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Fine. So what is it?
The Geocentric Model places Earth atthe center of the Universe. Everything
(Sun, Moon, stars, etc.) revolves aroundthe Earth.
Parallax and the Geocentric Model
• Things at different distances line up differently when you move around
• 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.
Opposition to the Geocentric Model
• Sun-centered model proposed by Aristarchus
– 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
A “Conspiracy” 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)
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
• “Wandering Stars” exhibited a strange feature
• Move forward, reverse briefly, resume going forward
• Notice that the size and brightness change
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
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 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
• 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 Revolution
• Copernicus knew about Aristarchus’s sun-centered system
• Went farther with mathematical details
• Went from philosophical arguments to predictive geometry
The Copernican Revolution
• Was not very accurate
• Was made as complicated as Ptolemaic model to make reasonable predictions– He added epicycles
The Copernican Revolution
• It didn’t work because:
Copernicus held onto ancient belief that heavenly motion must occur in perfect circles.
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 Brahe
• Took large amounts of high quality data at his observatory
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
• 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 Kepler
Rather than brushing aside small errorsto validate a preconceived notion,Kepler trusted the carefully taken
data and revolutionized astronomy.
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 Galilei
• The surface of the Sun:
Galileo Galilei
• Resolved individual stars in band of Milky Way:
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.