Classical Greek Astronomy

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• Classical Greek Astronomy

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Quick History

• The first traces of settlement date to around
  3,000 B.C,
• Several major civilizations rose and fell over
  the centuries
• Minoans 2700-1450 B.C., mainly on Crete
• Mycenaeans 1550-1050 B.C, mainland Greece
• Classical Greece 776-323 B.C.
• Macedonians 323-88 B.C.
• Romans/Byzantines 88 B.C.-1453 A.D.

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Problems

• The biggest problem of studying Greek astronomy
  is the lack of the original works
• This is due to both the destruction of libraries,
  such as at Alexandria, and the early Christian
  Church's attitudes toward Greco-Roman culture
• Most writings have been completely lost and are
  known only through writings of later authors
• Since the original works no longer exist, only
  the general ideas, not the specific details are
  known

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The Beginning

• The earliest astronomical references in Greece
  were found in the works of Hesiod and Homer, from
  nearly 3,000 years ago.
• These pieces give practical advice for navigation
  and farming
• When Orion and Sirius are come to the middle of
  the sky and the rosy fingered dawn confronts
  Arcturus, cut off all your grapes and bring them
  home with you-Hesiod, 700 B.C.

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The First Scientists

• The Greeks were notable in that they were the
  first people that attempted to explain the world
  in terms of natural phenomenon, leaving out gods
  and myths
• Obviously, many of the following theories are
  wrong and may seem funny today, but they were
  closer to explaining the world than the stories
  of gods used by all previous cultures
• The key innovation is the Greeks' attempt to
  explain the world rationally

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• The Astronomers of Miletus

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Thales

• Lived 624-547 B.C.
• One of the legendary Seven Sages, the first known
  Greek to explain the world naturally
• Originally a merchant who gathered knowledge on
  his business travels, especially to Egypt
• Learned to calculate distances/heights by using
  geometry, which would later factor into astronomy
• Was said to have predicted a solar eclipse

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Anaxamander

• Lived 611-547 B.C.
• Said to have been a student of Thales
• Often called The Father of Cosmology
• Gave the first physical explanations of the
  celestial realm
• The earth was a cylinder floating freely in space
• The sun, moon, and stars were fire filed wheels
  with holes that allowed their light to escape

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Anaximenes

• Lived 585-526 B.C.
• Also gave mechanical explanations of the universe
• Believed that the stars were like nails, fixed to
  the interior of a crystalline vault surrounding
  the earth.
• This idea would eventually grow into the
  celestial sphere, which would dominate
  astronomical thought for almost 2000 years

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• Pythagoras

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Pythagoras

• Lived 582-500 B.C.
• Best known for his theorem on right triangles and
  his 5 perfect solids
• Left no writings of his own, so it's hard to
  determine what ideas were his and what ideas were
  those of his followers,

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Astronomical Innovations

• Believed that the Celestial bodies revolved in
  circles around a hidden central fire, which was
  rendered invisible by a counter Earth
• Believed that the Earth and all other celestial
  bodies were spheres
• These two ideas were quickly accepted
• Also believed that all of the celestial motion
  makes a lot of noise, but that we can't recognize
  it because we have no silence for comparison

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The Pythagoras Model
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• Eudoxus

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Universe Model Updated

• Lived 408-355 B.C.
• Expanded greatly on the idea of the crystalline
  sphere by using many spheres to explain the
  motions of celestial objects
• The stars' motion was explained by the rotation
  of a single sphere
• The sun needed two spheres, one rotates Westward
  once a day to account for rising/setting and the
  other rotates Eastward to account for motion
  through the Zodiac

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Universe Model Updated

• Planets were even more complicated in this model,
  each needing 4 spheres
• Spheres 3 and 4 rotate in the planet's synodic
  period in opposite directions and slightly tilted
  to account for the cyclical figure eight motion
• Sphere 2 rotates Eastward to explain motion
  through the Zodiac. This, combined with spheres
  3/4, accounts for retrograde motion
• Sphere 1 rotates Westward once a day
• What this model fails to account for is the
  changing brightness of planets

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The Eudoxus Model
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• Aristotle

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Proofs of Earlier Ideas

• Lived 384-322 B.C.
• Offered proofs of earlier ideas
• Earth is a sphere
• All falling objects move to the center of the
  Earth, which is straight down. If the Earth were
  a cube, this wouldn't always be true.
• The Earth casts a circular shadow on the moon
  during an eclipse
• Some stars are visible in Greece that aren't in
  Egypt
• Other celestial objects are spheres
• The shapes of the shadow on the moon in an
  eclipse can only be true with a spherical moon

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Long Lasting Ideas

• Aristotle had the best reasoned argument for a
  geocentric solar system
• Taking the earlier idea of Empedocles that the
  universe is made of earth, air, fire, and water,
  Aristotle reasoned that since all of the heavy
  materials are in the Earth, it is heavy and
  therefore must be stationary
• This reasoning helped cement the idea of an
  Earth-centered solar system, which would dominate
  for almost 2000 years

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Argument for Spherical Earth
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Spherical Earth
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Flat Earth
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• Aristarchus

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Astronomy and Geometry

• Lived 310-230 B.C.
• Used geometry to calculate the distance to the
  moon and the ratio of the distances between the
  earth and sun and earth and moon
• Used sound geometry, but wrong data for the
  distances, which resulted in estimates that
  weren't even close
• Once the size of the Earth was determined (later)
  new calculations were made that were remarkably
  accurate

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Heliocentricism

• Aristarchus was one of the few astronomers to
  support the idea of a sun centered solar system
• Aristarchus calculated the sun to be 7 times
  bigger than the earth
• To Aristarchus, it made no sense to place the
  much smaller Earth at the center
• To account for the motion of the sky, Aristarchus
  reasoned that the motion was only apparent,
  caused by the Earth spinning on its axis once a
  day

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Objections

• The heliocentric model was largely rejected, the
  only other astronomer known to support it was
  Seleucus of Selucia
• One objection was that, if the Earth moved, there
  should be a wind caused by its motion
• A second was that anything dropped should land to
  the West of where it was dropped from as Earth
  rushed by during the fall
• A third was that if the Earth orbited the sun,
  two stars should appear different distances
  relative to each other in different seasons (the
  other option of stars being extremely far away
  wasn't apparently considered)

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Objection
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• Eratosthenes

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Earth Measured

• Lived 276-195 B.C.
• Used geometry to calculate the circumference of
  the Earth
• The drive to do this came from reports that the
  Sun cast no shadows at noon in Syene, located in
  Southern Egypt, on the first day of summer
• At the same time on the same day in Alexandria,
  located in Northern Egypt, the Sun cast shadows

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The Method

• Knowing the sun was directly 90 degrees overhead
  in Syene, Eratosthenes measured the sun's height
  in Alexandria, which was 82.8 degrees, meaning
  that the latitudes were 7.2 degrees, 1/50th of a
  circle in difference
• He then had the distance between the two cities
  measured, which was done by men specially trained
  in pacing out the distance.
• The end result came within a few percent of the
  Earth's actual circumference

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Measuring the Earth
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• Hipparchus

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Antiquity's Greatest Astronomer

• Lived 190-120 B.C.
• Is responsible for recording the first variable
  star, which drove him to make a catalog of the
  sky, noting stars' position and brightness.
• We can thank Hipparchus for magnitudes
• When charting positions, he noticed that all
  stars were about 2 degrees off from older
  reference materials he was using
• With this observation, Hipparchus discovered the
  Precession of the Equinoxes

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Precession Pictured
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• Claudius Ptolemy

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Last Great Classical Astronomer

• 90-168 A.D.
• Worked in Alexandria, Egypt
• His greatest accomplishment was the perfection of
  the geocentric system
• The earlier model of Eudoxus failed to account
  for the changing brightness of the planets, the
  fact that different planets retrograde
  differently, and was very complex.
• Ptolemy's system was far simpler and worked even
  better for explaining planets' motion

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Ptolemy's Universe

• Geocentric with the Earth at the center, planets
  and sun orbit.
• Each of the planets, in addition to orbiting the
  Earth, moves in circles, called epicycles, on
  their own orbits, which explains both retrograde
  motion and planets' changing brightness
• Since the system wasn't totally accurate for
  predicting planetary motion, Ptolemy modified the
  model, moving the Earth slightly out of center
• This system goes unchallenged for 1500 years.

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Ptolemy's Model
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In Motion
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• The Decline of Classical Astronomy

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Reasons

• Largely due to the increasing political and
  religious instability.
• All of the astronomical achievements presented
  took place in a time where the Greco-Roman
  culture was unquestionably dominant.
• Shortly after the death of Ptolemy, the Roman
  Empire began a long, steady decline in its power.
  Learning was cast aside as a priority as
  barbarian attacks on the Empire increased.
• Religiously, Christianity became the dominant
  religion and its leaders sought to suppress the
  traditional Greco-Roman culture, which included
  early science, and instead explain the world
  through theology.