Announcements Wednesday Sep 6

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AnnouncementsWednesday Sep 6

• PRS
• Will start Today
• Please bring to class all lectures
• Homework
• Astronomy tutorial schedule (on website)
• 665 VAN
• Mon,Fri 12pm-1pm
• Tue,Thu 1030am-1230pm
• Clear Sky patrol
• Mon-Thur 9pm 11pm, VAN roof (correction)

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Todays Lecture

• Homework
• Calendars
• Julian vs. Gregorian
• Leap years
• Ancient Greek Astronomy
• Erastosthenes
• Used shadows to measure radius of Earth (10
  modern vlaue!)
• Ptolemy
• First comprehensive star catalog
• Geocentric model of Solar System
• Epicycles, deferents

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Q. What team did the Hawkeyes play last Saturday?

• Michigan
• LSU
• Western Kentucky
• Montana
• UCLA

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Homework 2
Answer Would not change Elevation would still
be 41.5 deg latitude
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Eratostheness Method for Finding the
Circumference of the Earth
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Q. If the distance from Alexandria to Syene was
2x the distance estimated by Erastothenes, how
would this affect his estimate of the
circumference of the Earth?

• It would double the circumference
• It would half the circumference
• It would not change the circumference estimate at
  all

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Retrograde motionPlanets drift eastward among
stars, but sometimes move westward
Click on movie to play
What causes retrograde motion?
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Claudius Ptolemy (c.150 AD)

• Worked at library in Alexandria, wrote "Almagest"
  
• Model of planetary motion epicycle carried by a
  deferent. This explains retrograde motion,
  brightness variation.
• Elaboration Earth and motion centers offset
  (equant). This explains the variable angular
  velocity.

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Ptolemys Model of Motion of a Planet (explains
retrograde motion)
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Ptolemys explnation of retrograde motion
(circles on circles) - Geocentric model
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The Complete Geocentric Model of Ptolemy
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Ptolemaic Geocentric Model

• Successes
• First model with predictive power
• In use for 1500 years!
• Shortcomings
• relative sizes of planet orbits arbitrary
• due to the equant, the model was not quite
  geocentric
• likewise it was not quite uniform circular motion
  
• extensions of the model for motion perpendicular
  to the ecliptic was independent of the model for
  motion along the ecliptic.
• The model contains unexplained coincidences

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Claudius Ptolemy (c. 85-165 AD)

• Greatest ancient astronomer
• Worked in Alexandria at the great library
• Ptolemy published the Almagest, an encyclopedia
  in which he used centuries of Babylonian
  observations of the motions of the planets to
  justify a geocentric (Earth centered) model of
  the universe.
• The ideas of the Ptolemaic system ruled the
  world of astronomy for 1,500 years, until
  Copernicus and Galileo (c. 1600)

9th cent. Greek MSS of the Almagest (Vatican
Library)
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Ptolemy devised the longest used geocentric model
to explain retrograde loops by putting planets on
epicycles and deferents.
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Epicycles explain retrograde motion in geocentric
models
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Nicolaus Copernicus (c. 1580) devised the first
comprehensive heliocentric (Sun-centered) model

• Copernicus imagined a universe where the Sun was
  at the center instead of Earth.
• He suggested that Earths motion around the Sun
  provided a more natural explanation for
  retrograde loops as Earth passed the other
  planets.

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Heliocentric planetary position terminology is
stated relative to Earth
Opposition Inferior conjunction Superior
conjunction Greatest eastern elongation (appears
east of the Sun in the sky) Greatest western
elongation (appears west of the Sun in the sky)
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In this heliocentric model, the planets just
appear to move backwards as the faster moving
Earth laps the more distant planet once each
year when it is at opposition.
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Copernican View of Retrograde motion