Ancient%20Astronomy:%20The%20Geocentric%20View

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Ancient AstronomyThe Geocentric View

• People have been fascinated by the night skies
  since the beginning of civilization.
• Astronomy must be counted among the first of the
  sciences.
• It was intimately related with the development of
  basic mathematics.

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Telling Time and Seasons

• Sundials were used all over the world (Egyptian
  obelisk)
• Crescent moon angles predicted rainy seasons
  (Nigeria)

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Special Seasonal Alignments

• Stonehenge (England -- and others) had many
  alignments for solstices, equinoxes and bright
  stars
• Sun Dagger (New Mexico) shows noon on summer
  solstice other effects in winter equinoxes

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Mesopotamian Astronomy

• MESOPOTAMIANS built observatories starting 6000
  years ago
• the ziggurats had seven levels, one for each
  wandering object in the sky
• Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn
• Thus 7 days to the week
• They tracked stars --- groups rising before sun
  at different times of year implied seasonal
  beginnings for planting and harvesting
  (zodiac).
• Divided circles in 360 degrees,
  each degree into 60 minutes and
  each minute into 60 seconds
  -- we still use!
• Left written records in cuneform so we understand
  them better

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Mesopotamian Astronomy and Influence

• By 2000 BC Ur and other Sumerian and Babylonia
  cities had large temples, or ziggurats, usually
  aligned N-S, like most Egyptian pyramids
• Egyptian and Mesopotamian knowledge spread to
  Europe, Asia and Africa

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Other Ancient Accomplishments

• Mesopotamians could predict planetary positions
  -- synodic periods, e.g., Mars returns to same
  location roughly every 780 days 22 synodic
  periods 47 years, so
• records of old planetary positions could give
  good locations.
• Knew about the SAROS cycle 2700 years ago lunar
  eclipses definitely occurred every 18.6 years.
• Chinese, Indians and Mayans also knew these
  patterns
• Egyptians used astronomical events to forecast
  Nile floods and harvest times.

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PEER INSTRUCTION QUESTION

• You see a 1st Quarter moon about 45?above the
  Eastern horizon. The time is roughly A.
  noon B. 3 PM C. 6 PM D. 9
  PM E. 3 AM

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PEER INSTRUCTION QUESTION

• You see a 1st Quarter moon about 45?above the
  Eastern horizon. The time is roughly A.
  noon B. 3 PM C. 6 PM D. 9
  PM E. 3 AM
• 1st Quarter Moon rises at noon, highest at 6PM
  and sets at midnight (roughly)

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Greek Astronomy The Earth at the Center

• While they may have built upon Egyptian
  Mesopotamian results (not much preserved beyond
  calendars and orientation of temples),
• Greeks tried to EXPLAIN and UNDERSTAND, not
  just
• PREDICT based upon repetitive cycles of motions.
• Thales (624--547 BCE) was claimed to have
  predicted a solar eclipse.
• Anaxamander (611--547 BCE) of Miletus (Asia
  Minor) produced a model Earth as a cylinder,
  Sun, Moon and stars are fire filled wheels --
  precursor of non-mythical explanations.

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Early Greek Ideas of the Cosmos

• Anaximenes of Miletus (585--526 BCE) believed
  stars were fixed to a solid, crystalline vault
  surrounding the Earth -- the concept of the
  Celestial Sphere.
• Pythagoras (582--500 BCE) and his students in
  Croton (S. Italy) argued that
• Earth and all heavenly bodies are perfect
  SPHERES.
• All celestial motions were perfect CIRCLES.
• By then, it was understood that moonshine was
  reflected sunlight.
• Eudoxus (408--355 BCE) had planets moving on
  multiple spheres, all surrounding the Earth.
  These could explain RETROGRADE LOOPS in the
  orbits of MARS, JUPITER and SATURN -- but didn't
  account for diversity thereof or for variations
  in brightness of planets, since their distance
  from Earth was fixed.

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Retrograde Loops
Planets usually go in same direction as stars but
at different speeds. BUT sometimes go
backwards. Mars in main figure and time lapse of
several planets (in planetarium) above. Greek
Model
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Heavenly Spheres Geocentrism

• Earth at Center, then
• Moon
• Mercury
• Venus
• Sun
• Mars
• Jupiter
• Saturn
• Fixed stars on the celestial sphere

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Aristotle The Authority

• Aristotle (384--322 BCE) gave PROOFS that the
  Earth was SPHERICAL objects all fell towards
  its center yet perpendicular to ground ?sphere
  (but it could still be a cylinder).
• Noted shadows cast on moon during eclipse were
  always round -- they sometimes wouldn't be if the
  earth were disk-like (or cylindrical).
• But he also argued that since everything fell
  toward the earth, it was the heaviest thing
  around, therefore it shouldn't move -- EARTH at
  CENTER of the UNIVERSE -- the GEOCENTRIC COSMOS
• This was certainly LOGICAL, but by no means a
  PROOF (as he thought it was).
• His opinions on this and many other subjects
  which he studied and wrote about were considered
  authoritative between 12th 17th centuries in
  the Western world.

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Aristarchus Real Distance Measurements

• Aristarchus (310--230 BCE) of Samos applied
  Euclid's geometry to get the distance to the
  Moon.
• The angular diameter is measured directly the
  linear diameter comes from seeing how much of the
  Earth's shadow the moon occupies during a lunar
  eclipse (about 3/8).
• If the Earth's diameter is known, this allows the
  Moon's to be found at about 3/8ths of Earth's.
• Used geometry to estimate that the Sun was 19
  times further than the Moon (19 times larger,
  since angular sizes are the same).
• Aristarchus then could estimate that Sun was
  about 7 times the diameter of the Earth (19 x
  3/8)
• THIS LED HIM TO PROPOSE A HELIOCENTRIC COSMOLOGY
  --- with the BIG SUN at REST, SMALL EARTH MOVING
  AROUND IT.
• His lunar size was a little too big and his
  distance to the sun much too small, because of
  inaccurate measurements, but the techniques were
  clever and were major advances.

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Lunar Eclipse Geometry
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Ancient Objections to a Heliocentric Picture

• A moving earth should yield a powerful wind that
  would blow us off.
• Stars didn't show measurable parallax (Greeks
  couldn't think of them being SO much further away
  than planets).
• It sure seems like we're standing still and
  everything in the sky is moving, doesn't it?

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The Size of the Earth

• Eratosthenes (276--195 BCE) used geometry and
  simple astronomy to make an accurate
  measurement of the Earth's radius.
• He realized the difference in the altitude of the
  noonday Sun in Syene and Alexandria equaled
  the latitude difference between the cities.
• That gave the ratio
• circumference of the Earth / 360? distance /
  7.2?
• Accuracy determined by distance in stadia ---
  measured by foot and uncertain, but around
  40,000 km, and probably good to 10
• (Correct value 40,074 km or 24,890 miles)

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Eratosthenes Measurement
7.2O/360O Alexandria--Syene distance/Earth
circumference
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Stellar Catalogs and Precession

• Hipparchus (190--125 BCE) utilized
• Aristarchus' method to get the Moon to be 59
  Earth radii away (60 is correct!)
• He made a better measurement of the length of the
  year.
• Hipparchus also saw a NOVA and this caused him to
  make a CATALOG of bright stars.
• Comparing his locations to those recorded about
  170 years earlier he found a difference of about
  2 degrees,
• and concluded that there was PRECESSION -- his
  estimate of a 28,000 year period was very good.

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Ptolemy The Peak of Greek Astronomy

• Ptolemy worked in Alexandria from 127--151 CE.
• As a geographer, he is the first one known to
  have used latitude and longitude on earth.
• His astronomy book, "???????-- megiste -- or
  The Greatest'' -- Arabic name Almagest.
• Catalog of over 1000 stars w/ brightnesses, using
  the MAGNITUDE SYSTEM.
• Ptolemy's influence on astronomy was immense for
  he published a detailed GEOCENTRIC MODEL.
• (Its not clear how much was original to him.)
  

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Improved Geocentric Model
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Detailed Geocentric Model

• Ptolemy showed the simple system of Hipparchus,
  with just a DEFERENT and EPICYCLE was
  inadequate.
• His model added an EQUANT -- the motion of the
  center of the epicycle is uniform only if
  viewed from the equant.
• However, Ptolemy's greatest contribution was the
  publication of his text, a summary of all
  earlier Greek astronomical knowledge.
• While complex, IT WORKED (to the accuracy the
  Greeks could measure) and was USED for 1500
  years! Certainly OPERATIONALLY successful.

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Advanced Geocentric Model
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Arabic Astronomy Knowledge Transmission

• Hypatia (370--415 CE) of Alexandria, built better
  instruments and made more accurate positional
  measurements. She was murdered by monks who
  objected to her paganism and her astrology.
• After the burning of the Alexandria library and
  the fall of Rome, Astronomy in Europe withered,
  with only parts of Greek and Roman knowledge
  retained.
• The rise of Islam (esp. the need to know
  direction to Mecca) led to large observatories in
  Samarkand, Persia, Turkey and eventually Spain
  with more careful observations and improved
  instruments.
• Big catalogs were produced more stars, more
  accurate locations
• Greek and Indian knowledge (e.g., zero) were
  combined and preserved in centers like Baghdad.

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Turkish and Chinese Instruments
Angles and relative positions of stars and
planets were measured and recorded, as were new
stars
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How was careful observation of the sky used in
early cultures?

• To determine the seasons
• To decide when to plant crops
• To navigate on long voyages
• All of the above
• 1 and 2 only

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How was careful observation of the sky used in
early cultures?

• To determine the seasons
• To decide when to plant crops
• To navigate on long voyages
• All of the above
• 1 and 2 only