A Historical Perspective

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A Historical Perspective

• 2950
• Dr Bryce

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Class notices

• Labs begin this week
• Remember drop/add slips are signed in the Physics
  General Office Room 203 Van Allen Hall

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Fig.02.15
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Next Stop

• A Historical overview of Astronomy!
• What did ancient people believe about the Heavens
• Daily timekeeping
• Tracking the seasons and calendar
• Monitoring lunar cycles
• Monitoring planets and stars
• Predicting eclipses
• And more

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Our observations

• Seasons
• Sun, Moon and Planets appear to move westwards
  during the course of the day and night (rising
  and setting)
• Sun, Moon and Planets appear to move Eastwards
  relative to background stars. i.e. it takes the
  Sun one year to make a full trip around the
  ecliptic

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The Mesopotamians

• Region around the Euphrates and Tigris rivers
• Made astronomical observations which have
  somewhat survived to the modern day
• Noted the Zodiac
• Used a sexagesimal numeral system (base 60)
• Many of our star names come from Mesopotamian
  astronomers, Betelgeuse
• Predicted planetary orbits and eclipses, the
  origination of astrology (Magi)

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Egyptian Astronomy

• We know that the Ancient Egyptians made
  astronomical observations from for example the
  alignments of the pyramids
• No written record.
• Most likely for practical (ie time keeping)
  reasons

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Modern Science and the Greeks

• Greeks were the first people known to make
  models of nature.
• They tried to explain patterns in nature without
  resorting to myth or the supernatural.

Greek geocentric model (c. 400 B.C.)
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A distant Massive Sun

• If the Sun is more massive than the Earth it is
  natural to think of it as stationary
• If the Earth is revolving why dont we feel a
  Westwards wind?
• And why arent the stars brighter when we are
  close to them?

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Eratosthenes measures the Earth (c. 240 BC)
Measurements Syene to Alexandria distance
5000 stadia angle 7
Calculate circumference of Earth 7/360 ?
(circum. Earth) 5000 stadia ? circum. Earth
5000 ? 360/7 stadia 250,000 stadia
Compare to modern value ( 40,100 km) Greek
stadium 1/6 km ? 250,000 stadia 42,000 km
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Development of Science
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• The most sophisticated geocentric model was that
  of Ptolemy (A.D. 100-170) the Ptolemaic model
• Sufficiently accurate to remain in use for 1,500
  years.
• Arabic translation of Ptolemys work named
  Almagest (the greatest compilation)

Ptolemy
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Ptolemys Universe
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Westwards motion?

• We see the Sun, Moon and the Superior Planets
  moving Eastwards relative to the Stars
• But occasionally we see Westwards motion

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Mars
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The explanation
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So how does the Ptolemaic model explain
retrograde motion? Planets really do go backward
in this model..
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Copernicus (1473-1543)

• Proposed Sun centre-ed Heliocentric Solar System
  model in 1543
• Planets still moved on perfect circles
• No contemporary observations could differentiate
  between the Geo and Helio centric models

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Tycho Brahe (1546-1601)

• Compiled the most accurate (one arcminute) naked
  eye measurements ever made of planetary
  positions.
• He could not detect stellar parallax, and thus
  still thought Earth must be at center of solar
  system (but recognized that other planets go
  around Sun)
• Hired Kepler, who used Tychos observations to
  discover the truth about planetary motion.

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Stellar Parallax
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Johannes Kepler

• Used Brahes planetary observations.
• Found that the orbits did not match perfect
  circles.
• Compiled 3 LAWS

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Keplers First Law The orbit of each planet
around the Sun is an ellipse with the Sun at one
focus.
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What is an ellipse?
An ellipse looks like an elongated circle
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Keplers Second Law As a planet moves around its
orbit, it sweeps out equal areas in equal times.

• means that a planet travels faster when it is
  nearer to the Sun and slower when it is farther
  from the Sun.

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Keplers Third Law

• More distant planets orbit the Sun at slower
  average speeds, obeying the relationship
• p2 a3
• p orbital period in years
• a avg. distance from Sun in AU

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Thought Question An asteroid orbits the Sun at
an average distance a 4 AU. How long does it
take to orbit the Sun?

• 4 years
• 8 years
• 16 years
• 64 years
• Hint Remember that p2 a3

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Galileo Galilei (1564-1642)
Galileo overcame major objections to Copernican
view. Three key objections rooted in Aristotelian
view were

• Earth could not be moving because objects in air
  would be left behind.
• Non-circular orbits are not perfect as heavens
  should be.
• If Earth were really orbiting Sun,wed detect
  stellar parallax.

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In Pisa

• According to legend he dropped balls of different
  masses from the tower to show that they fell at
  the same rate.
• In reality his experiment used rolling balls to
  show that a moving object remains in motion
  unless a force acts to stop it.

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The telescope

• Galileo did not invent the telescope.
• He did build his own
• And use it in a logical and systematic way
• He made the first recorded observations of many
  celestial objects.

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One of his first targets

• Was the Sun
• Not recommended, Galileo lost his own sight
• Sunspots, the Sun is rotating and not a perfect
  sphere!

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Galileo and the Moon

• Again not a perfect sphere
• Moon

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Galileo and Stars

• The ancient world had a heaven that did not
  change but observations such as a supernova and
  comet during Tychos lifetime were contradictory
  to that.
• Galileo observed that the Milky Way was made up
  of many stars rather than being a diffuse cloud
  (or creamy nougat and caramel).
• Stars were further away hence no stellar
  parallax measurements

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Galileo and Jupiter

• Observed stars close to Jupiter.
• Overtime the stars positions changed.
• Are in fact Moons.
• How can Earth have the Universe orbit around it
  when there are objects orbiting Jupiter

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Galilean Moons

• Io
• Europa
• Ganymede
• Callisto
• And the Great Spot was first recorded by Galileo

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Saturn and Galileo

• 1610 large Moons on both sides
• 1612 the objects have disappeared, Earth was
  crossing the same plane as the rings
• 1616 Two half ellipses
• 1655 Huygens proposes that the objects are rings
• 1883 First photograph of Saturns rings

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Venus

• Venus goes through phases
• Which only makes sense if it is orbiting the Sun
  and not the Earth
• Was the observation that the Copernican
  revolution needed!
• Landed Galileo in hot water.

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