Ancient Astronomy

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Ancient Astronomy

• Objects in the Sky move in cycles
• Stars, Sun, Moon, eclipses, etc.
• Why did most ancient people care?
• Agriculture
• Religion
• Egyptians aligned pyramids with cardinal
  directions (2700 - 2100 B.C.)
• Natural philosophy of Greece strongly influenced
  astronomy (500 B.C - 150 A.D)

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Music of the Spheres

• Plato (428-347 B.C) argued that the sphere and
  the circle were perfect shapes because of their
  symmetry
• The heavens being the creation of the god would
  have to be perfect
• The heavens were assumed to contain spheres in
  uniform (constant speed) circular motions
• Earth was the center of motion

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Geocentric Universe

• Aristotle (384-322 B.C.) expanded on the ideas of
  Plato and Eudoxus of Cnidus
• Aristotle placed the Earth at the center of 56
  concentric spheres
• Spheres rotated as to explain the observed
  motions of the Moon, Sun, planets, and stars in
  the sky
• Stars were on the celestial sphere

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Aristotles Model
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Earth the Central Sphere

• The Earth always cast a round shadow on the Moon
  when there was an eclipse
• Eratosthenes measured circumference of the Earth
  (c. 240 B.C)
• 250,000 stadia isabout 41,700 km
• Modern value is 40,000 km

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Aristarchus (310-230 B.C)

• Hypothesized the Sun was the center of motion for
  the planets and Earth
• Retrograde motion of Mars, Jupiter, Saturn
• Believed Sun was bigger than Earth
• Heracleides (388-315 B.C) suggested that Mercury
  and Venus orbited the sun
• Heliocentrism was dismissed for 1800 years

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Ptolemaic Model

• Ptolemy (100-170 AD.) explained retrograde motion
  with epicycles

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Heliocentrism

• Copernicus (1473-1543) developed a theory that
  explained retrograde motion without epicycles
• De Revolutionibus Orbium Caelestium was published
  near his death
• Completed in 1530 not published until his death
• fear of Church
• inaccurate predictions for planets required
  epicycles
• Copernicus put the Sun at the center of motion
• The Copernican Model was inaccurate for planet
  positions, but heliocentrism is correct

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Tychos Data

• Tycho Brahe (1546-1601) was a Danish nobleman who
  studied Astronomy
• As student, noticed inaccurate planet positions
  of earlier models
• In 1572, observed a Supernovae and deduced that
  celestial sphere was not constant
• Tycho performed accurate measurements of
  positions for 777 stars and all the planets, the
  Moon, and the Sun (better than arcminutes)
• Before his death, named Johannes Kepler to be his
  successor

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Kelpers Laws

• Kepler was brilliant at mathematics
• Kepler supported heliocentrism
• Noticed that predictions for Mars were off by up
  to 8 arcminutes using circular orbits
• Believed Keplers data was accurate
• Only other explanation was elliptical orbits
• Discovered 3 laws of planetary Motion

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Circles vs. Ellipses
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Keplers First Law of Planetary Motion
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Keplers Second Law of Planetary Motion
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Keplers Third Law of Planetary Motion
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Galileo and the Telescope

• The telescope was invented by Hans Lippershy in
  1608 (Holland)
• Galileo (1564-1642) was the first to use the
  telescope for astronomical observations
• Galileo supported heliocentrism but not publicly
  in his early years
• The first 3 of his 5 biggest discoveries were
  published in the Sidereal Messenger (1610)

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Galileo's Telescopic Discoveries

• The Moon was not a perfect sphere because of
  shadows from mountains and valleys
• Milky Way was composed of many stars that are too
  faint to see individually without a telescope
• Observed 4 "planets" orbiting Jupiter
• These are now known as the 4 Galilean moons (Io,
  Europa, Ganymede, Callisto)
• Proved there could be other centers of motion
• Observed sunspots on Sun
• Venus had phases much like the Moon

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Phases of Venus

• Supports heliocentrism(Copernicus Kelper)

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