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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

AST 111 Lecture 3

Recent History of Astronomy

fine so what is it
Fine. So what is it?

The Geocentric Model places Earth atthe center of the Universe. Everything(Sun, Moon, stars, etc.) revolves aroundthe Earth.

the geocentric model1
The Geocentric Model

But it’s wrong. So why did it survivefor 2000 years?

opposition to the geocentric model
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
conspiracies in the geocentric model
“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.
conspiracies in the geocentric model1
“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
the geocentric model2
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”
retrograde motion
Retrograde Motion
  • “Wandering Stars” exhibited a strange feature
  • Move forward, reverse briefly, resume going forward
  • Notice that the size and brightness change
geocentric model
Geocentric Model
  • “Wandering Stars” drove ancient astronomers up the wall, especially retrograde motion
  • Was extremely difficult to fit to geocentric model
    • Became more and more complicated until simply unreasonable
the ptolemaic model
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
the ptolemaic model1
The Ptolemaic Model
  • Little circle: Epicycle
  • Big circle: Deferent
the ptolemaic model2
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”
the copernican revolution
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
the copernican revolution1
The Copernican Revolution
  • Copernicus knew about Aristarchus’s sun-centered system
  • Went farther with mathematical details
  • Went from philosophical arguments to predictive geometry
the copernican revolution2
The Copernican Revolution
  • Was not very accurate
  • Was made as complicated as Ptolemaic model to make reasonable predictions
    • He added epicycles
the copernican revolution3
The Copernican Revolution
  • It didn’t work because:

Copernicus held onto ancient belief that heavenly motion must occur in perfect circles.

tycho brahe
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
tycho brahe1
Tycho Brahe
  • Took large amounts of high quality data at his observatory
tycho brahe2
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
johannes kepler
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
johannes kepler1
Johannes Kepler

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

johannes kepler2
Johannes Kepler

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

kepler s laws
Kepler’s Laws
  • Newton was later able to deriveKepler’s Laws from more fundamental principles
galileo galilei
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
galileo galilei1
Galileo Galilei
  • Moons of Jupiter
galileo galilei2
Galileo Galilei
  • The surface of the Sun:
galileo galilei3
Galileo Galilei
  • Resolved individual stars in band of Milky Way:
galileo galilei4
Galileo Galilei
  • Phases of Venus
galileo galilei5
Galileo Galilei
  • Phases of Venus
wandering stars
“Wandering Stars”
  • The “Wandering Stars” are PLANETS.
  • They “wander” because much closer to Earth than stars



(Planets are not stars!)


stellar parallax
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.