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Note that the following lectures include
animations and PowerPoint effects such as fly ins
and transitions that require you to be in
PowerPoint's Slide Show mode (presentation mode).
2
The Origin of Modern Astronomy

• Chapter 4

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Guidepost
The sun, moon, and planets sweep out a beautiful
and complex dance across the heavens. Previous
chapters have described that dance this chapter
describes how astronomers learned to understand
what they saw in the sky and how that changed
humanitys understanding of what we are. In
learning to interpret what they saw, Renaissance
astronomers invented a new way of knowing about
nature, a way of knowing that we recognize today
as modern science. This chapter tells the story
of heavenly motion from a cultural perspective.
In the next chapter, we will give meaning to the
motions in the sky by adding the ingredient
Renaissance astronomers were missinggravity.
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Outline
I. The Roots of Astronomy A. Archaeoastronomy B.
The Astronomy of Greece C. The Ptolemaic
Universe II. The Copernican Revolution A.
Copernicus the Revolutionary B. Galileo the
Defender III. The Puzzle of Planetary Motion A.
Tycho the Observer B. Tycho Brahe's Legacy C.
Kepler the Analyst D. Kepler's Three Laws of
Planetary Motion E. The Rudolphine Tables IV.
Modern Astronomy
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The Roots of Astronomy

• Already in the stone and bronze ages, human
  cultures realized the cyclic nature of motions in
  the sky.
• Monuments dating back to 3000 B.C. show
  alignments with astronomical significance.
• Those monuments were probably used as calendars
  or even to predict eclipses.

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Stonehenge

• Constructed 3000 1800 B.C.

Summer solstice
Heelstone

• Alignments with locations of sunset, sunrise,
  moonset and moonrise at summer and winter
  solstices
• Probably used as calendar.

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Other Examples All Over the World
Big Horn Medicine Wheel (Wyoming)
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Other Examples All Over the World (2)
Caracol (Maya culture, approx. A.D. 1000)
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Ancient Greek Astronomers (1)

• Unfortunately, there are no written documents
  about the significance of stone and bronze age
  monuments.
• First preserved written documents about ancient
  astronomy are from ancient Greek philosophy.
• Greeks tried to understand the motions of the sky
  and describe them in terms of mathematical (not
  physical!) models.

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Ancient Greek Astronomers (2)
Models were generally wrong because they were
based on wrong first principles, believed to be
obvious and not questioned

1. Geocentric Universe Earth at the Center of the
   Universe.
2. Perfect Heavens Motions of all celestial
   bodies described by motions involving objects of
   perfect shape, i.e., spheres or circles.

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Ancient Greek Astronomers (3)

• Eudoxus (409 356 B.C.) Model of 27 nested
  spheres
• Aristotle (384 322 B.C.), major authority of
  philosophy until the late middle ages Universe
  can be divided in 2 parts

1. Imperfect, changeable Earth,
2. Perfect Heavens (described by spheres)

• He expanded Eudoxus Model to use 55 spheres.

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Eratosthenes ( 200 B.C.)Calculation of the
Earths radius
Angular distance between Syene and Alexandria
70 Linear distance between Syene and Alexandria
5,000 stadia ? Earth Radius 40,000 stadia
(probably 14 too large) better than any
previous radius estimate.
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Eratostheness Experiment
(SLIDESHOW MODE ONLY)
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Later refinements (2nd century B.C.)

• Hipparchus Placing the Earth away from the
  centers of the perfect spheres

• Ptolemy Further refinements, including epicycles

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Epicycles
Introduced to explain retrograde (westward)
motion of planets
The Ptolemaic system was considered the standard
model of the Universe until the Copernican
Revolution.
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Epicycles
(SLIDESHOW MODE ONLY)
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The Copernican Revolution
Nicolaus Copernicus (1473 1543) Heliocentric
Universe (Sun in the Center)
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Copernicus new (and correct) explanation for
retrograde motion of the planets
Retrograde (westward) motion of a planet occurs
when the Earth passes the planet.
This made Ptolemys epicycles unnecessary.
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Galileo Galilei (1594 1642)

• Invented the modern view of science Transition
  from a faith-based science to an
  observation-based science.

• Greatly improved on the newly invented telescope
  technology. (But Galileo did NOT invent the
  telescope!)

• Was the first to meticulously report telescope
  observations of the sky to support the Copernican
  Model of the Universe.

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Major Discoveries of Galileo

• Moons of Jupiter
• (4 Galilean moons)

(What he really saw)

• Rings of Saturn

(What he really saw)
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Major Discoveries of Galileo (2)

• Surface structures on the moon first estimates
  of the height of mountains on the moon

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Major Discoveries of Galileo (3)

• Sun spots (proving that the sun is not perfect!)

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Major Discoveries of Galileo (4)

• Phases of Venus (including full Venus),
  proving that Venus orbits the sun, not the Earth!

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Johannes Kepler (1571 1630)

• Used the precise observational tables of Tycho
  Brahe (1546 1601) to study planetary motion
  mathematically.

• Found a consistent description by abandoning both

1. Circular motion and

1. Uniform motion.

• Planets move around the sun on elliptical paths,
  with non-uniform velocities.

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Keplers Laws of Planetary Motion

1. The orbits of the planets are ellipses with the
   sun at one focus.

c
Eccentricity e c/a
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Eccentricities of Ellipses
1)
2)
3)
e 0.1
e 0.2
e 0.02
5)
4)
e 0.4
e 0.6
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Eccentricities of Planetary Orbits
Orbits of planets are virtually indistinguishable
from circles
Most extreme example Pluto e 0.248
Earth e 0.0167
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Planetary Orbits (2)

• A line from a planet to the sun sweeps over equal
  areas in equal intervals of time.

• A planets orbital period (P) squared is
  proportional to its average distance from the sun
  (a) cubed

(Py period in years aAU distance in AU)
Py2 aAU3
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Historical Overview
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New Terms
archaeoastronomy eccentric uniform circular
motion geocentric universe parallax retrograde
motion epicycle deferent equant heliocentric
universe paradigm ellipse semimajor axis,
a eccentricity, e hypothesis theory natural law  
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Discussion Questions
1. Historian of science Thomas Kuhn has said that
De Revolutionibus was a revolution-making book
but not a revolutionary book. How was it an
old-fashioned, classical book? 2. Why might
Tycho Brahe have hesitated to hire Kepler? Why do
you suppose he appointed Kepler his scientific
heir? 3. How does the modern controversy over
creationism and evolution reflect two ways of
knowing about the physical world?
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Quiz Questions
1. Why are Stonehenge and The Big Horn Medicine
Wheel thought to be ancient astronomical
observatories? a. Petroglyphs at each site
describe how they were used to make
observations. b. Ancient Greek writings list the
important discoveries made at each of these two
sites. c. Stones at each site aligned with
significant rising and setting positions. d. Both
a and c above. e. All of the above.
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Quiz Questions
2. Plato proposed that all heavenly motion is a.
constantly changing b. circular c. uniform d.
Answers a and b above. e. Answers b and c above.
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Quiz Questions
3. How did Claudius Ptolemaeus account for the
retrograde motion of the planets? a. Planets
slow down, stop, and then reverse their orbital
direction around the Sun. b. Inner planets orbit
the Sun faster and pass outer planets as they
orbit around the Sun. c. Each planet moves on an
epicycle, that in turn moves on a deferent that
circles around Earth. d. The Sun and Moon orbit
Earth, whereas all the other planets orbit the
Sun. e. None of the above.
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Quiz Questions
4. Who accurately determined the size of Earth by
considering Sun angles at Syene and
Alexandria? a. Thales of Miletus (c. 624-547
BC) b. Pythagoras (c. 570-500 BC) c. Eudoxus
(409-356 BC) d. Aristotle (384-322 BC) e.
Eratosthenes (c. 200 BC)
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Quiz Questions
5. One of the first principles of ancient
astronomy is that the heavens beyond _____ are
perfect, and the Earth is corrupt. a. the
atmosphere b. the Sun c. the Moon d. Saturn e.
Pluto
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Quiz Questions
6. Who taught that the Earth is stationary and at
the center of the universe with the Sun, the
Moon, and the planets moving around Earth in
perfect circles? a. Thales of Miletus (c.
624-547 BC) b. Pythagoras (c. 570-500 BC) c.
Eudoxus (409-356 BC) d. Aristotle (384-322 BC) e.
Eratosthenes (c. 200 BC)
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Quiz Questions
7. How did Nicolaus Copernicus account for the
retrograde motion of the planets? a. Planets
slow down, stop, and then reverse their orbital
direction around the Earth. b. Inner planets
orbit the Sun faster and pass outer planets as
they orbit around the Sun. c. Each planet moves
on an epicycle, that in turn moves on a deferent
that circles around Earth. d. The Sun and Moon
orbit Earth, whereas all the other planets orbit
the Sun. e. None of the above.
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Quiz Questions
8. What feature of Aristotle's model of the
universe was included in the model proposed by
Copernicus? a. Earth is stationary and at the
center. b. Mercury and Venus move around the
Sun. c. Mars, Jupiter, and Saturn move around
Earth. d. Uniform circular motion. e. Elliptical
orbits.
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Quiz Questions
9. Why did the model of the universe proposed by
Copernicus gain support soon after its
publication? a. It more accurately predicted
the position of planets. b. It gave a better
explanation for the phases of the Moon. c. It was
a more elegant explanation of retrograde
motion. d. The old system of Ptolemy was never
very popular. e. It displaced Earth from the
center of the universe.
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Quiz Questions
10. When Tycho observed the new star of 1572, he
could detect no parallax. Why did that result
undermine belief in the Ptolemaic system? a.
This star is closer than the Moon, and thus stars
are not all at the same distance. b. This star is
closer than the Moon, and thus smaller than other
stars. c. This star is farther away than the
Moon, and thus the heavens are perfect and
unchanging. d. This star is farther away than the
Moon, and thus the heavens are not perfect and
unchanging. e. This star is planet-like.
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Quiz Questions
11. What was the most important contribution of
Tycho Brahe to modern astronomy? a. The
invention of the optical telescope. b. The
discovery of four moons orbiting Jupiter. c. A
model of the universe that was part Aristotelian
and part Copernican. d. The study of the
Supernova of 1572. e. Twenty years of accurate
measurements of planetary positions.
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Quiz Questions
12. How was Tycho Brahe able to make more
accurate astronomical measurements than had been
made before his time? a. He used a telescope to
magnify the image and spacing of celestial
objects. b. He designed and used large devices to
measure small angles. c. His island observatory
was hundreds of miles offshore, under very dark
skies. d. His observatory was at high elevation
and thus above much of Earth's atmosphere. e. All
of the above.
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Quiz Questions
13. How did Kepler's first law of planetary
motion alter the Copernican system? a. It
changed the perfect circles to ellipses. b. It
added epicycles to the perfect circles. c. It
placed the Sun at one focus of each orbit. d.
Answers a and c above. e. Answers b and c above.
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Quiz Questions
14. Which of the "First Principles of Ancient
Astronomy" did Kepler's laws contradict? a.
Earth is at the center of the universe. b. The
heavens are perfect and Earth is imperfect. c.
All heavenly motion is uniform and circular. d.
Both a and b above. e. Both a and c above.
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Quiz Questions
15. What does Kepler's second law indicate about
the orbital speed of a planet? a. The orbital
speed of each planet is constant. b. A planet
moves at its slowest when it is closest to the
Sun. c. A planet moves at its fastest when it is
closest to the Sun. d. The orbital speed of a
planet varies in no predictable way. e. None of
the above.
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Quiz Questions
16. If the semimajor axis of a planet is 4 AU,
what is its orbital period? a. 4 years. b. 8
years. c. 16 years. d. 64 years. e. It cannot be
determined from the given information.
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Quiz Questions
17. Galileo's discovery of four moons orbiting
Jupiter showed that planetary bodies could move
and carry moons. This supports the model of the
universe presented by a. Aristotle b. Claudius
Ptolemaeus c. Nicolaus Copernicus d. Both a and b
above. e. All of the above.
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Quiz Questions
18. What phases of Venus are predicted by the
Ptolemaic system? a. New and Crescent phases
only. b. Quarter and Gibbous phases only. c.
Gibbous and Full phases only. d. Crescent and
Gibbous phases only. e. New, Crescent, Quarter,
Gibbous, and Full phases.
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Quiz Questions
19. What phases of Venus were observed by
Galileo? a. New and Crescent phases only. b.
Quarter and Gibbous phases only. c. Gibbous and
Full phases only. d. Crescent and Gibbous phases
only. e. New, Crescent, Quarter, Gibbous, and
Full phases.
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Quiz Questions
20. The phases of Venus observed by Galileo
support the model of the universe presented
by a. Aristotle b. Claudius Ptolemaeus c.
Nicolaus Copernicus d. Both a and b above. e. All
of the above.
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Answers
1. c 2. e 3. c 4. e 5. c 6. d 7. b 8. d 9. c 10. d
11. e 12. b 13. d 14. e 15. c 16. b 17. c 18. a 19
. e 20. c