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The Origin of Modern Astronomy
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• Chapter 4

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Guidepost
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The preceding chapters gave you a modern view of
Earth. You can now imagine how Earth, the moon,
and the sun move through space and how that
produces the sights you see in the sky. But how
did humanity first realize that we live on a
planet moving through space? That required the
revolutionary overthrow of an ancient and honored
theory of Earths place. By the 16th century,
many astronomers were uncomfortable with the
theory that Earth sat at the center of a
spherical universe. In this chapter, you will
discover how a Polish astronomer named Nicolaus
Copernicus changed the old theory, how a German
astronomer named Johannes Kepler discovered the
laws of planetary motion, and how the Italian
Galileo Galilei changed what we know about nature.
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Guidepost (continued)
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• Here you will find answers to four essential
  questions
• How did classical philosophers describe Earths
  place in the Universe?
• How did Copernicus revise that ancient theory?
• How did astronomers discover the laws of
  planetary motion?
• Why was Galileo condemned by the Inquisition?

This chapter is not just about the history of
astronomy. As they struggled to understand Earth
and the heavens, the astronomers of the
Renaissance invented a new way of understanding
nature a way of thinking that is now called
science.
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Outline
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I. The Roots of Astronomy A. Archaeoastronomy B.
The Astronomy of Greece C. Aristotle and the
Nature of Earth D. The Ptolemaic Universe II.
The Copernican Revolution A. The Copernican
Model B. De Revolutionibus III. Planetary
Motion A. Tycho Brahe B. Tycho Brahe's
Legacy C. Kepler An Astronomer of Humble
Origins D. Joining Tycho E. Kepler's Three Laws
of Planetary Motion E. The Rudolphine Tables
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Outline (cont.)
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IV. Galileo Galilei A. Telescopic
Observations B. Dialogo and Trial V. Modern
Astronomy
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The Roots of Astronomy
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• 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.

Newgrange, Ireland, built around 3200 B.C.
Sunlight shining down a passageway into the
central chamber of the mount indicates the day of
winter solstice.
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Stonehenge
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Summer solstice
Heelstone

• Constructed 3000 1800 B.C.

• 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 Around the World
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Chaco Canyon, New Mexico
Slit in the rock formation produces a sunlit
dagger shape, indicating the day of summer
solstice
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Other Examples All Around the World (2)
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Mammoth tusk found at Gontzi, Ukraine
Inscriptions probably describing astronomical
events
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Ancient Greek Astronomers (1)
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• 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)
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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

Greeks assumed the Earth was not moving because
they did not observe parallaxes in the sky.
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Ancient Greek Astronomers (3)
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• 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
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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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Later refinements (2nd century B.C.)
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• Hipparchus Placing the Earth away from the
  centers of the perfect spheres

• Ptolemy Further refinements, including epicycles

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Epicycles
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Introduced to explain retrograde (westward)
motion of planets
The Ptolemaic model was considered the standard
model of the Universe until the Copernican
Revolution.
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The Copernican Revolution
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Nicolaus Copernicus (1473 1543) Heliocentric
Universe (Sun in the Center)
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Copernicus New (and Correct) Explanation for the
Retrograde Motion of the Planets
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Retrograde (westward) motion of a planet occurs
when the Earth passes the planet.
This made Ptolemys epicycles unnecessary.
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Tycho Brahe (1546 1601)
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• High precision observations of the positions of
  stars and planets

• Measurement of the nightly motion of a new
  star (supernova) showed no parallax

• Evidence against Aristotelian belief of
  perfect, unchangeable heavens

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Tycho Brahes Legacy
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New World model

• Still geocentric (Earth in the center of the
  sphere of stars)

• Sun and Moon orbit Earth
• Planets orbit the sun.

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Johannes Kepler (1571 1630)
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• Used the precise observational tables of Tycho
  Brahe to study planetary motion mathematically.

• Found a consistent description by abandoning both

1. Circular motion

1. Uniform motion

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

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Keplers Laws of Planetary Motion
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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
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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
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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)
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2. A line from a planet to the sun sweeps over
equal areas in equal intervals of time.
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Planetary Orbits (3)
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3. 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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Galileo Galilei (1594 1642)
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• 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
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• Moons of Jupiter (4 Galilean moons)

• Rings of Saturn

(What he really saw)
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Major Discoveries of Galileo (2)
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• Surface structures on the moon first estimates
  of the height of mountains on the moon

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Major Discoveries of Galileo (3)
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• Sun spots (proving that the sun is not perfect!)

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Major Discoveries of Galileo (4)
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• Phases of Venus (including full Venus), proving
  that Venus orbits the sun, not the Earth!

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Historical Overview
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