Chapter Four Mr. Saks Astronomy

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Chapter FourMr. SaksAstronomy

• The Origin of Modern Astronomy

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-Anonymous

• "There are many things that are known and things
  that are unknown in between is exploration."

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Science

• 16th and 17th Century astronomers tried to
  understand the motions in the sky
• They invented science
• They invented a new way to understand nature
• A logical way of thinking about nature
• A way of understanding what we are and where we
  are

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• Pre-Copernican Astronomy

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The Aristotelian Universe

• Philosophers of ancient world attempted to
  explain the true structure of the universe by
  reasoning from 1st principles.
• First Principle was something that was obviously
  true
• We usually think of making observations first and
  make our deductions or what we call scientific
  thinking
• This chapter is the invention of science in
  knowing our world

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Artistotelian Universe

• These ancient astronomers had three ideas that
  really threw them off as they began to study the
  Universe around them
• First They accepted as a first principle that
  the Earth was located at the center of the
  universe
• Second Philosophers and astronomers accepted
  that the heavens were perfect and the Earth was
  imperfect
• Third Claudius Ptolemy involved math into the
  model of the universe. He moved the Earth from
  the direct center and adjusted the speed of the
  planets

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In the Beginning

• Plato (Greek philosopher)
• Argued the heavens were perfect
• Since the only perfect shape was a circle he
  concluded that all heavenly motions must be a
  circle
• Uniform Circular Motion all motions in heaven are
  circular motions turning at uniform rates

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Platos Student Aristotle

• Aristotle argued that the Earth was the center of
  the universe (geocentric)
• How did he know the Earth was not moving?
• Parallax If the Earth moved then constellations
  should look distorted in our sky as we moved and
  we should seethe sky from different locations at
  different times of the year
• They saw no parallax because the stars are too
  far and the parallax is too small to see with the
  unaided eye
• He devised a model that had 55 spheres turning at
  different rates and at different angles

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Geocentric Problems

• Planetary Motion is the eastward motion of the
  planets against the background of fixed stars
• Planet is a Greek word meaning wanderer.
• However, the planets speed changed and every so
  often these planets would stop and move westward
  for few months then return to their eastward
  path
• Retrograde motion

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Claudius Ptolemy

• Ptolemy was the man who moved Earth off center in
  the universe and adjusting the speed of the
  planets using the equant
• Ptolemy lived 5 centuries after Aristotle and yet
  still championed his ideas on the universe
• But wanted to add a mathematical reason
• To him first principle took second chair to
  mathematical precision
• Mathematical Syntaxis (140 A.D.) contained the
  details of his model
• Islamic astronomers preserved the book and called
  it Al Majisti (The Greatest).

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Trying to Fix a sinking ship

• To fix retrograde motion
• Epicycles are the small circles that planets
  moved as the orbited
• Deferent the large circle or orbit the planet
  followed around the Earth

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Confusion to Confusion

• TO fix the problem of speed changes of planets
• Equant
• To adjust the speed of the planet Ptolemy
  supposed the Earth was slightly off center and
  that the center of the epicycle moved such that
  it appeared to move at a constant rate as seen
  from the point called the equant

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Accuracy

• For some time Ptolemaic universe worked
• The position of the planets were fairly accurate
• But this changed over time, centuries
• Arabians and later European astronomers had to
  update the system
• Compute new constants, change the epicycles

• Alfonsine Tables
• Middle of 13th Century
• Team of astronomers supports by King Alfonso X of
  Castile worked 10 years to rework the Ptolemaic
  system
• It was the last great adjustment to the Ptolemaic
  system

Either way Geocentric ideas lasted for 1500
years from Aristotle and Plato to Ptolemy.
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Nicolaus Copernicus (born 1473)

• A Polish church official with a talent for math
• Most of his life, astronomy was based on the
  Ptolemaic system
• Aristotelian universe charged that the heavens
  were perfect and imperfection was within the
  Earths center
• Geocentric philosophy supported heaven and hell
• To challenge this would be to challenge the
  church and thus commit heresy

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Copernicus Belief

• Believed in a Sun centered model of our universe
• Heliocentric
• In about 1507, at 34, he wrote a pamphlet
  outlining his hypothesis
• Sun centered
• Earth rotated on axis
• Earth revolved around Sun
• He submitted his summary in hand written form and
  often time anonymously

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De Revolutionibus

• De Revolutionbus Orbium Coelestium was finished
  in 1530
• Sun centered benefit
• Retrograde motion immediately explained
• Speed of Earths rotations compared to planets
  that are further away
• SIMPLE

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Copernicus Problem

• Could not totally disprove geocentricity
• His model did not accurately predict the position
  of the planets any better than Ptolemys
• WHY?
• Copernicus still believed in uniform circular
  motion
• Totally disagreed with equants
• Small variations in planet motion were not solved
• Copernicus added epicycles to his model to make
  adjustments
• Prutenic Tables (1551) were based on the
  Copernican model but were not any more accurate
  than the 13th century Alfonsine Tables

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Why was it inaccurate?

• Could not precisely describe planetary motion
• Used uniform circular motion instead of an
  elliptical orbit
• Why was it gaining acceptance?
• Simple
• Symmetry among the motions of the planets
• No longer an empty point in space (equant) in
  which things revolve around
• All planets revolve around the Sun

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The Publishing

• De Revolutiopnbus was not published until
  Copernicus knew he was dying
• Although his hypothesis was correct the model was
  wrong
• His hypothesis put the Sun at the center of
  universe
• Sun is at center of our solar system
• Hypothesis also included uniform circular motion
  (incorrect)
• With the Earth in the center, planetary motion
  was still uncertain.
• Copernicus a mathematician could not explain
• Maybe we just needed a great observational
  astronomer
• (HMWK 1)

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Tycho Brahe

• A Danish nobleman was born in 1543, 3 years after
  De Revolutionibus was published
• Tycho studied planets and the stars with great
  accuracy
• His contribution to astronomy was the exact
  observations and instruments he made to make
  these observations.

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Tychos Supernova (1572)

• A new star appeared in the sky shining more
  brightly than Venus
• Tycho could measure no parallax to this new star
• This put this new star at a distance greater than
  the moon
• Challenging Aristotle and Ptolemy which stated
  that the spheres beyond the Moon were perfect and
  unchanging
• De Stella Nova (The New Star) was published in
  1573

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Tychos Findings

• Tycho dismissed Ptolemys model stating the
  starry sphere was not perfect
• Tycho put the Earth in the center of the universe
  once again
• He stated that the moon and the Sun revolve
  around the Earth which is a stationary object in
  space
• All the other planets revolve around the Sun

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Rudolphine Tables

• Tycho became very popular with his new book
• The King gave him lots of money to build
  observatories and study the stars
• He revised the Alfonsine Tables and formed these
  new tables in honor of the Holy Emperor Rudolph
  II
• In his efforts Tycho hired mathematicians and
  astronomers
• One math guy was Johannes Kepler

• Kepler was trying to help solve the problem of
  planetary motion for Tycho when Tycho died
• Tycho tried to resolve the problem of Earths
  place by assuming the universe was geocentric
• Tycho made great observations by doing several
  trials and taking the average
• This method had not been practiced prior to this
  point although it is common today

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Johannes Kepler

• Born in 1571 to a poor family
• While in college found and began to believe in
  the Copernican hypothesis
• 1596, the same year Tycho came to Prague, Kepler
  was ready to solve the mysteries of the universe

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Mysterium Cosmographicum

• Keplers book contains almost no value except to
  prove he knew a lot about math and astronomy
• He sent copies to Tycho and Galileo, both of
  which could see his genius
• He tried to state that spacing of the planetary
  orbits are caused by their spacers (a cube,
  tetrahedron, dodecahedron, icosahedrons, and
  octahedron).
• Since he only recognize these five regular solids
  there could only be 6 planets (Mercury, Venus,
  Earth, Mars, Jupiter, and Saturn)

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Kepler Joins Tycho

• Kepler jumped at the chance to work with the
  great Danish astronomer
• When Tycho died, he appointed Kepler his
  replacement
• Little did Tycho know that Kepler did not believe
  in geocentricity
• Kepler used Tychos careful observations and
  notes and began to study Mars to solve how the
  planet moved

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Astronomia Nova

• Kepler finally found that the orbit of Mars was
  an ellipse and not a circle
• He finally abonded the 2000 year old belief of
  circular motion
• Kepler also found their speed changed
• Closer to the Sun the faster the orbit
• Further from the Sun the slower the orbit
• Kepler also discussed the force that holds the
  planets in orbit and was extremely close to
  stating the principles of mutual gravitation

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Keplers Discoveries

• Keplers Supernova (1604)
• Harmonice Mundi (The Harmony of the World) 1619
• Wrote about supernovas, comets, and Copernican
  astronomy

• Keplers Three Laws of Planetary Motion
• The orbits of the planets are ellipses with the
  Sun at one focus
• A line from a planet to the Sun sweep over equal
  areas in equal intervals of time
• A planets orbital period squared is proportional
  to its average distance from the Sun cubed

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Rudolphine Tables Published

• Printed in 1627, Kepler dedicated the publication
  to Tycho
• Based on the heliocentric model of Copernicus
• Included elliptical orbits
• These publications were Keplers masterpiece
• But still did not know why the planets moved or
  why they followed ellipses
• That would have to wait for Newton
• Kepler died in 1630

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Galileo Galilei

• Born in Pisa, 1564
• Studied medicine at a local university but loved
  math
• In 1597 he wrote a letter to Kepler stating he
  did not believe in Copernicanism because of the
  criticism he might endure
• That all changed with the telescope

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The Telescope

• First invented in 1608 by a Dutch lens maker Hans
  Lippershey
• 1609 Galileo built his own
• The first to observe the sky systematically and
  apply his observations to the issue
• Where is the true place of the Earth?

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Sidereus Nuncius

• Galileo published a book on his observations
• The moon was not perfect, it had mountains,a and
  valleys
• Aristotle held the moon was perfect
• The Milky Way was made of billions of stars, some
  too faint to see with the naked eye
• He discovered four new planets circling Jupiter
• The Galilean moons of Jupiter
• These moons moved and stayed with Jupiter
• Not all heavenly motions were centered around the
  Earth as stated by Aristotle

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Galileos Continued Discoveries

• He found sunspots on the Sun
• Sun was less than perfect
• Further conclude the Sun was a sphere and rotated
  on its own axis
• Venus moved through phases
• Proved it did indeed revolve around the Sun

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Dialogue Concerning the Two Chief World Systems

• Galileos book comparing Aristotles method to
  Copernicus method in 1632
• Allowed as long as it was as a hypothesis
• Galileo was charged with heresy
• He eventually plead that the Copernican ideas
  were wrong in order to avoid imprisonment
• Galileo died January 1642
• HMWK 2

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Isaac Newton

• Born Christmas day 1642
• Attended Trinity College to study physics and
  mathematics
• He is the fruit that the first four astronomers
  planted the seeds for in astronomy

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Black Plague

• 1665 the Black Plague swept through England
• Newtons college closed
• He spent his time working on discoveries in
  optics, mechanics, and mathematics
• It was during this time some of his greatest
  discoveries were found

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Principia (1687)

• During the plague
• New optics
• Developed three laws of motion
• Divined the nature of gravity
• Invented differential calculus
• He published his finding in his book in 1687

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Newtons First Law of Motion

• Newton observed the Moon
• He thought there must be some force pulling the
  moon toward the center of the Earth
• If there was no force the moon would continue
  moving in a straight line and leave the Earth
• He observed that a body at rest or in motion will
  remain as such unless a force acts upon it.

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Newtons Second Law of Motion

• Newton then made the observation that a bodys
  change in motion is a direct result of a force
• The force of the Earth pulling on the moon toward
  the Earth
• Apple falling from a tree toward the center of
  the Earth

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Newtons Third Law of Motion

• Newton noticed that forces act in pairs
• If one body attracts another, the second body
  must also attract the first
• He concluded that all masses have gravity
• Gravity will also depend on the masses of the
  bodies involved
• Newtonian gravitation
• Universal mutual gravitation

Mass the amount of matter in object
Newton also noticed the closer to objects were
together the stronger the gravitational force
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Orbital Motion

• These laws solved the longing question why the
  planets move
• Suppose we fire a cannon on a mountain top
• Eventually it falls due to gravity
• If we use enough gun powder it might not fall
  even though gravity pulls on it, why?
• Gravity will pull it toward the center, but
  Earths surface is curved
• The surface curves away at the same rate at which
  it falls
• It is in orbit

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Circular Velocity

• The velocity needed to maintain orbit
• Used to put satellites into space
• Just above the Earths atmosphere must travel
  about 7,800 m/s, about 17,000 mph
• Newtons laws remade astronomy into an analytical
  science
• It could be measured
• We could calculate forces that celestial bodies
  were imposing on each other

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The Foundation Effect

• It is nature of science to build on the
  discoveries of the past
• Newton admitted this
• If I have seen farther than other men, it is
  because I stood upon the shoulders of giants.