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  1. Announcements • Exam 1 is one week from today. It will cover all of the Five Ages of the Universe and Chapter 1 & 2 from Foundations of Modern Cosmology. Probably 15 multiple choice questions worth 3 points each and 3 essay questions worth 18 points each. I’ll give you five essays to choose from. There will be questions about the cosmological myths that you are presenting on today. • Your presentations first then I’ll start Chapter 2.

  2. Chapter 2: Cosmology Becomes a Science

  3. To the early Greeks, the universe was small and centered on the Earth They observed that all the planets moved in a band near the path of the Sun: the Ecliptic. The earliest models kept the planets on the ecliptic. Later models had the planets moving slightly off the ecliptic.

  4. The Greeks: Plato • 427-347 BCE • More philosopher than astronomer • Codified the requirement of circular motions in the heavens • Theory is truth, observations are subject to human frailties

  5. The Greeks: Eudoxus • 400 – 347 BCE • Student of Plato • First to use nested spheres to explain planetary motions • Took 27 spheres to explain observed motions

  6. The Greeks: Aristotle • 384 – 322 BCE • Like Plato, his teacher, more philosopher than astronomer • Developed a cosmology similar to Eudoxus but with 55 spheres • Codified “Physics”Terrestrial realm obeys different physical laws than celestial realm and is made of different stuff: earth, air, fire & water versus quintessence

  7. Despite being wrong, most people “believe” the physics of Aristotle The arrow is given an impetus by the bow. The impetus provides the force as it flies through the air but it is used up as the arrow pushes the air out of the way “All motion is either Natural motion or Forced motion. Falling is a natural motion. All bodies fall at a constant speed. The speed of a falling body is proportional to its mass”

  8. Aristarchus: ahead of his time 310 – 230 BCE Most of his writings are lost Measured the Earth-Moon distance (not too far off) and the Earth-Sun distance (way off) along with the relative sizes of the Moon and Sun. Found that the Sun is much larger than the Earth so he proposed the Earth went around the sun

  9. Hipparchus 190 – 120 BCE Proposed a geocentric model with an epicycle and eccentric. The distance from Earth to the center was called the eccentric.

  10. Claudius Ptolemy: an Alexandrian • 87 – 150 AD • Put finishing touches on geocentric model of the heavens • Complete mathematical model for calculating planetary positions at any time into the future (almost) • Wrote an extensive text on astronomy, astrology and mathematics now called the Almagest

  11. Ptolemy placed the Earth at the center (almost) center The Equant was equidistant from the center of the deferent and on the opposite side of the center from the Earth.

  12. With Ptolemy’s system, the retrograde motion of a planet could be explained

  13. The Ptolemaic Model of the Universe Each planet had its own eccentric so the center of the deferent was different for each planet. The size of the deferent , the size of the epicycle and the speed of the planet on each could be adjusted to match the observed motion of the planet fairly closely.

  14. Using geometry and recent observations, planetary positions could be recalculated

  15. Nicholas Copernicus • 1473 – 1543 • The Sun is the center • Mathematical model no better than Ptolemy’s • Model had predictive power • Watch video from Museo Galileo on the Copernican System

  16. The Copernican Universe The Sun is at the center of the solar system and Earth is just another planet going around it. The only thing that went around Earth was the Moon.

  17. Copernicus publishes his book in the year he dies De RevolutionibusOrbiumCoelestium is published in 1543 and the first printing is delivered to him on his death bead. He had delayed publication for fear of excommunication.

  18. Tycho Brahe: last great naked-eye astronomer • 1546 – 1601 • Made detailed observations of Mars • Observed a “nova stella”

  19. Tycho’s Uraniborg and Stjerneborg

  20. Tycho’s Mural Quadrant Tycho directs while his apprentices make the observation, read the scale and record the measurements. Others work elsewhere on calculations and instrument making

  21. Johannes Kepler • 1571 – 1630 • Hired by Tycho Brahe in 1600 to do calculations on the orbit of Mars • Appointed Imperial Astronomer after Tycho’s death