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Chapter 2

Chapter 2. Grab your textbooks. Origins of Modern Astronomy. Chapter 2 The Copernican Revolution. Units of Chapter 2. 2.1 Ancient Astronomy 2.2 The Geocentric Universe 2.3 The Heliocentric Model of the Solar System The Foundations of the Copernican Revolution

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Chapter 2

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  1. Chapter 2 Grab your textbooks

  2. Origins of Modern Astronomy

  3. Chapter 2 The Copernican Revolution

  4. Units of Chapter 2 2.1 Ancient Astronomy 2.2 The Geocentric Universe 2.3 The Heliocentric Model of the Solar System The Foundations of the Copernican Revolution 2.4 The Birth of Modern Astronomy

  5. Units of Chapter 2, continued 2.5 The Laws of Planetary Motion Some Properties of Planetary Orbits 2.6 The Dimensions of the Solar System 2.7 Newton’s Laws 2.8 Newtonian Mechanics Weighing the Sun

  6. White Board Question • What are some ancient structures that people used the sun and stars to help keep track of time and seasons?

  7. 2.1 Ancient Astronomy • Ancient civilizations observed the skies • Many built structures to mark astronomical events Summer solstice sunrise at Stonehenge:

  8. 2.1 Ancient Astronomy Spokes of the Big Horn Medicine Wheel are aligned with the rising and setting of the Sun and other stars

  9. 2.1 Ancient Astronomy This temple at Caracol, in Mexico, has many windows that are aligned with astronomical events

  10. Debunking the mythDec. 21, 2012 • End of an era, beginning of a new • http://www.foxnews.com/science/2012/10/01/experts-meet-to-debunk-mayan-calendar-end-world-stories/ • The Classic Maya had almost no tradition of cataclysmic endings For them, 2012 is just a year when several of their calendars reset, like 2000 for modern calendars.

  11. Mayans classic period lasted from 250 to 900 AD • the Long Count was linear rather than cyclical, and kept time roughly in units of 20: 20 days made a uinal, 18 uinals (360 days) made a tun, 20 tuns made a k'atun, and 20 k'atuns (144,000 days or roughly 394 years) made up a b'ak'tun.

  12. What Happened On December 21, 2012? • December 21 is the winter solstice, and in 2012 the Sun on the solstice was almost perfectly aligned with the plane of the galaxy (the Milky Way galaxy).

  13. Planetary alignments? • There are no planetary alignments in the next few decades, Earth will not cross the galactic plane in 2012, and even if these alignments were to occur, their effects on the Earth would be negligible. Each December the Earth and sun align with the approximate center of the Milky Way Galaxy but that is an annual event of no consequence.

  14. NASA explains • http://www.nasa.gov/topics/earth/features/2012-alignment.html

  15. It is really not that special • Every year on the winter solstice, our Sun has a Declination of -23.5 degrees, and a Right Ascension of 18 hours. • On December 21, 2012, the alignment was right along the plane of the entire galaxy. • Precession goes in a complete circle and happens only once every 26,000 years. The winter solstice moves 360 degrees every 26,000 years, or 0.01 degrees each year. • it takes the winter solstice between 700 hundred and 1,400 years to cross the plane of the galaxy! So 2012 was just one year amidst a span of 700 years.

  16. 2.2 The Geocentric Universe Earth based Ancient astronomers observed: Sun Moon Stars Five planets: Mercury, Venus, Mars, Jupiter, Saturn

  17. 2.2 The Geocentric Universe Sun, Moon, and stars all have simple movements in the sky • Planets: • Move with respect to fixed stars • Change in brightness • Change speed • Undergo retrograde motion

  18. Star movement in the sky http://physics.weber.edu/schroeder/ua/StarMotion.html http://www.youtube.com/watch?v=z6AgLOmxdww http://www.youtube.com/watch?v=z6AgLOmxdww

  19. Golden Age600 BC-AD 150Aristotle • Greek philosopher (384-322 BC) • Earth is round • Casts a curved shadow when passes between sun and moon His belief was abandoned during the Middle Ages

  20. Eratosthenes276-194 BC • First successful attempt to figure out the size of the earth Hipparchus 2nd Century BC • Divided stars into six groups according to brightness • Method for predicting lunar eclipses

  21. 2.2 The Geocentric Universe • Inferior planets: Mercury, Venus • Superior planets: Mars, Jupiter, Saturn Now know: Inferior planets have orbits closer to Sun than Earth’s Superior planets’ orbits are farther away

  22. 2.2 The Geocentric Universe • Early observations: • Inferior planets never too far from Sun • Superior planets not tied to Sun; exhibit retrograde motion

  23. Geocentric model • Moon, sun and planets orbit the earth • Celestial sphere (stars) orbits the earth • Incorrect

  24. 2.2 The Geocentric Universe Earliest models had Earth at center of solar system Needed lots of complications to accurately track planetary motions

  25. Carl Sagan on epicycles http://www.dailymotion.com/video/xerwsh_carl-sagan-videos-epicycles-of-ptol_tech

  26. White board question • On your white board make a Venn Diagram comparing Heliocentric and Geocentric views of the universe.

  27. Heliocentric vs. Geocentric Model • First heliocentric astronomer--Aristarchus (Greek 312-230 BC) • Earth and other planets orbit the sun

  28. 2.3 The Heliocentric Model of the Solar System Sun is at center of solar system. Only Moon orbits around Earth; planets orbit around Sun. This figure shows retrograde motion of Mars.

  29. Understanding retrograde motion • http://www.youtube.com/watch?v= • http://astro.unl.edu/classaction/animations/renaissance/retrograde.html • Interactive • http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::800::600::/sites/dl/free/0072482621/78780/Retro_Nav.swf::Retrograde%20Motion

  30. Discovery 2-1: The Foundations of the Copernican Revolution • Earth is not at the center of everything. • Center of Earth is the center of Moon’s orbit. • All planets revolve around the Sun. • The stars are very much farther away than the Sun. • The apparent movement of the stars around the Earth is due to the Earth’s rotation. • The apparent movement of the Sun around the Earth is due to the Earth’s rotation. • Retrograde motion of planets is due to Earth’s motion around the Sun.

  31. 2.4 The Birth of Modern Astronomy • Telescope invented around 1600 • Galileo built his own, made observations: • Moon has mountains and valleys • Sun has sunspots, and rotates • Jupiter has moons (shown) • Venus has phases

  32. 2.4 The Birth of Modern Astronomy Phases of Venus cannot be explained by geocentric model

  33. 2.5 The Laws of Planetary Motion Kepler’s laws were derived using observations made by Tycho Brahe

  34. 2.5 The Laws of Planetary Motion 1. Planetary orbits are ellipses, Sun at one focus

  35. 2.5 The Laws of Planetary Motion 2. Imaginary line connecting Sun and planet sweeps out equal areas in equal times

  36. 2.5 The Laws of Planetary Motion 3. Square of period of planet’s orbital motion is proportional to cube of semimajor axis

  37. More Precisely 2-1: Some Properties of Planetary Orbits Semimajor axis and eccentricity of orbit completely describe it Perihelion: closest approach to Sun Aphelion: farthest distance from Sun

  38. 2.6 The Dimensions of the Solar System Astronomical unit: mean distance from Earth to Sun First measured during transits of Mercury and Venus, using triangulation

  39. 2.6 The Dimensions of the Solar System Now measured using radar: Ratio of mean radius of Venus’s orbit to that of Earth is very well known

  40. 2.7 Newton’s Laws Newton’s laws of motion explain how objects interact with the world and with each other.

  41. 2.7 Newton’s Laws Newton’s first law: An object at rest will remain at rest, and an object moving in a straight line at constant speed will not change its motion, unless an external force acts on it.

  42. 2.7 Newton’s Laws Newton’s second law: When a force is exerted on an object, its acceleration is inversely proportional to its mass: a = F/m Newton’s third law: When object A exerts a force on object B, object B exerts an equal and opposite force on object A.

  43. 2.7 Newton’s Laws Gravity On the Earth’s surface, acceleration of gravity is approximately constant, and directed toward the center of Earth

  44. 2.7 Newton’s Laws Gravity For two massive objects, gravitational force is proportional to the product of their masses divided by the square of the distance between them

  45. 2.7 Newton’s Laws Gravity The constant G is called the gravitational constant; it is measured experimentally and found to be G = 6.67 x 10-11 N m2/kg2

  46. 2.8 Newtonian Mechanics Kepler’s laws are a consequence of Newton’s laws; first law needs to be modified: The orbit of a planet around the Sun is an ellipse, with the center of mass of the planet–Sun system at one focus.

  47. More Precisely 2-3: Weighing the Sun Newtonian mechanics tells us that the force keeping the planets in orbit around the Sun is the gravitational force due to the masses of the planet and Sun. This allows us to calculate the mass of the Sun, knowing the orbit of the Earth: M = rv2/G The result is M = 2.0 x 1030 kg (!)

  48. 2.8 Newtonian Mechanics Escape speed: the speed necessary for a projectile to completely escape a planet’s gravitational field. With a lesser speed, the projectile either returns to the planet or stays in orbit.

  49. Summary of Chapter 2 • First models of solar system were geocentric but couldn't easily explain retrograde motion • Heliocentric model does; also explains brightness variations • Galileo's observations supported heliocentric model • Kepler found three empirical laws of planetary motion from observations

  50. Summary of Chapter 2 (cont.) • Laws of Newtonian mechanics explained Kepler’s observations • Gravitational force between two masses is proportional to the product of the masses, divided by the square of the distance between them

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