Ancient Astronomy

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Ancient Astronomy
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Before the Greeks

• Mesopotamian Civilizations
• Identification of the zodiac constellations
• Division of a circle into degrees, minutes,
  seconds
• Introduction of leap months
• Popularization of ASTROLOGY
• Observation of the motion of planets
• Conjunctions (inferior, superior)
• Oppositions
• Retrograde motion

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Retrograde Motion
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Early Greek Astronomy

• Pythagoras (c520 B.C.)
• Developed a model of the universe
• Most celestial objects, including earth, are
  spheres
• Celestial bodies move in circular paths
• Earth moves about a stationary, unseen central
  fire
• A counter earth remains between Earth and the
  fire
• With all the movement, a great deal of sound is
  produced, but because weve never heard silence
  we cant tell.

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Advancements Brought by Pythagoras

• The spherical shape of celestial bodies was
  quickly accepted
• Earths shadow during a lunar eclipse is always
  curved
• Objects always fall straight down
• Different stars visible from different areas
• The idea of motion on perfect circles became the
  basis of all other models

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Early Greek Astronomy

• Eudoxus (c375 B.C.)
• Attempt to account for the retrograde motions of
  planets
• All motion was explained by the rotation of a
  series of spheres with a common center
• Each sphere rotates in a certain direction and at
  a certain rate
• Four spheres are needed to account for the motion
  of each planet

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Problems with Eudoxuss Model

• Although it roughly estimates the motion of the
  planets, it assumes each retrograde loop for a
  planet to be the same (not observed)
• Doesnt work very well for some planets
• Doesnt account for the varying brightness of the
  planets

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Later Greek Astronomy

• Aristarchus (c250 B.C.)
• Application of geometry to astronomy
• Small angle equation
• Distance and size of the moon, sun
• Proposed the earth spins to produce the daily
  motion of the sky
• Rejected because of no great wind rushing to the
  west, and that objects fell straight down, not at
  an angle
• Proposed the earth revolves around the sun
• Rejected because the brightness of the stars did
  not change throughout the year

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Later Greek Astronomy

• Eratosthenes (c225 B.C.)
• Used geometry to calculate the size of the Earth
• Observed the altitude of the sun at local noon on
  the summer solstice in two different locations
  (Alexandria and Syene in Egypt)

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Later Greek Astronomy

• Hipparchus (134 B.C.)
• Created a catalog of stars
• Noted both position and brightness
• Noted that positions had changed over previous
  records
• Attributed the difference to precession
• Estimated the precessional cycle to be 28,000
  years (actually 26,000 years)

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The Ptolemaic Model

• Claudius Ptolemy (c140 A.D.)
• Wrote a book on Astronomy The Almagest
• Contained improved methods for finding the
  distances to the Sun and Moon
• Presented a catalog of more than 1000 stars
• Created the magnitude system (still in use)
• Created a model of the Universe
• Was able to closely match the observed motions of
  the sky and each planet
• Could predict the future position of planets

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The Ptolemaic Model

• Geocentric
• Combination of Motions on Circular Paths
• Deferent
• The general eastward motion (prograde)
• Epicycle
• Occasional westward motion (retrograde)
• Remained in regular use for 1500 years

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Jupiter Saturn
Sun
Mercury
Moon
Venus
Mars