The History of Astronomy

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The History of Astronomy
http//www.phys.uu.nl/vgent/babylon/babybibl_intr
o.htmhttp//mason.gmu.edu/jmartin6/howe/Images/p
ythagoras.jpghttp//www.russellcottrell.com/greek
/aristarchus.htmhttp//www.mesopotamia.co.uk/astr
onomer/homemain.htmlplato.lib.umn.edu/http//web
.hao.ucar.edu/public/education/sp/images/aristotle
.htmlhttp//web.hao.ucar.edu/public/education/sp/
images/ptolemy.htmlhttp//www.windows.ucar.edu/to
ur/link/people/ancient_epoch/hipparchus.htmlhttp
//copernicus.atspace.com/http//www.danskekonger
.dk/biografi/andre/brahe.htm/http//antwrp.gsfc.n
asa.gov/apod/ap960831.htmlhttp//www.lucidcafe.co
m/library/95dec/newton.html
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The Model of the Cosmos

• Its a natural thing for people to want to
  understand the world around them.
• People build models, a (simplified?) conceptual
  framework that represents the real world and
  operates in a manner consistent with
  observations.
• To be successful, a model MUST be able to do two
  things

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Model Building

• A model must accept and incorporate all careful,
  accurate observations. No observations may be
  conveniently discarded, simply because they
  contradict the model. The model must be
  refined!
• A model must be able to make accurate predictions
  of future events.

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Models Change

• Take note of how the model of the cosmos changes
  as new observations become available.
• Take note also of the forces which resisted
  logical changes mental inertia, political
  censorship and religious dogma.

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The First Astronomers

• The first astronomers were the Babylonians.
• Other cultures observed the stars, but the
  Babylonians were the first to keep records of the
  positions of objects in the sky.

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Babylonian Astronomy

• The Babylonians kept careful records on clay
  tablets for 1400 years, from 1600 B.C. until 200
  B.C., even though they were repeatedly conquered,
  and their culture declined.

The tablet aboverecords a lunar eclipse.
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Religious Motivation

• To the Babylonians, the objects moving regularly
  or erratically through the sky were deities, gods
  who could influence the events and futures of
  men.
• Marduk was their chief.

Mardukhttp//www.ancientneareast.net/religion
_mesopotamian/gods/marduk.html
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Religious Motivation

• Because the motivation for the Babylonians
  astronomy was religious rather than scientific
  (they wouldnt even have recognized the word!),
  the Babylonians never bothered to make a model of
  what the cosmos (universe to them, solar system
  to us) was like.

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The Ancient Greeks

• The Greek culture overlapped the end of the
  Babylonian period (600 to 100 B.C.)
• Certain Greek sub-cultures (notably the
  Athenians) came to prize logic and philosophy as
  the way to understand the world.
• You might recognize some of the names
  Pythagoras, Plato, Aristotle, Democritus,
  Aristarchus, and Hipparchus.

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Pythagoras

• The philosopher Pythagoras was the first to
  propose a model of the cosmos.
• Because of their symmetry, Pythagoras thought
  that some shapes were more perfect than others.

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Pythagoras (2)

• Because the sphere was perfectly symmetrical in
  every direction, it was considered to be the most
  shape.
• The sky and everything in it appeared to be
  spherical. It was also thought that the earth,
  being the center of life and thought, was
  spherical and therefore perfect.
• It follows that things that are perfect dont
  ever need to change.

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Pythagoras (3)

• Pythagoras proposed a series of nested,
  concentric, crystalline spheres which rotated at
  constant speeds around the earth.
• Each sphere would carry a single object on its
  inner surface.
• The moon was on the closest sphere, followed by
  Mercury, Venus, the Sun, the outer planets.
• The stars were painted on the outermost sphere.
• The spheres were each able to rotate
  independently at different, but uniform speeds.

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http//csep10.phys.utk.edu/astr161/lect/retrograde
/aristotle.html
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Plato

• Plato was an extremely influential person in
  ancient Greece. Because he was so highly
  regarded, what he said was often taken as
  absolute truth.

http//plato.lib.umn.edu/Images/plato.jpg
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Plato (2)

• Plato didnt change Pythagoras model of the
  cosmos, but Platos contribution to the
  model-building process was his insistence that
  any model save the appearances. By this, Plato
  meant that any model of the universe had to
  accurately match the positions of the objects in
  the sky.

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Plato (3)

• Platos belief in the Pythagorean model and his
  insistence that a model must save the
  appearances became driving forces in the early
  understanding of the arrangement of the cosmos.
• (Note, however, that Plato himself couldnt live
  up to his own pronouncements Pythagoras model
  couldnt explain retrograde motion of the
  planets!)

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Aristotle

• Aristotle was a student of Platos.
• He was the first to try to understand not just
  the way the cosmos was arranged, but the why and
  how of its functioning.

http//www.seanet.com/realistic/chpt4.html
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Aristotle (2)

• A Greek philosopher, Democritus, had proposed
  that every different substance in the universe
  had its own type of atom. Imagine cheese,
  wood, hair atoms!
• A competing idea was that there were only 5
  elements, fire, air, water, earth, and ether and
  that every different substance was composed of
  different ratios of these elements.

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Aristotle (3)

• Aristotle believed that the elements would always
  try to return to their sources.
• No forces were necessary for this to occur
  rather it was natural for these movements to
  happen.

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Aristotle (4)

• Some examples
• Smoke rises, because it is mostly air.
• Flames rise because they were trying to return to
  the sun.
• Water flows downhill because its attempting to
  return to the sea.
• A thrown rock falls because its returning to the
  earth.

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http//www.24hourmuseum.org.uk/nwh_gfx_en/ART23268
.htmlhttp//www.fs.fed.us/r9/mnf/index.shtml
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Natural Motion

• Aristotle used arguments like these to explain
  that Pythagoras crystalline spheres turned
  naturally no force or engine was required to
  drive the revolution.
• Today, we reject natural motion and accept
  something equally non-intuitive action at a
  distance, forces like gravity and magnetism,
  transmitted with no visible mechanism or
  connection.

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Aristotle (5)

• Aristotle also had other reasons for believing
  that the earth didnt move, but that the heavens
  did
• He reasoned that if the earth were moving, a
  stone thrown into the air would fall back to the
  earth along a parabolic track, not a vertical
  track.

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Aristotle (6)

• An arrow fired directly north, would appear to
  veer to the west as the earth under the arrow
  rotated eastward.
• Since neither of these things appeared to happen,
  it was quite natural for Aristotle to believe
  that the earth did not rotate.

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A competing idea

• Aristarchus of Samos proposed a heliocentric
  model of the cosmos.
• It incorporated a rotating earth which revolved
  around the sun, along with all the other planets.

http//www.krzysiek.finka.pl/szkoly/foto/rel02_fot
10_aristarchus.jpg
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A Prediction

• Why wasnt Aristarchus model accepted?
• It made the prediction of heliocentric stellar
  parallax, which wasnt observed.
• Parallax is the apparent movement of a nearby
  object that is really due to the movement of the
  observer.

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http//www.physics.carleton.ca/watson/410_notes/H
istory_of_Astronomy/410_Astro_history.html
Angle a gt Angle b
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A Modern Explanation of Parallax
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alx.jpg
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Try these animations

• http//instruct1.cit.cornell.edu/courses/astro101/
  java/parallax/parallax.html
• http//physics.bgsu.edu/layden/Anim/Parallax/para
  llax.htm

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Parallax (2)

• Today, with large telescopes, we are able to
  detect and measure the parallax of nearby stars.
• Even the nearest stars show parallaxes of less
  than about 0.3 arcsecond, an apparent motion far
  too small to see with the naked eye.
• Parallax is real, but the ancients couldnt see
  it without the technology of the telescope.

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Problems begging for solutions

• After the time of Aristotle, there were several
  problems that the model of the cosmos couldnt
  explain.
• Retrograde motion of the planets.
• The apparent speeding up and slowing down of the
  sun and planets at different times of the year.
• Varying shapes and durations of the planetary
  retrograde motions.

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Hipparchus of Rhodes

• Hipparchus worked from about 160 to 130 B.C.
• He was a mathematician who used geometry to try
  to solve the problem of retrograde motion.

http//universe-review.ca/I08-18-Hipparchus.jpg
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A Clockwork Cosmos

• Hipparchus extended the idea of the crystalline
  spheres. The main path or orbit of the planet
  was termed the deferent.
• Attached to and centered on the deferent was a
  second, smaller orbit called the epicycle. The
  planet revolved as the deferent and epicycle both
  revolved.

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http//faculty.uml.edu/awalters/43.311/lecturesf2k
2/Slide9.GIF
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Real backward motion

• As the deferent and epicycle both turned
  independently, the planet would actually move
  backward during the retrograde (westward) portion
  of its motion.
• With a correctly sized deferent and epicycle, the
  predicted positions of the planets would match
  the actual positions within naked-eye accuracy
  limits!!!

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http//pl.wikipedia.org/wiki/GrafikaEpicycle_et_d
eferent.png
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Attacking problem 2

• To try to solve the problem of the sun and
  planets traveling faster at some times of the
  year than others, Hipparchus proposed the
  eccentric.
• Despite the requirement that the earth be at the
  center of the cosmos, Hipparchus placed the earth
  off-center by a small distance.

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The Eccentric

• The off-center placement allowed the sun and
  planets to appear to speed up when they were
  closer to the earth and appear to slow down when
  they were farther away. (The angular velocity no
  longer appears to be uniform.)

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The Eccentric

• Imagine standing in the exact center of the
  infield of a race track. Walk towards the
  tracks inner edge and the cars appear to be
  moving faster on the side youre closer to, and
  slower on the opposite side.

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http//www-astronomy.mps.ohio-state.edu/pogge/Ast
161/Unit3/Images/epicycle.gif
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The 3rd Problem

• The last problem to be solved was that of
  different shape duration planetary retrograde
  motions from one year to the next.

http//www.xtec.es/recursos/astronom/articulos/ret
ro/indexe.htmhttp//jcboulay.free.fr/astro/sommai
re/astronomie/univers/galaxie/etoile/systeme_solai
re/mars/page_mars3.htm
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Ptolemy

• Hipparchus never solved this last problem. It
  had to wait for a Greek astronomer working in
  Alexandria, Egypt around 125 A.D. Claudius
  Ptolemy

http//www.livius.org/a/1/greeks/ptolemy.jpg
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The Equant

• Ptolemy proposed a point in space opposite the
  eccentric point, called the equant, where the
  angular speeds of the sun and planets would
  appear to be uniform.

http//www-astronomy.mps.ohio-state.edu/pogge/Ast
161/Unit3/Images/equant.gif
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The Equant (2)

• While this helped solve the problem of
  differently shaped retrograde loops, it also
  violated the premise that the crystalline spheres
  turned with uniform speeds. Now they were
  required to actually speed up and slow down.
• How does this happen when no force or engine
  drives the crystalline spheres?

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A Special Problem - Epicycles of Venus Mercury

• Ptolemy also realized that Hipparchus model had
  another problem with Mercury, Venus, and the
  Sun all revolving around the earth, Mercury and
  Venus should sometimes appear in opposition to
  the sun (180o from the sun in our sky).
• However this never happened. Venus was never
  more than 46o from the sun, and Mercury never
  more than 28o.

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The Solution for Mercury Venus

• Ptolemy proposed that the epicycles of Mercury
  Venus be pinned to a line drawn between the Sun
  and the Earth.
• In this way, those two planets could oscillate
  from one side of the sun to the other, yet
  continue orbiting the earth.

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The Epicycles of Venus and Mercury, pinned to a
line drawn fromthe Sun to the Earth.
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A Prediction

• Ptolemys setup for the epicycles of Mercury and
  Venus makes a prediction each planet should be
  able to show crescent and new phases as seen from
  the earth, but never a full phase.
• Later, well see that we actually do see full
  phases for Mercury and nearly-full phases for
  Venus.

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Ptolemys 2 other accomplishments

• Ptolemy calculated what he believed to be the
  size of the cosmos 20,000 earth radii or
  134,000,000 kilometers (radius).
• Ptolemy wrote the first astronomy textbook, the
  Almagest (the Majestic Book).

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The Almagest
http//www.er.uqam.ca/nobel/r14310/Ptolemy/Images/
Regiomontanus/1496.g.jpg
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Why does an idea persist?

• Because these ideas were now in print and were
  published at then Great Library in Alexandria,
  these ideas became institutionalized.

http//ils.unc.edu/dpr/path/alexandria/
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Heres the kicker!

• Despite the complex geometry and logical
  inconsistencies, this model worked well enough to
  accurately predict the positions of the planets
  to within a few minutes of arc!
• The Ptolemaic model works well enough that the
  planetarium projector mechanism is based on it!
• Its no wonder that this system wasnt seriously
  challenged for 1400 years!