of revolutionin years  a= average distance from sun  ○ Ex. An asteroid orbits the sun at an average distance a = 4au  It would take 8 years to orbit the sun ○ Comments  Kepler only described planetary motions □ In 1685,Isaac Newton explained them  Kepler's orbits fitted observations much better than those of Copernicus □ But not perfectly  Kepler's laws would be perfect if the Sun was the only body attracting planets □ Newton discovered that all planets also attract each other Galileo Galilei • Born February 15, 1564,Pisa, Italy • Died January 8, 1642, Acetri, Italy • Accomplishments ○ Discoveredphases of Venus ○ Provided the fatal blow to the Ptolemaicview of the universe  Wrote a book ○ Discoveredsunspots  Lookedat the sun through a telescope □ He died blind because of such ○ Identified craters on the moon ○ Individual stars in the Milky Way  Before that they thought it was just a big grey spot in the sky How did Galileo solidify the Copernican revolution? • Galileo overcamemajor objections to the Copernican view • Three key objections rooted in the Aristotelian view were the following: 1. Earth could not be moving because objects in air (birds, clouds, etc.) would be left behind  BUSTED--> Galileo's experimentsshowed that objects in air would stay with a moving Earth  Aristotle thought that all objects naturally come to rest at the center of the Universe  2. Heavenly perfection--Noncircularorbits  BUSTED--> Tycho's observationsof comet and supernova already challenged this idea of unchanging heavens  Using his telescope, Galileo saw □ Sunspots on the sun  Imperfections □ Mountains and valleys on the moon  Proving it is not a perfect sphere 3. Parallax  BUSTED-->Tycho thought his observed lack of stellar parallax ruled out an orbiting Earth  Galileo showed stars must be much farther than Tycho thought  • Galileo also saw four moonsorbiting Jupiter, proving that not all objects orbit Earth! • Galileo also observed the phases of Venus proving that it orbits the Sun and not the Earth • Galileo condemnedto Heresy ○ In 1633 the catholic church ordered Galileo to recant his claims that Earth orbits the Sun ○ His book on the subjects (the Starry messenger)was banned ○ Galileo was formerlyvindicated by the Church in 1992by pope John Paul II  Took quite some time  Took quite some time ○ Galileo's middle finger is preserved in a glass jar in Italy 3.4 The Nature of Science Friday, September 9, 2016 1:42 PM How can we distinguishscience from nonscience? • Defining science can be surprisingly difficult • Science comes from the Latin Scientia, meaning knowledge • But not all knowledge comes from science What is scientifictheory? • The idealized scientific method 1. Make observations 2. Ask a question 3. Suggest a hypothesis 4. Make a prediction 5. Performa test: experimentor additional observation • Sometimeswe star by "just looking" then coming up with possible explanations • Sometimeswe follow our intuition rather than a particular line of evidence • Modern sciences seeks explanations for observed phenomena that rely solely on natural causes • #2 maximal simplicity ○ Science progresses through the creation and testing of modelsin nature….. • 3 testable predictions ○ A scientific model must make testable predications • The world theory has a different meaning in science than in everydaylife • In science, a theory is NOT the same as a hypothesis


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