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AST 111 Lecture 3

AST 111 Lecture 3. Recent History of Astronomy. The Geocentric Model. IT IS WRONG!. Fine. So what is it?. The Geocentric Model places Earth at the center of the Universe. Everything (Sun, Moon, stars, etc.) revolves around the Earth. The Geocentric Model.

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AST 111 Lecture 3

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  1. AST 111 Lecture 3 Recent History of Astronomy

  2. The Geocentric Model IT IS WRONG!

  3. Fine. So what is it? The Geocentric Model places Earth atthe center of the Universe. Everything(Sun, Moon, stars, etc.) revolves aroundthe Earth.

  4. The Geocentric Model But it’s wrong. So why did it survivefor 2000 years?

  5. Opposition to the Geocentric Model • Sun-centered model proposed by Aristarchus • All stars thought to be on surface of a sphere • At equal distances! • Assertions: • If Earth orbits sun, angular separations of stars change • If they don’t, stars must be unrealistically far away • Observation: • The angular separations of stars don’t seem to change

  6. “Conspiracies” in the Geocentric Model • This change in angular separation is called parallax • Hold your two index fingers at different distances and move your head around. • Lack of observable parallax kept geocentric model alive.

  7. “Conspiracies” in the Geocentric Model • Venus maintains about the same brightness • Suggests it orbits Earth! • Should get dimmer as it gets farther away? • This is a truly wretched coincidence. • As it gets farther, it gets dimmer. • BUT its phase increases (just like the Moon) • Ancients didn’t understand phases • Thought the planets were stars that glowed on their own

  8. The Geocentric Model • Greeks placed “Wandering Stars” at different distances • Stars all on outer sphere • Wandering stars “do their own thing in their own sphere”

  9. Retrograde Motion • “Wandering Stars” exhibited a strange feature • Move forward, reverse briefly, resume going forward • Notice that the size and brightness change

  10. Geocentric Model • “Wandering Stars” drove ancient astronomers up the wall, especially retrograde motion • ULTIMATELY BROKE THE GEOCENTRIC MODEL. • Was extremely difficult to fit to geocentric model • Became more and more complicated until simply unreasonable

  11. The Ptolemaic Model • Greek knowledge of astronomy represented by the Ptolemaic Model • “Wandering Stars” move around on small circles that rotate around a large circle

  12. The Ptolemaic Model • Little circle: Epicycle • Big circle: Deferent

  13. The Ptolemaic Model • Still did not predict “Wandering Star” motion accurately enough • Smaller circles added to the small circles • Positioned some of the larger circles off-center • Accurate to “within a hand at arms length”

  14. The Copernican Revolution • This is where they started to get it right. • Nicholas Copernicus born February 19, 1473 in Poland • By then, tables of planetary motion from Ptolemaic model inaccurate

  15. The Copernican Revolution • Copernicus knew about Aristarchus’s sun-centered system • Went farther with mathematical details • Went from philosophical arguments to predictive geometry

  16. It didn’t work very well.

  17. The Copernican Revolution • Was not very accurate • Was made as complicated as Ptolemaic model to make reasonable predictions • He added epicycles

  18. The Copernican Revolution • It didn’t work because: Copernicus held onto ancient belief that heavenly motion must occur in perfect circles.

  19. Tycho Brahe • Good data was hard to come by • No telescopes! • Built a naked-eye observatory • Measured to within an arcminute • 30 years of measurements

  20. Tycho Brahe • Took large amounts of high quality data at his observatory

  21. Tycho Brahe • Aristotle : patterns of stars are “immutable” (unchanging) • One blew up in 1572 • Tycho Brahe made parallax measurements • It was determined to be as far away as the stars

  22. Johannes Kepler • Tycho Brahe’s apprentice • Worked to match circular motions to Tycho’s data • Didn’t work • Trusted Tycho’s data • Assumed sun-centered system

  23. Johannes Kepler Kepler deduced that planetary orbits arein the shape of ellipses, not circles.

  24. Johannes Kepler Rather than brushing aside small errorsto validate a preconceived notion,Kepler trusted the carefully taken data and revolutionized astronomy.

  25. Ellipses and Circles

  26. Kepler’s Laws • Newton was later able to deriveKepler’s Laws from more fundamental principles

  27. Galileo Galilei • Opponents of sun-centered system with elliptical orbits argued: • Non-circular orbits mean celestial realm imperfect • Stars don’t shift position as Earth orbits the Sun • Galileo used the telescope to counter these arguments

  28. Galileo Galilei • Moons of Jupiter

  29. Galileo Galilei • The surface of the Sun:

  30. Galileo Galilei • Resolved individual stars in band of Milky Way:

  31. Galileo Galilei • Phases of Venus

  32. Galileo Galilei • Phases of Venus

  33. “Wandering Stars” • The “Wandering Stars” are PLANETS. • They “wander” because much closer to Earth than stars

  34. “WANDERING STARS” ARE NOT STARS! THEY ARE PLANETS! (Planets are not stars!) WE WILL NEVER USE THE TERM AGAIN!

  35. Stellar Parallax • Stellar parallax was finally observed in 1838 • Measured in arcseconds • 1/60 of the thickness of a fingernail at arm’s length • Can be used to find the distance to nearby stars • This marked the definitive end of the Earth-centered model.

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